From the Department of Biochemistry and Molecular
Biology, Louisiana State University Health Sciences Center,
Shreveport, Louisiana 71130-3932 and the ¶ Department of
Medicine, University of California, San Diego, California 92103
Received for publication, December 5, 2002
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ABSTRACT |
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The shutoff of host protein synthesis by certain
picornaviruses is mediated, at least in part, by proteolytic cleavage
of eIF4G-1. Previously, we developed a cleavage site variant of
eIF4G-1, termed eIF4G-1SM, that was 100-fold more
resistant to in vitro cleavage by Coxsackievirus 2A
protease (2APro) than wild-type eIF4G-1
(eIF4G-1WT), but it was still digested at high protease
concentrations. Here we identified a secondary cleavage site upstream
of the primary site. We changed Gly at the P1'-position of the
secondary site to Ala to produce eIF4G-1DM.
eIF4G-1DM was 1,000-10,000-fold more resistant to cleavage
in vitro than eIF4G-1WT. Full functional
activity of eIF4G-1DM was demonstrated in vitro
by its ability to restore cap-dependent translation to a
2APro-pretreated rabbit reticulocyte system. An isoform
containing the binding site for poly(A)-binding protein,
eIF4G-1eDM, was more active in this assay than an isoform
lacking it, eIF4G-1aDM, but only with polyadenylated
mRNA. Functional activity was also demonstrated in vivo
with stably transfected HeLa cells expressing eIF4G-1DM
from a tetracycline-regulated promoter. Cap-dependent green
fluorescent protein synthesis was drastically inhibited by
2APro expression, but synthesis was almost fully restored
by induction of either eIF4G-1aDM or
eIF4G-1eDM. By contrast, encephalomyocarditis virus
internal ribosome entry site-dependent green fluorescent
protein synthesis was stimulated by 2APro; stimulation was
suppressed by eIF4G-1eDM but not
eIF4G-1aDM.
Upon infection of mammalian cells with picornaviruses of the
rhino-, aphtho-, and enterovirus genera, most host protein synthesis is
shut off coincident with the appearance of viral proteins (1). The
shutoff is thought to be mediated by a switch from
cap-dependent to cap-independent translation. Extracts from
poliovirus-infected cells are unable to carry out
cap-dependent translation, but this ability can be restored
by addition of initiation factors from uninfected cells (2, 3),
particularly preparations containing eIF41 polypeptides (4, 5).
eIF4F, a complex of eIF4E, eIF4A, and eIF4G that is isolated from high
salt extracts of ribosomes, was shown to restore
cap-dependent translation in lysates of poliovirus-infected HeLa cells (6, 7). eIF4F from uninfected cells stimulates cap-dependent but not cap-independent translation in the
messenger RNA-dependent lysate (MDL) system, whereas the
opposite is true for eIF4F from poliovirus-infected cells (8).
eIF4G was discovered as a result of its proteolysis coincident with the
loss of cap-dependent initiation during poliovirus infection (9). eIF4G is cleaved by the 2A protease (2APro)
of Coxsackievirus B4 (CVB4) or human rhinovirus 2 (HRV2) into two
functional domains, an N-terminal fragment (cpN) that binds eIF4E and poly(A)-binding protein (PABP) and a C-terminal fragment (cpC) that binds eIF4A and eIF3 (10-12) (see Fig.
1). The 5'-untranslated region of
picornavirus mRNA contains an internal ribosome entry site (IRES)
that allows translation to be initiated by the direct binding of the
40 S ribosomal subunit (13, 14). Cleavage of eIF4G drastically
inhibits translation of capped mRNAs in vitro, whereas
initiation of IRES-containing and uncapped mRNAs is either unaffected or even stimulated (15-17).
INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
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Fig. 1.
