From the Department of Biochemistry, School of Medicine, Case Western Reserve University, Cleveland, Ohio 44106-4935
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ABSTRACT |
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Eukaryotic initiation factor (eIF) 4A is the
prototypic member of the DEAD box family of proteins and has been
proposed to act as an RNA helicase to unwind secondary structure in the
5'-untranslated region of eukaryotic mRNAs. Previous studies have
shown that the RNA helicase activity of eIF4A is dependent on the
presence of a second initiation factor, eIF4B. In this report, eIF4A
has been demonstrated to function independently of eIF4B as an
ATP-dependent RNA helicase. The biochemical and kinetic
properties of this activity were examined. By using a family of RNA
duplexes with an unstructured single-stranded region followed by a
duplex region of increasing length and stability, it was observed that
the initial rate of duplex unwinding decreased with increasing
stability of the duplex. Furthermore, the maximum amount of duplex
unwound also decreased with increasing stability. Results suggest that
eIF4A acts in a non-processive manner. eIF4B and eIF4H were shown to
stimulate the helicase activity of eIF4A, allowing eIF4A to unwind
longer, more stable duplexes with both an increase in initial rate and maximum amount of duplex unwound. A simple kinetic model is proposed to
explain the mechanism by which eIF4A unwinds RNA duplex structures in
an ATP-dependent manner.
Initiation of protein synthesis in mammalian systems is a complex
process in which the 40 S and 60 S ribosomal subunits are joined with
mRNA and initiator methionyl-tRNA
(Met-tRNAi)1 to
form a translationally competent 80 S initiation complex (for reviews
on translation, see Refs. 1-3). Prior to the formation of an 80 S
complex, a 48 S pre-initiation complex composed of the mRNA, 40 S
ribosomal subunit, initiator methionyl-tRNA, and several initiation
factors (eIFs) must be formed. The first major event in the formation
of this 48 S complex is the binding of eIF2, GTP, and
Met-tRNAi to the 40 S ribosomal subunit, which is
facilitated by eIF1A (1). The second major event is the binding of
mRNA to this 40 S ribosomal subunit. This is a key regulatory step
in the initiation of protein synthesis and involves eIF3, eIF4A, eIF4B,
eIF4F, and possibly eIF4H, a novel initiation factor demonstrated to
interact with RNA (1, 3, 4).
Eukaryotic mRNAs are typically recognized via their
7-methylguanosine (m7G) cap structure by eIF4E, the small
subunit of eIF4F (5). eIF4F also contains a large subunit (170 kDa),
eIF4G, and a subunit that is eIF4A (thus eIF4A may exist as part of
eIF4F or in free form) (6). Following binding of the eIF4F to the 5'
end of the mRNA, initiation factors eIF4A and eIF4B interact with
the mRNA and disrupt secondary/tertiary structure existing in the 5'-untranslated region (UTR) through the helicase activity of eIF4A
(7-9). This facilitates the binding of the 40 S ribosomal subunit to
the 5' end of the mRNA and allows for subsequent scanning by the
40 S subunit to the initiating AUG codon and correct placement of the
Met-tRNAi. Although the general sequence of events is
known, the molecular mechanism by which these steps take place is still poorly understood.
eIF4A is the prototypic member of the DEAD box family of proteins.
These proteins are highly conserved from bacteria to humans and are
involved in a wide range of cellular processes including translation,
ribosome biogenesis, and mRNA splicing (for reviews, see Refs. 10
and 11). Since these proteins have been implicated in a variety of
cellular functions that require unwinding of RNA secondary structures,
it has been proposed that these proteins act as RNA helicases. This is
further supported by the fact that all DEAD box proteins studied
in vitro exhibit an RNA-dependent ATPase
activity and share amino acid sequence similarity to DNA helicases
(11-13). Whereas several DEAD box (and related DEAH and DExH box)
proteins have been shown to unwind RNA duplex structures in
vitro (8, 9, 14-17), relatively little is known about the
biochemical properties of these proteins or the molecular mechanism of
their unwinding activity.
eIF4A has been previously characterized as an
ATP-dependent RNA-binding protein and an
RNA-dependent ATPase and is thought to unwind secondary
structures in the 5'-UTR of mRNAs (7, 18-21). Prior studies with
eIF4A have shown that an additional initiation factor, eIF4B, is a
strict requirement for RNA duplex unwinding activity of eIF4A in
vitro (8, 9, 22, 23). It was hypothesized, however, that eIF4A may
be able to unwind RNA duplex structures independently of other
initiation factors based on the following: (i) eIF4A displays inherent
ATP-dependent RNA binding and RNA-dependent ATPase activities in the absence of other initiation factors; (ii) the
sequence of eIF4A contains the common motifs associated with DEAD box
(and related DEAH and DExH box) proteins, and in several cases it has
been demonstrated that these proteins are capable of unwinding RNA
duplexes in vitro (14-17, 24); (iii) eIF4A alone has been
shown to alter secondary and/or tertiary structure in reovirus
mRNAs and also to have a limited ability in disrupting duplexes
formed by annealing short DNA oligonucleotides to globin and reovirus
mRNAs (25). Furthermore, previous studies of the helicase activity
of eIF4A used concentrations of duplex RNA that were roughly 4 orders
of magnitude below the reported Kact2
value needed to stimulate the ATPase activity of eIF4A (9, 19, 20, 22,
23), and thus failure to observe helicase activity may have been the
result of the poor affinity of eIF4A for the RNA duplex substrate. In
addition, the duplex employed in these studies had a relative stability
( To directly examine these possibilities, as well as to understand the
mechanism of RNA duplex unwinding by DEAD box proteins in general, an
investigation was undertaken to examine the biochemical and kinetic
properties of the helicase activity of eIF4A by using a family of RNA
duplexes of limited stability. Evidence is presented that, in contrast
with previous studies, eIF4A can act in the absence of eIF4B as an
ATP-dependent helicase. The biochemical and kinetic
parameters of the RNA duplex unwinding activity of eIF4A are examined,
and a mechanism is proposed by which eIF4A unwinds RNA duplexes.
Finally, preliminary results are given concerning how this activity is
affected by eIF4B, eIF4F, and eIF4H.
Materials
Reagents were purchased from the following suppliers: rabbit
reticulocyte lysate from Green Hectares, Oregon, WI; ATP, ADP, ADPNP,
and bovine serum albumin from Sigma; [ Methods
Purification of eIF4A, -4B, -4F, and -4H from Rabbit Reticulocyte
Lysate--
Purification of eIF4A, -4B, and -4F follows the standard
procedures used to purify protein translation initiation factors that
have been previously published by this laboratory (26, 27).
Purification of eIF4H is described (4).