Schematic diagram of the primary structure of
human eIF4G-1. eIF4G-1a, eIF4G-1e, and eIF4G-1f are the 154- (21),
172- (25, 26), and 176-kDa (24) forms, respectively. N-terminal
(cpN) and C-terminal (cpC) cleavage products
are generated during infection of cells with picornaviruses of the
entero-, rhino-, and aphthovirus genera or by in vitro
cleavage with 2APro. The primary (1°) and
secondary (2°) cleavage sites of CVB4 2APro
are shown with arrows. Binding sites for other proteins are
indicated by shaded boxes, bounded by aa residue
numbers.
cDNAs for eIF4G have been cloned from numerous sources, including
human, rabbit, Drosophila melanogaster, wheat,
Saccharomyces cerevisiae, and Schizosaccharomyces
pombe (reviewed in Ref. 18). There are at least two different
genes for eIF4G in yeast, wheat, and mammals (19-23). In humans, the
two different genes encode eIF4G-1 (also known as p220, eIF-4,
eIF4G-I, and eIF4GI)2 and
eIF4G-2 (also known as eIF4G-II and eIF4GII), which share 46% identity
at the amino acid (aa) level (21, 23). The human cDNA initially
isolated for eIF4G-1 contained an open reading frame for a protein of
154 kDa (21), referred to as eIF4G-1a (Ref. 24; see Fig. 1).
Subsequently, two co-linear but longer cDNAs were isolated
that diverged at the 5'-end (25, 26). These extended the open reading
frame by an additional 156 codons to produce a theoretical polypeptide
of 172 kDa, termed eIF4G-1e (24). Recently, an even longer form,
eIF4G-1f, was detected by mass spectrometric analysis (24). eIF4G-1e
and eIF4G-1f contain the binding site for PABP, but eIF4G-1a does not
(Fig. 1).
Cleavage of eIF4G-1 by 2APro was initially thought to be the major mechanism utilized by entero- and rhinoviruses to shut down host protein synthesis and allow viral protein synthesis to continue by IRES-driven translation (1). Subsequently, evidence has been presented that eIF4G-1 cleavage is not responsible, or is only partially responsible, for the shutoff of host protein synthesis. When cells are infected with poliovirus in the presence of a viral replication inhibitor, eIF4G-1 is completely degraded, but host protein synthesis is only partially shut off (27, 28). Similarly, ionophores block the host shutoff but not eIF4G cleavage (29). Expression of poliovirus 2APro in COS-1 cells has a much greater inhibitory effect on transcription by RNA polymerase II than on translation, suggesting that both processes may be involved in the host shutoff (30). In vivo expression of 2APro alone activates viral mRNA translation, independent of its role in the host shutoff (31, 32). Complete cleavage of eIF4G-1 in Xenopus oocytes (33) or HeLa cells (34) by introduction of exogenous 2APro results in only a modest reduction of protein synthesis. Cleavage of eIF4G-2 appears to correlate better with host shutoff than that of eIF4G-1 (35). Finally, PABP is also cleaved during Coxsackievirus B3 (CVB3; Ref. 36) and poliovirus (37) infection; partial cleavage coincides with the most rapid period of viral protein synthesis, whereas complete cleavage coincides with the cessation of viral protein synthesis (36). Thus, it is possible that events accompanying picornavirus infection other than eIF4G-1 cleavage are responsible, wholly or partly, for the host shutoff.
Creating an eIF4G-1 variant that is resistant to 2APro would provide a useful tool to study the mechanism of host protein synthesis shutoff upon viral infection. Previously, we determined the cleavage site in eIF4G-1 for HRV2 and CVB4 2APro (10) and developed a variant form in which the Gly in the P1'-position was changed to Glu (38). Even though the variant (eIF4G-1aSM) was 100-fold more resistant to cleavage than eIF4G-1aWT, it was still digested, at sufficiently high protease concentrations and extended incubation times, into fragments of approximately the same size as those derived from eIF4G-1aWT. This raised the question of whether cleavage occurred at the same or a different site.
In the present study, we have mapped the CVB4 2APro
cleavage site in eIF4G-1SM and found it to be different
from the site in eIF4G-1WT. We used this information to
create a second variant in which both cleavage sites were altered,
eIF4G-1DM. This variant is 1,000-10,000-fold more
resistant to CVB4 2APro than eIF4G-1WT in
vitro. eIF4G-1DM was capable of restoring
cap-dependent protein synthesis after 2APro
expression both in vitro and in vivo.