DNA and RNA Oligonucleotides--
DNA oligonucleotides were
synthesized by the Case Western Reserve University Molecular Biology
Core Laboratory, purified using an oligonucleotide purification
cartridge and stored in double-distilled H2O. RNA
oligonucleotides were synthesized by Cybersyn, purified by
polyacrylamide gel electrophoresis, lyophilized, and resuspended in
double-distilled H2O. Quantitation of each
oligonucleotide was performed by UV spectroscopy, and a value of 33 µg per 1 A260 was used in determining
concentration. Integrity and proper size of each oligonucleotide was
assessed by 32P end labeling each oligonucleotide
(described below) and running on a denaturing (7 M urea)
20% polyacrylamide gel with known size standards.
Transcription of RNA-1--
RNA-1, a 50-nucleotide RNA
oligonucleotide (see Table I), is synthesized by in vitro
transcription using T7 RNA polymerase. The template for transcription
is composed of the following synthetic DNA oligonucleotides:
5'-GAATTTAATACGACTCACTATAG-3' and
3'-CTTAAATTATGCTGAGTGATAT*CCCCTCT(TTTTG)5ATCGTGGCATTTCGTGCG-5'. (* denotes the transcription start site.) Transcription reactions are performed using Ambion's MegashortscriptTM
transcription kit per the manufacturer's instructions. Integrity and
proper size of the transcript was confirmed by running a sample on a
denaturing (7 M urea) 20% polyacrylamide (19:1
bis:acrylamide) gel. The reaction was then made to be 50 µl total and
passed over a Bio-Rad P6 spin column to remove salts and unincorporated
nucleotides. The recovered volume was phenol-extracted once and
ether-extracted three times. Concentration of RNA-1 was measured by
using Molecular Probes RibogreenTM RNA quantitation assay
kit, using rRNA of a known concentration to generate a standard curve.
32P End Labeling of Oligonucleotides--
Forty
picomoles of RNA-10, -11, -12, -13, -14, or -15 oligonucleotides (see
Table I) and 10 pmol of [ Helicase Substrates--
Duplexes used in the helicase reactions
are made by combining RNA-1 and 32P-labeled RNA-10, -11, -12, -13, -14, or -15 oligonucleotide in a 1.25:1 ratio, respectively.
This excess of RNA-1 is to ensure that a majority of the labeled
oligonucleotide is hybridized to the RNA-1. The complementary strands
are combined in 1× hybridization buffer (1× Tris-EDTA plus 100 mM KCl), and the concentration of duplex was 0.5 pmol/µl.
Samples were heated to 95 °C for 5 min then slow cooled to 4 °C
over 90 min (0.1 °C/5 s) using a programmable thermocycling
instrument. Under these conditions, approximately 85-90% of the
labeled oligonucleotide hybridizes to the RNA-1. The duplex was stored
at Helicase Assay--
Unwinding of duplex substrates was monitored
by following displacement of the radiolabeled strand from the duplex.
In general, 20-µl reactions contained 20 mM HEPES, pH 7.5 (final pH 7.2), 70 mM KCl, 2 mM dithiothreitol,
1 mg/ml bovine serum albumin, and 1 mM magnesium acetate.
The concentration of ATP was 1 mM; duplex concentration was
1.7-1.8 nM (34-36 fmol), and eIF4A concentration was 0.4 µM (8 pmol), unless stated otherwise. Concentrations of eIF4B (monomer), eIF4F, and eIF4H (when applicable) were 0.2 µM. Time courses of reactions without added eIF4A were
performed to quantitate the background unwinding, or thermal
melting,3 of each duplex in
the absence of protein. All reactions were mixed in siliconized tubes
to minimize the amount of material sticking to tube walls and kept on
ice throughout the assembly process. Reactions were incubated at
35 °C (unless otherwise stated) for the duration of the time course.
In most circumstances, eIF4A was the last component added to the
reaction, just before incubation at 35 °C; however, equivalent
results were obtained when either duplex or ATP was added as the last
component. Reactions were terminated by adding 5 µl of a solution
containing 50% glycerol, 2% SDS, 20 mM EDTA, and 0.01%
bromphenol blue and xylene cyanol dyes. The products of unwinding
reactions were analyzed by separation of displaced strand (labeled
RNA-10, -11, -12, -13, -14, or -15 oligonucleotide) from duplex species
by electrophoresis on 12% native polyacrylamide gels (19:1
bis:acrylamide) for 1.5 h at 200 V at 4 °C in 1× Tris
borate-EDTA buffer. Gels were pre-electrophoresed at 4 °C for 20-30
min. Gels were scanned directly using an Ambis radioanalytical scanner
for 30-90 min. Gels were then exposed to Kodak X-Omat AR film at
Quantitation of the Helicase Assay--
Quantitation of cpm in
duplex and displaced/single-stranded RNA bands was performed using the
Ambis software. The degree of unwinding for each reaction was performed
by measuring the percent of cpm in duplex and displaced/single-stranded
RNA bands using the following formulas: % duplex = cpm
duplex/(cpm duplex + cpm displaced strand) and % displaced strand = cpm displaced strand/(cpm duplex + cpm displaced strand). This method
of quantitation was used to account for slightly different yields in
total cpm due to variations in pipetting and gel loading steps. The
total yield in cpm among different reactions did not vary by more than
±10% of the average cpm value of all the reactions in the given
experiment. Furthermore, all reagents and protein preparations used in
the assay were judged to be free of RNase contamination. A single reaction without ( Data Treatment--
Time courses of unwinding RNA duplexes are
the average of at least three separate experiments. All plots were fit
using Prism software by GraphPad. Duplex stability ( Design of the Helicase Substrates--
As noted in the
Introduction, it was hypothesized that eIF4A may display an
ATP-dependent helicase activity in the absence of accessory
initiation factors (eIF4B and eIF4F). To test this hypothesis, a family
of RNA duplex substrates of limited stability was designed that
contained an unstructured 5' single-stranded region of 31 nucleotides,
followed by a duplex region of 10-15 base pairs. The long strand,
designated RNA-1, includes a short sequence to optimize transcription
by T7 polymerase, followed by five A4C repeats that assume
an unstructured conformation. This is followed by an arbitrary sequence
of 18 nucleotides designed to minimize the formation of any
intramolecular secondary structure. Short oligonucleotides of
10-15 nucleotides, designated RNA-10 to RNA-15, with complementary
sequences to RNA-1 are then hybridized to RNA-1 to form duplexes of
increasing length and stability. The characteristics of these
substrates in terms of sequence, length, and stability are summarized
in Table I.
The nearest neighbor method of Turner et al. (29) was used
to calculate the relative stability (
It was reasoned that the initial rate of unwinding of RNA
duplexes by eIF4A should decrease as the length and stability of the
duplex region is increased, to a point where eIF4A is unable to unwind
duplexes of a given stability. Whereas the helicase activity of eIF4A
has been characterized as bi-directional (9), a 5' single-stranded
region was used as this best represents the 5'-UTR of mRNAs
undergoing translation initiation. Previous studies have estimated an
RNA-binding site size of 15-20 nucleotides for eIF4A, which is also
required for maximal stimulation of the ATPase activity of eIF4A (19,
30); thus a single-stranded region of 31 nucleotides ensures a
sufficient binding site size for eIF4A.
eIF4A Acts Independently of Other Initiation Factors as an
ATP-dependent RNA Helicase--
Preliminary results showed
that under conditions used in the helicase assay (see "Methods"),
eIF4A was able to unwind the RNA-1/RNA-10 duplex in an
ATP-dependent fashion in the absence of eIF4B. Typical raw
experimental data of the unwinding reaction using the RNA-1/RNA-10
duplex is shown in Fig. 1A. It
is apparent that the labeled RNA-10 oligonucleotide is released
(unwound) from the RNA-1 strand in the presence of eIF4A and ATP (Fig.
1A, lanes labeled +4A, +ATP) but not in the
presence of eIF4A when ATP was omitted (Fig. 1A, lanes labeled
+4A,
Once eIF4A was shown to possess an ATP-dependent helicase
activity, the biochemical properties of this activity were investigated using the RNA-1/RNA-11 duplex. This duplex was chosen as it displays the highest ratio between the initial rates of
eIF4A-dependent unwinding and background thermal melting
and thus allows for the most accurate measurement of differences in the
unwinding activity of eIF4A as reaction conditions are changed.
Addition of ATP·Mg2+ to the reaction showed saturation at
1-2 mM (Fig. 2A).
A Km value for ATP·Mg2+ in the
helicase reaction is calculated to be 440 µM, which is in
good agreement with the Km value of 330 µM for ATP in the RNA-dependent ATPase
reaction (21). It is stressed, however, that this is an apparent
Km, as duplex substrate under these conditions is
subsaturating. Titration of Mg2+ (using 1 mM
ATP) into the reaction shows optimal helicase activity at 1 mM or when Mg2+ is equimolar to ATP (Fig.
2B). This suggests that excess Mg2+ inhibits the
helicase activity of eIF4A, probably due to enhanced stabilization of
the duplex by free Mg2+. The helicase activity was optimal
under a pH range of 6.4-7.0 (Fig. 2C), and the optimal
concentration of monovalent cation (KCl) was 50 mM (Fig.
2D). The helicase activity of eIF4A was inhibited by ADP, as
a 50% inhibition of the initial rate of unwinding was seen with a 1/10
molar ratio (100 µM ADP) of ADP to ATP (data not
shown).
Effects of protein and substrate concentration on the helicase activity
of eIF4A were investigated using the RNA-1/RNA-12 substrate, as this
allowed accurate measurement of both increases and decreases in the
initial rate of the reaction as a function of eIF4A or duplex
concentration. Addition of eIF4A into the reaction displayed a linear
response in initial rate of unwinding over the range tested (0-1.6
µM, 0-32 pmol, data not shown). Furthermore, increasing
the concentration of protein also increased the maximal amount of
duplex that is able to be unwound (data not shown and Fig. 6).
Titration of duplex into the reaction displayed a linear response in
initial rate over the range tested (0-1.8 nM, 0-36 fmol,
data not shown).
Kinetic Characterization of the Helicase Activity of
eIF4A--
Whereas many DEAD box proteins have been shown to display
helicase activity in vitro, the molecular mechanism of RNA
duplex unwinding is poorly understood. In order to begin elucidating this mechanism, simple kinetic experiments using the duplexes listed in
Table I were performed. It was reasoned that longer, more stable duplex
structures should take longer to unwind, and thus the initial rate of
unwinding by eIF4A of these substrates should decrease as the length
and stability is increased. Therefore, time course experiments using
RNA-1/RNA-10 through RNA-1/RNA-15 were performed. Fig. 1A
demonstrates the rapid unwinding of the RNA-1/RNA-10 duplex
(lanes labeled +4A, +ATP), whereas in contrast, Fig. 1B shows that the RNA-1/RNA-15 duplex is barely unwound
by eIF4A in the presence of ATP (Fig. 1B, lanes labeled
+4A, +ATP). Raw data time points from each duplex substrate
were quantitated and plotted as the amount of duplex unwound (fmol)
versus time. The unwinding reaction may be described by a
pseudo-first order kinetic model and the data fit to a single-phase
exponential equation (see "Experimental Procedures" and Fig.
3). Observed initial rates of the
unwinding reaction were calculated using the linear portion (0-2 min)
of each time course (Fig. 3). Initial rates of the thermal melting
(unwinding of duplex in the absence of eIF4A) of each duplex were also
measured (data not shown) and subtracted from eIF4A-dependent initial rates of unwinding. It is clear
from these plots that the initial rate of eIF4A-dependent
unwinding decreases as the stability of the duplex substrate increases.
Furthermore, the maximum amount of substrate unwound during the time
course also decreased as stability of the duplex increased. The values of the initial rates of thermal melting and eIF4A-dependent
unwinding and maximum percentage of duplex unwound for each substrate
are summarized in Table II.
In order to establish a relationship between the initial rate of
unwinding and duplex stability, the ln of the initial rates of both
eIF4A-dependent unwinding and thermal melting were plotted versus the stability of the duplex (
Whereas the result that the initial rate of unwinding by eIF4A
decreases with increasing duplex stability was expected, the fact that
maximum amount of duplex unwound also decreased was not. Under
conditions used ([eIF4A]
The possibility was then considered that eIF4A may becoming inactive
over the time course of the experiment. To test this hypothesis, eIF4A
was incubated for 30 min under usual reaction conditions without duplex
substrate. When duplex was added, it unwound with the same initial rate
and reached the same plateau value as determined without preincubation
of eIF4A (data not shown). Furthermore, a duplex addition experiment
was performed in which eIF4A was allowed to unwind the RNA-1/RNA-15
duplex for 15 min, and then the RNA-1/RNA-11 duplex was added (Fig.
5). The added RNA-1/RNA-11 duplex was
unwound to the same maximal value (
It was then postulated that the unwinding reaction may be approaching
equilibrium due to a second process in which eIF4A is "stalled" in
the process of unwinding and that a build-up of these stalled,
non-productive complexes prevented the helicase reaction from reaching
completion. Furthermore, it was reasoned that the amount of these
putative non-productive complexes formed should be directly
proportional to the length or stability of the duplex. An attempt was
made to isolate these non-productive complexes by nitrocellulose filter
binding assays and gel shift experiments. In the filter binding assays,
relatively little eIF4A-dependent retention of duplexes
(5% or less) was observed, and there was no detectable difference in
retention among the less stable duplexes, more stable duplexes, or even
single-stranded RNA (RNA-15) (data not shown). Gel shift experiments
were performed in which no SDS or EDTA was used in the quench solution,
and gels were run under reaction buffer conditions. Results yielded no
evidence of a shifted band corresponding to the postulated stalled
complex containing eIF4A (data not shown). This is in agreement with
previous results obtained by Sonenberg and co-workers (32), who were
also unable to detect a complex of eIF4A and duplex RNA in similar
experiments. This hypothesis was further tested by the addition of an
equal amount (8 pmol) of eIF4A to the reaction after the maximal value of unwinding the RNA-1/RNA-12 duplex by eIF4A had been reached (15 min
time point, Fig. 6). It can be seen in
Fig. 6 that addition of 8 pmol eIF4A to obtain 0.8 µM
eIF4A total gave the same final maximal amount of duplex unwound
( Activation Energy of the Helicase Reaction--
As a means to
examine the thermodynamic nature of the helicase activity of eIF4A,
kinetic measurements were taken at different temperatures. Time courses
of eIF4A-dependent unwinding using the RNA-1/RNA-11
substrate at 35, 30, and 25 °C were performed (data not shown).