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EXPERIMENTAL PROCEDURES |
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Materials-- The C4 column for reverse phase purification of cpC was obtained from Vydac (Hesperia, CA). HPLC-grade acetonitrile and trifluoroacetic acid were obtained from Fisher and Pierce, respectively. MDL was prepared from New Zealand White rabbits (39). Globin mRNA was prepared from high salt-washed ribosomal pellets from rabbit reticulocytes (39). The vectors pIRES2-EGFP, pEGFP-C1, and pTRE2pur were from Clontech (Palo Alto, CA). The vectors pET29A and pCITE2a as well as S-Protein-agarose were from Novagen (Madison, WI). The vector pFastBac HTb was purchased from Invitrogen. A plasmid expressing the VP-1-2APro domain of CVB3 from an encephalomyocarditis virus (EMCV) IRES was described previously (40) and is here referred to as pVP12A. Benzamidine, pepstatin, Pefabloc, and ExpandTM High Fidelity PCR systems were purchased from Roche Molecular Biochemicals. T7 RNA polymerase, T4 polynucleotide kinase, RNase H, 5-bromo-4-chloro-3-indolyl phosphate/nitro blue tetrazolium, all restriction enzymes unless otherwise specified, RQ DNase, 5-bromo-4-chloro-3-indolyl phosphate, and nitro blue tetrazolium chloride were from Promega (Madison, WI). [35S]Met was obtained from ICN Biochemicals (Irvine, CA). Oligonucleotides used for site-directed mutagenesis and sequencing were acquired from Invitrogen. [5'-32P]pCp was obtained from ICN Biochemicals (Irvine, CA). SgrAI and T4 RNA ligase were purchased from New England Biolabs (Beverly, MA). The T7-1 primer 5'-AATACGACTCACTATAG-3' was provided by the DNA sequencing facility at Iowa State University. The production of an antibody against peptide 7 of eIF4G-1 was described previously (21).
Construction of Recombinant Plasmids--
The plasmids pSN1
and pS
N1E were constructed by inserting the
SmaI/XhoI fragments from pAD4GWT and
pAD4GG486E (38),3
respectively, into pET29A between the EcoRV and
XhoI sites. This resulted in plasmids encoding a portion of
eIF4G-1 (aa 590-1601) containing either the wild-type Gly or the
variant Glu at aa position 683, each protein also containing an
N-terminal tag consisting of the S-peptide of RNase A. These proteins
are named S-eIF4G-1(590-1601)WT and
S-eIF4G-1(590-1601)SM, respectively.
Construction of the plasmid pCITE4G-1aDM (double mutant), which encodes eIF4G-1a with a second aa substitution (G675A), was achieved by site-directed mutagenesis of pCITE4GG486E (38). A mutated PCR product from nucleotide (nt) 1221-2033 was synthesized from pCITE4GG486E using the sense primer 5'-AGCCCCCTCGCCACCAG-3' (nt 1221-1237) and the antisense primer 5'-GTTGTGCGCGCAAGGTTGGCAAAGGATGGAG-3' (nt 2003-2033). A second mutated PCR product from nt 2016-2952 was produced from pCITE4GG486E using the sense primer 5'-CAACCTTGCGCGCACAACCCTTAGCACCCG-3' (nt 2016-2045) and the antisense primer 5'-CACGTCCTGCAGCATAAAGCGG-3' (nt 2931-2952). PCR was conducted with the ExpandTM High Fidelity PCR system (Roche Molecular Biochemicals) for 20 cycles at 95 (1 min), 61 (30 s), and 72 °C (30 s), with a final extension at 72 °C (5 min). Two DNA fragments, one derived from the PCR product of nt 1221-2033 by cutting with BamHI and BssHII, and the other from the PCR product of nt 2016-3052 by cutting with BssHII and NsiI, were ligated into pCITE4GG486E that had been digested with BamHI and NsiI. The mutagenesis not only changed the encoded aa at position 675 from Gly to Ala but also created a restriction site for BssHII at nt 2023-2028. Therefore, plasmids from colonies were initially screened by restriction digestion with BssHII.