Pseudo-first order initial rate constants were calculated as described
under "Methods" and plotted versus 1/T (in
Kelvin) (data not shown). The slope from this Arrhenius plot (r2 value of 0.998) yields a value of 24.7 kcal/mol as the activation energy for the unwinding reaction.
Effects of eIF4B, eIF4F, and eIF4H on Helicase Activity of
eIF4A--
Previous investigation of the helicase activity of eIF4A,
eIF4B, and eIF4F showed that eIF4A or eIF4F (which contains eIF4A as a
subunit) were able to function as ATP-dependent RNA
helicases only in the presence of eIF4B (eIF4B has no helicase or
ATPase activity by itself) (8, 9, 22, 23). However, since the above
results have demonstrated that eIF4A is capable of unwinding RNA
duplexes in the absence of eIF4B, it was necessary to reinvestigate the
effect of eIF4B on the activity of eIF4A and to ascertain if eIF4F was
also able to function as an RNA helicase in the absence of eIF4B. These
experiments were extended to study the effect eIF4H on the helicase
activity of eIF4A, since eIF4H has been shown to stimulate the ATPase
activity of eIF4A in a manner similar to eIF4B (4). Furthermore, eIF4H
contains an RNA recognition motif (RRM) that is 45% identical to the
RRM in eIF4B that is required for maximal stimulation of the helicase
activity of eIF4A (4, 33).
To this end, the helicase assay was performed with 0.2 µM
eIF4B (monomer), eIF4F, or eIF4H, both in the presence and absence of
ATP. Then the assay was performed with the addition of 0.4 µM eIF4A to each factor in the presence and absence of
ATP. The RNA-1/RNA-15 duplex was used as a substrate since it was
barely unwound by eIF4A alone and thus allowed measurement of any
enhancement of the helicase activity of eIF4A by these other factors.
Fig. 7A shows the raw data
obtained when eIF4A, eIF4B, eIF4F, or eIF4H are incubated with the
RNA-1/RNA-15 duplex in the presence of ATP (lanes 3-6,
respectively), and when eIF4A is added to eIF4B, eIF4F, or eIF4H in the
presence of ATP (lanes 8-10, respectively). Fig.
7B shows the quantitation of the unwinding reactions for these combinations tested. As seen before, eIF4A alone (lane
3) shows a small amount of unwinding activity toward the
RNA-1/RNA-15 duplex ( The results presented here demonstrate that eIF4A is able to
function as an ATP-dependent RNA helicase independently of
other initiation factors. This was accomplished using eIF4A purified from rabbit reticulocyte lysates (greater than 95% homogenous) and a
family of RNA duplex substrates that contained a 5' single-stranded unstructured region of 31 nucleotides followed by a duplex region of
increasing length (10-15 base pairs) and stability
( Therefore, experiments were initially designed to have approximately
the same reaction conditions as those used previously, except that an
RNA duplex substrate of lesser stability (RNA-1/RNA-10, These results suggest that, by itself, eIF4A is an inefficient,
non-processive helicase. This finding is consistent with the fact that
eIF4A alone is a relatively slow ATPase, maximally hydrolyzing ATP on
the order of 3 per min (21). In addition, studies by Lorsch and
Herschlag (21, 34) demonstrated that the association and dissociation
rates of RNA and ATP for eIF4A are very fast compared with the ATP
hydrolysis event, which evokes a conformational change in eIF4A that
likely allows for RNA duplex unwinding. Thus, under reaction conditions
used in this study (0.4 µM eIF4A, 1.7-1.8 nM
RNA duplex), the ATPase/helicase activity of eIF4A is most likely a
single turnover event for any one eIF4A·RNA substrate complex
(calculations indicate that only 1-2% of the RNA duplex would be
bound to protein at any given time). These results strongly suggest
that eIF4A is only able to unwind a minimal number of base pairs during
the slow ATP hydrolysis step before dissociating from the duplex
substrate. The non-processive nature of eIF4A under these conditions is
further supported by the fact that addition of an excess of poly(A)
after initiation of the reaction abrogates any further unwinding of
substrate (data not shown).
The results from the experiments reported in this article combined with
the results of Lorsch and Herschlag (21, 34) suggest a model for the
helicase activity of eIF4A that has eIF4A unwinding a limited number of
base pairs, which may vary depending on the sequence's contribution to
stability. This model is summarized schematically in Fig.
8 and depicts two kinetic steps of
unwinding; the first step is the ATP-dependent minimal
duplex unwinding by eIF4A, and the second step consists of the
remaining base pairs of the duplex unwinding at the rate of thermal
melting (see below). Thus, under the reaction conditions used, eIF4A
acts as a helicase by unwinding only a few base pairs of the duplex,
and this substrate subsequently behaves as a shorter, less stable
duplex in a thermodynamic sense. This minimal unwinding is sufficient
to completely destabilize the RNA-1/RNA-10 and RNA-1/RNA-11 duplexes,
and unwinding of these substrates is rapid and complete (Fig. 8,
path c). However, with longer, more stable duplexes (12-15
base pairs), this minimal unwinding is insufficient to completely
destabilize these substrates (Fig. 8, path d). Thus, as soon
as eIF4A dissociates, only a fraction of these partially unwound
duplexes continue to unwind (thermally), whereas the other fraction of
partially unwound duplexes reanneal. This is supported by the fact that
the initial rate of unwinding decreases for more stable substrates.
Although these results do not allow the explicit calculation of a
specific number of base pairs unwound per binding event, it is
hypothesized that it is probably small, on the order of 4-5 base
pairs. This would be in agreement with the kinetic data obtained for
the step size of unwinding by the UvrD helicase (35) and is also
supported by the structural data obtained for the hepatitis C NS3
helicase (the sequence of eIF4A is 44% identical and 63% similar to
the NS3 helicase sequence in conserved motifs), which has a binding cavity occupied by 5 nucleotides (36).
INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
G) of
25.7 kcal/mol, which may have been too stable a
duplex for eIF4A to unwind independent of eIF4B or other initiation
factors. Therefore, it was reasoned that eIF4A may possess an inherent
RNA duplex unwinding activity in the absence of eIF4B or other
initiation factors given either increased concentrations of duplex
substrate or a duplex substrate of lesser stability.
EXPERIMENTAL PROCEDURES
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
-32P]ATP and
[
-32P]CTP from NEN Life Science Products; DNA
oligonucleotides from Case Western Reserve University's Molecular
Biology Core Laboratory; RNA oligonucleotides from Cybersyn, Lenni, PA;
MegashortscriptTM In Vitro transcription kit
from Ambion; RibogreenTM RNA oligonucleotide quantitation
kit from Molecular Probes; P-6 spin columns from Bio-Rad; and T4
polynucleotide kinase from New England Biolabs.
-32P]ATP (specific activity
3000 Ci/mmol, 10 mCi/ml) were combined with 10 units of T4
polynucleotide kinase (10 µl final volume), and reactions were
performed as described in Ref. 28. Percent incorporation of the label
into these oligonucleotides is 90-95%, and specific activities of the
oligonucleotides are on the order of 1 × 106
cpm/pmol.
20 °C and diluted in 1× hybridization buffer to 0.02 pmol/µl
for use in the helicase assay.
80 °C for 30-120 min using intensifying screens.
) eIF4A was incubated at 4 °C during the course of each experiment, and the amount of duplex in this control reaction (typically 34-36 fmol) was normalized to 100%, and the amount of
(background) single-stranded RNA (typically 4-6 fmol) was normalized to 0%. All other reactions were scaled to these values and then converted to femtomoles of duplex or single-stranded RNA species. Incubation of the minus eIF4A sample at 4 °C indicates the amount of
starting material (duplex RNA) present at the start of the reactions.
Monitoring the disappearance of duplex RNA (or the appearance of
single-stranded RNA) at 35 °C versus the control reaction
at 4 °C allows for the measurement of both
eIF4A-dependent unwinding and thermal melting over the time
course. Furthermore, incubation at 4 °C (t = 0 and
t = 30 min) indicates that the amounts of duplex and
single-stranded RNA species are preserved during the time course of the reaction.
G)
values for RNA duplex substrates were calculated for 35 °C using the
nearest neighbor method of analysis as described by Turner et
al. (29). The unwinding reaction may be approximated by the
pseudo-first order rate equation Vobs = k'[duplex], where Vobs is the
observed rate of duplex unwinding (or the rate of single-stranded RNA
formation) by eIF4A in fmol/min, k' is the pseudo-first
order rate constant, and [duplex] is the concentration of duplex at
time t. k' is a macroscopic pseudo-first order
rate constant composed of terms including the concentration of eIF4A
(which is constant), and microscopic constants reflecting eIF4A binding
to ATP, eIF4A·ATP binding to the RNA duplex, the ATP hydrolysis rate
constant, and the duplex unwinding rate constant. In the time course
experiments, ATP is saturating (Fig. 2A), and it is assumed
that there is only one eIF4A molecule per unwinding event due to the
poor affinity of eIF4A single-stranded RNA (20, 21). Since
Vobs also equals d[ssRNA]/dt (the
appearance of single-stranded RNA with respect to time), then
d[ssRNA]/dt = k'[duplex], which when
integrated and evaluated yields [ssRNA]t = [duplex]0 (1
e
k't) + c, where
[ssRNA]t = the amount of single-stranded RNA formed at time
t (i.e. amount of duplex unwound),
[duplex]0 = the initial amount of duplex at
t = 0, k' = pseudo-first order rate
constant, and c = amount of single-stranded RNA
(background) at time 0. This equation describes a single-phase
exponential curve, and kinetic data were fit to this equation. A single
phase exponential equation describes the data obtained since [eIF4A]
[duplex], and a decrease in the amount of duplex reflects a decrease in the rate of the reaction. Observed initial rates were calculated by linear regression using the linear portion (0-2 min) of
each plot. Initial rates were used to describe the relationship between
unwinding and duplex stability because (i) the reaction is being
performed under conditions of limiting substrate (duplex) and does not
reflect steady-state kinetics and (ii) correction of the forward rate
of unwinding due to the reverse (reannealing) reaction is not needed,
as its rate is negligible (less than 5% of the forward unwinding
rate), and the amount of single-stranded species which could reanneal
is quite small in the initial phase (0-2 min) of the unwinding
reaction. The observed initial rates for the thermal melting of
duplexes were calculated as above and subtracted from the observed
initial rates of eIF4A-dependent duplex unwinding.
RESULTS
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
Characteristics of RNA duplex substrates
G value) of duplex
substrates. Although assay conditions used in this report differ
from the conditions used by Turner et al. (29) to determine
stability, it is emphasized that the objective of these experiments is
to focus on the relative differences in stabilities among
different duplexes (
G values) and the rate of
unwinding by eIF4A, rather than the determination of the absolute
G value of each duplex. Thus, even though the calculated
stabilities of the duplex substrates may not entirely reflect their
correct absolute
G values, they provide a reliable scale
for correlating the helicase activity of eIF4A with relative
increases/decreases in duplex stability.
ATP). In contrast, the RNA-1/RNA-15 duplex was barely
unwound by eIF4A under these same conditions (Fig. 1B, lanes
labeled +4A, +ATP). This difference is addressed below and under
"Discussion." Unwinding of the RNA-1/RNA-10 substrate was not seen
in the presence of the non-hydrolyzable ATP analog, ADPNP (data not
shown). This unwinding activity is due to eIF4A and not a contaminant
in the reticulocyte lysate preparation, as recombinant human eIF4AI
purified from Escherichia coli displays the same activity
(data not shown).
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Fig. 1.
eIF4A acts as an ATP-dependent
RNA helicase. Autoradiograms illustrating the unwinding of the
RNA-1/RNA-10 (A) and RNA-1/RNA-15 (B) duplexes by
eIF4A as a function of time. All time courses were performed at
35 °C unless noted otherwise. Lanes/reactions designated as
eIF4A (no protein) were incubated at 4 °C for the time
indicated.
denotes the duplex was heated to 95 °C for 3 min and
then incubated at 35 °C for the time indicated. The concentrations
of eIF4A and ATP·Mg2+ were 0.4 µM and 1 mM, respectively, when present in the reaction. The
concentration of duplex was 1.7-1.8 nM.
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Fig. 2.
Determination of optimal conditions for
eIF4A-dependent unwinding of RNA duplex substrates.
All optimizations were performed using the RNA-1/RNA-11 duplex, and
each point represents the amount of duplex unwinding in 2 min at
35 °C. The concentrations of eIF4A and duplex were 0.4 µM and 1.7 nM, respectively. A,
addition of ATP·Mg2+ into the helicase reaction.
B, titration of magnesium acetate into the helicase reaction
(the concentration of ATP was held constant at 1 mM).
C, pH curve of the helicase reaction. The following buffers
were used: pH 5.0-6.4, 40 mM MES; pH 6.7-7.9, 40 mM HEPES; pH 8.2-8.8, 40 mM Tris.
D, addition of KCl into the helicase reaction.
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Fig. 3.
Kinetic plots of eIF4A-dependent
unwinding of RNA duplex substrates. The graph illustrates the
entire time course of eIF4A-dependent unwinding of all RNA
duplex substrates tested. Unwinding reactions were quantitated and fit
to a single phase exponential equation as described under
"Experimental Procedures." The concentrations of eIF4A and duplex
were 0.4 µM and 1.7-1.8 nM, respectively.
, RNA-1/RNA-10;
, RNA-1/RNA-11; *, RNA-1/RNA-12;
,
RNA-1/RNA-13;
, RNA-1/RNA-14;
, RNA-1/RNA-15.
Kinetic parameters of eIF4A-dependent unwinding of RNA
duplex substrates
G in
kcal/mol, Fig. 4). It can be seen in Fig.
4 that a direct relationship exists between the rate of
eIF4A-dependent unwinding and duplex stability. It was
expected that plotting the ln(initial rate of
eIF4A-dependent unwinding) versus
G would yield a straight line with a negative slope due
to the following: the thermodynamics of duplex unwinding can be given
by
G =
RT ln Keq =
RT ln
k1/k2 where
k1 is the rate constant for unwinding/melting
and k2 is the rate constant for hybridization
(31).4 Since
k2 changes little under the experimental
conditions of the assay described here,
G =
RT ln k1 + C, where
C = RT ln k2. Since
the initial rate is directly proportional to k1,
then ln(initial rate) versus
G should also
yield a straight line with a negative slope, and this is what is
observed in Fig. 4. Furthermore, the slopes of
eIF4A-dependent unwinding (10-15 base pairs) and thermal
melting (10-12 base pairs) are approximately equal, suggesting that
the rate of eIF4A-dependent unwinding is related to the
rate of thermal melting. First it should be noted that thermal melting of the 13, 14, and 15 base pair duplexes is essentially undetectable using these assay conditions. This is due to the fact that for the 13, 14, and 15 base pair duplexes, equilibrium between duplex species and
single-stranded species has been achieved at t
0 and that
the Tm values of these duplexes are greater than the
reaction temperature of 35 °C (41.5, 43.1, and 46.3 °C, respectively). Second, extrapolation for predicted ln(initial rate of
thermal melting) for these more stable duplexes yields values that are
too small to be distinguished from background. Thus, the value of the
rate of thermal melting for the 12 base pair duplex represents the
lower limit of measurable unwinding/melting by this experimental
method. The observation that the slopes describing the processes of
eIF4A-dependent unwinding and thermal melting are parallel
provides insight into a model for the unwinding activity of eIF4A (see
"Discussion").
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Fig. 4.
ln(initial rate of duplex unwinding or
melting) versus duplex stability. The graph
illustrates that a linear relationship exists between the ln(initial
rate of eIF4A-dependent duplex unwinding) and duplex
stability ( ) and a linear relationship exists between the ln(initial
rate thermal duplex melting) and duplex stability (
). See text for
details of this relationship. Note that the slopes of
eIF4A-dependent unwinding (10-15 base pairs) and thermal
melting (10-12 base pairs) are approximately equal, suggesting that
the rate eIF4A-dependent unwinding is related to the rate
of thermal melting.
[duplex]), it was expected that the
unwinding of all duplexes should go to completion (90-100% unwound)
since the initial rates of unwinding by eIF4A for each duplex were
greater than those for thermal melting. One would predict that it
should simply take longer for eIF4A to completely unwind the more
stable duplex substrates, which have a slower initial rate of
unwinding. In order to investigate this observation, several further
experiments were performed. Since the kinetic unwinding curves resemble
approaches to equilibrium (Fig. 3 and Table II), it was postulated that
the curves simply demonstrated an equilibrium between eIF4A unwinding
and subsequent reannealing of displaced radiolabeled strands to the
RNA-1 strand. However, the rates of reannealing were measured
independently and found to be less than 5% of the initial rate of
eIF4A-dependent unwinding for each duplex tested (data not
shown). In order to clarify this, experiments were performed that
included a 10-fold excess of a DNA oligonucleotide complementary to the
radiolabeled RNA strand. The DNA oligonucleotide would then
"capture" the labeled RNA strand once it was displaced from
the RNA-1 by eIF4A. This would prevent reannealing and, subsequently,
"pull" the reaction to completion. Control experiments in which the
RNA-1/RNA-12 duplex was fully melted by heating to 95 °C showed that
the excess complementary DNA oligonucleotide captured the RNA-12 strand
efficiently and completely, with almost no reannealing to the RNA-1
strand (data not shown). However, inclusion of this capture strand in
the helicase reaction using the RNA-1/RNA-12 duplex did not increase
the maximum amount of duplex that eIF4A was able to unwind (
50%,
data not shown) and shows that reannealing is not responsible for the
unwinding reactions not reaching completion.
90%) and with a similar initial
rate to what had been determined for this duplex alone (Fig. 5). Again,
this indicated that eIF4A was not becoming inactive over the time
course of the experiment.
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Fig. 5.
eIF4A is not inactivated during the time
course of the unwinding reaction. eIF4A is able to unwind the
RNA-1/RNA-11 duplex (added at t = 15 min, dashed
line) while in the presence of the RNA-1/RNA-15 duplex. , time
course of eIF4A-dependent unwinding of the RNA-1/RNA-11
duplex.
, time course of eIF4A-dependent unwinding of
the RNA-1/RNA-15 duplex.
, eIF4A-dependent unwinding of
the RNA-1/RNA-15 duplex before the addition of the
RNA-1/RNA-11 duplex at t = 15 min.
,
eIF4A-dependent unwinding of the RNA-1/RNA-11 duplex added
at t = 15 min to the former reaction containing the
RNA-1/RNA-15 duplex. The concentration of eIF4A was 0.4 µM, and the concentrations of the individual duplexes
were 1.8 nM.
70%) as starting the reaction with 0.8 µM eIF4A. If
the entire amount of unwound duplex was sequestered in non-productive
complexes, then addition of more eIF4A to the reaction should have had
little or no effect on the maximal amount of duplex unwound. This
result suggests that the duplex is not sequestered in non-productive
complexes and that this is not the reason for the reactions not going
to completion. The results in Fig. 5 and Fig. 6 also implicitly
demonstrate that the reaction is not falling short of completion due to
a drop in the concentration of ATP from ATP hydrolysis or inhibition of
the reaction due to a concomitant increase in ADP concentration, as
both the addition of eIF4A or duplex increase the final maximal amount
of substrate unwound.
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Fig. 6.
Duplex substrates are not sequestered into
non-productive complexes by eIF4A during the unwinding reaction.
Addition of 8.0 pmol of eIF4A (0.8 µM total) during the
time course of unwinding (at t = 15 min) increases the
maximal amount of duplex unwound to the same level as starting the
reaction with 0.8 µM eIF4A. ,
eIF4A-dependent unwinding of the RNA-1/RNA-12 duplex, 0.8 µM eIF4A.
, eIF4A-dependent unwinding of
the RNA-1/RNA-12 duplex, 0.4 µM eIF4A.
,
eIF4A-dependent unwinding of the RNA-1/RNA-12 duplex,
0.4 µM eIF4A, before the addition of 8.0 pmol
eIF4A.
, eIF4A-dependent unwinding of the RNA-1/RNA-12
duplex, after the addition of 8.0 pmol eIF4A (0.8 µM total). The concentration of duplex was 1.8 nM.
5.6 fmol, 16%). eIF4F alone (lane
5) shows approximately the same magnitude (
7.0 fmol, 21%) of
unwinding as free eIF4A, indicating that it is able to function as a
helicase in the absence of eIF4B. When eIF4F was added to eIF4A
(lane 9), a simple additive effect (
13.2 fmol, 40.7%)
was seen between these two factors. Both eIF4B and eIF4H (lanes
4 and 6, respectively) showed minimal background
unwinding of this duplex in the presence of ATP (
2.9 fmol, 8.4%,
3.2 fmol, 9.2%, respectively). When eIF4B was added to eIF4A
(lane 8), a 3-fold stimulation in duplex unwinding (
16.9 fmol, 49.5%) was observed. eIF4H (lane 10) was also able to
enhance the helicase activity of eIF4A, showing a 4.5-fold stimulation (
23.7 fmol, 69.2%) of duplex unwinding. These results indicate that
(i) eIF4A, as a subunit of eIF4F, is also capable of unwinding RNA
duplexes in the absence of eIF4B and (ii) either eIF4B or eIF4H is
capable of enhancing the helicase activity of eIF4A. Furthermore,
preliminary results indicate that not only is the maximum amount of
duplex unwound increased by the addition of these factors, but in the
case of eIF4B and eIF4H, the initial rate of unwinding is increased
5-10-fold (data not shown).
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Fig. 7.
Effect of addition of eIF4B, eIF4F, and eIF4H
to eIF4A in the helicase reaction. A, autoradiogram
showing the effect of eIF4B, eIF4F, and eIF4H individually and in
combination with eIF4A in the helicase assay. The RNA-1/RNA-15 duplex
(1.8 nM) was used. eIF4A was present at 0.4 µM. eIF4B (monomer), eIF4F, and eIF4H were present at 0.2 µM. Lane 1 ( protein) indicates no
protein was added and was incubated at 4 °C for the duration of the
reaction. Lane 2 (
) denotes this reaction was heated to
95 °C for 3 min and then incubated at 35 °C for 15 min. All other
reactions were performed at 35 °C for 15 min. B,
quantitation of the degree of RNA-1/RNA-15 duplex unwinding in
A for eIF4B, eIF4F, and eIF4H individually and in
combination with eIF4A, either in the presence or absence of ATP.
None represents the level of duplex unwinding by eIF4A alone
both in the absence and presence of ATP. The concentration of
ATP·Mg2+ was 1 mM.
DISCUSSION
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
G =
16.1 to
24.7 kcal/mol, see Table I). These
results are in contrast with previous studies showing that either eIF4A
or eIF4F required eIF4B for unwinding activity (8, 9, 22, 23).
Sonenberg and co-workers (9, 22, 23) employed a single RNA strand of 40 bases that contained a 10-nucleotide region of alternating G and C
residues, making this RNA self-complementary. This 10-base pair G:C
duplex has a calculated stability (
G) of
25.7 kcal/mol. It was postulated that the failure to observe helicase activity by
eIF4A alone could have been due to the poor affinity of eIF4A for the
RNA duplex substrate (Kd
125 µM)
(21), or that previous studies may have used an RNA duplex with too
great a stability for eIF4A alone to unwind.
G =
15.7 kcal/mol) was used. As is evident in
Figs. 1 and 3 and Table II, the RNA-1/RNA-10 duplex is unwound rapidly
and to completion, whereas increasing the stability of the duplex
lowers both the initial rate of eIF4A-dependent unwinding
and the maximum amount of duplex able to be unwound. From these
results, it is concluded that the ability to demonstrate that eIF4A can
act as an RNA helicase in the absence of eIF4B is due to the difference in duplex stability. It should be noted that the stability of the
duplex used by Sonenberg and co-workers (9, 22, 23) (
G =
25.7 kcal/mol) is above the upper limit of
our most stable duplex (RNA-1/RNA-15,
G =
24.7
kcal/mol), which was barely unwound by eIF4A alone under the reaction
conditions used in this study. Further comparison of the data in this
report with that described previously shows that the rate of unwinding
RNA duplexes of eIF4A is dependent on duplex stability rather than
simply length of the duplex, as eIF4A was unable to unwind an RNA
duplex consisting of 10 G:C base pairs (
G =
25.7
kcal/mol) but is able to efficiently unwind 10-12 base pair duplexes
consisting of both A:U and G:C base pairs (
G =
16.1 to
21.4 kcal/mol, Table II). In addition, it was demonstrated
that eIF4A is most active as a helicase when the concentration of
Mg2+ is approximately 1 mM (in the presence of
1 mM ATP, Fig. 2B), and lower concentrations of
Mg2+ decrease the helicase activity of eIF4A (Fig.
2B). Since the previous studies used only 0.5 mM
Mg2+ (in the presence of 1 mM ATP), this
probably also contributes slightly to the differences in observations
between this investigation and those performed previously.
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Fig. 8.
Kinetic model of eIF4A-dependent
unwinding of short RNA duplexes. Schematic diagram showing the
kinetic steps involved in unwinding of RNA duplexes by eIF4A.
a, fast association of eIF4A·ATP to the single-stranded
region of the RNA duplex substrate (ATP is presumed to bind to eIF4A
first because it is at saturating levels, whereas the duplex is at
subsaturating levels). b, during the slow ATP hydrolysis
step (20-30 s), eIF4A translocates and unwinds a limited number
(e.g. 4) base pairs, which subsequently keeps the duplex at
n 4 base pairs for 20-30 s. Strand separation
occurs during this 20-30-s time period, and eIF4A·ADP subsequently
dissociates from the long RNA strand (c), or strand
separation does not occur during this 20-30-s time period, eIF4A·ADP
dissociates from the duplex, and the strands reanneal (d).
RNA-1/RNA-10, -11, and -12 follow the pathway outlined by c,
whereas RNA-1/RNA-13, -14, and -15 follow the pathway outlined by
d.
If the helicase activity of eIF4A makes a longer (more stable) duplex
behave like a shorter (less stable) duplex with respect to the rate of
thermal melting, then this model would predict that the slope of the
ln(initial rate of thermal melting) versus duplex stability
would be equal to the slope of the ln(initial rate of
eIF4A-dependent unwinding) versus duplex
stability. For example, the 14-base pair duplex unwound by eIF4A would
subsequently behave thermally as the 10-base pair duplex. This
relationship is what is observed in Fig. 4; the slopes characterizing
these two processes (eIF4A-dependent unwinding and thermal
melting) are the same. This fact supports the model proposed above in
which eIF4A unwinds a minimal number of base pairs in a non-processive fashion and then the duplex proceeds to thermally melt at a rate dependent on the number of base pairs remaining. The actual
"apparent" change in duplex length would be dependent on eIF4A
concentration, which yields increased rates of strand separation with
an increase in eIF4A concentration. For the eIF4A concentration used in
these studies (0.4 µM), it would appear that, on average,
eIF4A has unwound about 4-5 base pairs (G
8 kcal/mol). Furthermore, increasing the concentration of eIF4A would
shift the entire curve of the ln(initial rate of
eIF4A-dependent unwinding) versus duplex
stability up due to the linear increase in initial rate, while the
slope would remain unchanged.
Models of translation initiation hold that eIF4A, eIF4B, eIF4F, and
eIF4H all act at the "mRNA activation" step in protein synthesis. The results in Fig. 7 demonstrate that eIF4F is also able to
function as a helicase in the absence of eIF4B and suggest that eIF4F
is approximately twice as active on a molar basis as eIF4A, since the
molar amount of eIF4F (0.2 µM) was half that of eIF4A
alone (0.4 µM). This finding is in corroboration with previous data showing that eIF4F is more active in poly(A) binding and
ATPase activity than eIF4A on a molar basis (19, 37); however, it
cannot be ruled out that this difference in helicase activity may
simply reflect a difference in the percentage of active molecules in
eIF4A and eIF4F preparations. Including eIF4A with eIF4F in the
helicase reaction showed a simple additive effect between these two
proteins (Fig. 7). The results of adding eIF4B or eIF4H to eIF4A
indicate that either of these proteins are able to stimulate the
helicase activity of eIF4A, allowing a faster initial rate of unwinding
and a greater maximal value of duplex unwound (data not shown and Fig.
7). Previous studies have shown that eIF4B's stimulation of the ATPase
activity of eIF4A is primarily through increasing the affinity
(250-fold) of eIF4A for single-stranded RNA, whereas a small (25%)
increase was seen in the maximum rate of the reaction (20). Thus, by
analogy, we suggest that the ability of eIF4A to rapidly and
efficiently unwind more stable duplex structures (RNA-1/RNA-15) in the
presence of eIF4B is also in part due to eIF4B increasing the affinity
of eIF4A for the substrate. Increased affinity for RNA would also
explain why eIF4A is approximately twice as active as a helicase when
part of the eIF4F complex, as the eIF4G subunit has RRM motifs that
would allow eIF4F to be more tightly bound to the duplex than eIF4A alone. Thus, while it was demonstrated that eIF4B and eIF4H stimulate the helicase activity of eIF4A, further experimentation will need to be
performed in order to ascertain if this is due to an increased affinity
for substrate, a stimulation of the rate of unwinding, conferring
processivity to the activity of eIF4A, or a combination of these possibilities.
The observation that the maximal amount of duplex unwound decreases with increasing duplex stability was unexpected. Experimentation has demonstrated that this decrease in the maximal value unwound was not due to the following: (i) strands displaced by eIF4A subsequently reannealing, (ii) eIF4A becoming inactive over the time course, (iii) the rate of eIF4A-dependent unwinding slowing/stopping due to a decrease in ATP concentration or a concomitant increase in ADP concentration over the time course of the reaction, and (iv) the more stable duplexes were being sequestered into non-productive complexes (see "Results"). It was observed, however, that the maximum amount of duplex unwound may be correlated to the initial rate of unwinding. This is evident in the unwinding profiles of duplexes with different stabilities (Fig. 3), and furthermore, increases in the initial rate of the reaction due to increases in eIF4A concentration also lead to an increase in the maximal value of duplex unwound (Fig. 6, compare 0.4 µM eIF4A to 0.8 µM eIF4A). This effect is also seen when the initial rate decreases due to a lowering of reaction temperature, and subsequently, a decrease in the maximal value of unwound duplex is observed (data not shown). Thus, the maximum amount of duplex unwound is positively correlated to the initial rate of unwinding; however, the reason for this relationship will require further experimentation.
In conclusion, this report demonstrates that eIF4A is able to function
in the absence of eIF4B as an RNA helicase. The initial rate of this
unwinding activity and the maximal amount of duplex unwound is
dependent upon the stability of the duplex. Our model suggests a single
ATP hydrolysis event allows eIF4A to unwind a limited number of duplex
base pairs. eIF4F is also able to function as an RNA helicase in
the absence of eIF4B. eIF4B and eIF4H are capable of enhancing the
helicase activity of eIF4A, and experimentation is currently being
performed to address how these additional factors affect the kinetic
parameters of this helicase activity. Overall, this study provides
biochemical and kinetic characterization of the unwinding activity of
eIF4A that may offer a basic framework for understanding the helicase
mechanism of DEAD box proteins in general.
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ACKNOWLEDGEMENTS |
---|
We thank Dr. Nahum Sonenberg for supplying recombinant eIF4AI and Dr. Tatyana Pestova and Dr. Christopher Hellen for supplying recombinant eIF4B. We also thank Dr. Vernon Anderson, Dr. Pieter de Haseth, Dr. Joyce Jentoft, Dr. Jack Hensold, Dr. Timothy Lohman, and Dr. James Wetmur for their advice and discussions.
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FOOTNOTES |
---|
* This work was supported in part by Research Grant GM26796 from the National Institutes of Health (to W. C. M.) and by Research Training Grants GM08056 and DK07319 in Cell and Molecular Biology (to G. W. R.) and Metabolism (to N. J. R.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
To whom correspondence should be addressed: Dept. of Biochemistry,
School of Medicine, Case Western Reserve University, 10900 Euclid Ave.,
Cleveland, OH 44106-4935. Tel.: 216-368-3578; Fax: 216-368-3419;
E-mail: wcm2{at}po.cwru.edu.
2 Kact is defined as the concentration of single-stranded RNA activator required to stimulate eIF4A's ATPase activity to half-maximal velocity under saturating concentrations of ATP.
3 Thermal melting is defined as any strand separation of duplex RNA that occurs as a passive thermal process; the term unwinding is reserved for strand separation of duplex RNA that occurs in an eIF4A-dependent manner.
4 James G. Wetmur, personal communication.
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ABBREVIATIONS |
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The abbreviations used are: Met-tRNAi, initiator methionyl-tRNA; eIF, eukaryotic initiation factor; m7G, 7-methylguanosine; UTR, untranslated region; ADPNP, adenylylimidodiphosphate; Pi, inorganic phosphate; MES, 2'[N-morpholino]ethanesulfonic acid; RRM, RNA recognition motif.
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REFERENCES |
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