A plasmid expressing eIF4G-1eDM, termed pCITE4G-1eDM, was constructed by inserting the DNA sequence that encodes the 156-aa N-terminal extension of eIF4G-1e into pCITE4G-1aDM. This was accomplished by digesting a plasmid containing this sequence in pCR2.1 (Ref. 26; kindly donated by Peter Sarnow) with XmaI and EcoRV, treating the reaction mixture with the Klenow fragment of DNA polymerase I (Promega), and then ligating the gel-purified 501-bp fragment into pCITE4G-1aDM that had been digested with EcoRV.
pTRE2pur4G-1aDM and pTRE2pur4G-1eDM were constructed as follows. First, the plasmid pFastBac4G-1aDM was constructed by digesting pCITE4G-1aDM with EcoRV and XhoI. The gel-purified insert was ligated into pFastBac HTb after cutting with StuI and XhoI. pFastBac4G-1eDM was similarly constructed by digesting pCITE4G-1eDM with SgrAI and XhoI. pTRE2pur4G-1aDM and pTRE2pur4G-1eDM were constructed by digesting pFastBac4G-1aDM and pFastBac4G-1eDM, respectively, first with SspI and then partially with NheI. The gel-purified inserts, consisting of the entire eIF4G-1aDM or eIF4G-1eDM coding regions, were ligated into pTRE2pur that had been previously cut with PvuII and NheI.
Expression and Purification of the cpC
Fragment--
One-liter cultures of the Escherichia coli
expression strain BL21(DE3)pLysS bearing the plasmids pSN1 and
pS
N1E were grown in LB medium (41) containing 25 µg/ml kanamycin
and 34 µg/ml chloramphenicol to an A600
of 0.3. Expression of S-eIF4G-1(590-1601)WT or
S-eIF4G-1(590-1601)SM was induced with 1 mM
isopropyl-1-thio-
-D-galactoside for 3 h at
30 °C. Cells were cooled in an ice water bath, pelleted by centrifugation, and stored at
80 °C. Cells were thawed in the presence of 30 ml of Buffer A (50 mM Tris-HCl (pH 8.0), 50 mM NaCl, 5 mM dithiothreitol, 2 mM
EDTA) containing 1 mM phenylmethylsulfonyl fluoride, 1 mM benzamidine, 1 mg/ml leupeptin, 1 mg/ml pepstatin, and 1 µg/ml Pefabloc. Lysis was completed by sonication (3-6 bursts of
10 s each), and insoluble debris was removed by centrifugation at
30,000 × g for 30 min. The supernatant was combined
with 2 ml of S-Protein-agarose equilibrated in Buffer A and incubated with rotation at 4 °C for 2 h. The resin was washed with 80 volumes of Buffer A containing 0.1% Triton X-100. The protein, still
bound to S-Protein-agarose, was subjected to digestion in 4 ml of
Buffer A containing 50 µg/ml CVB4 2APro and 0.1% Triton
X-100 at 30 °C for 2 h. The resin was allowed to settle, and 2 ml of the supernatant containing cpC were removed and
further fractionated by reverse phase HPLC on a 4.5 × 15 cm C4
column equilibrated in Buffer B (0.1% aqueous trifluoroacetic acid).
The column was developed with 5 ml of Buffer B followed by a 45-ml
linear gradient of Buffer B to 80% Buffer C (0.1% trifluoroacetic acid in 95% acetonitrile). Peak fractions containing cpC
were subjected to automated Edman degradation at the University of Kentucky Macromolecular Structure Analysis Facility.
In Vitro Translation-- MDL was prepared from rabbit reticulocyte lysate by depletion of endogenous mRNA with micrococcal nuclease (42). In some cases, endogenous eIF4G was cleaved by pretreating MDL with recombinant CVB4 2APro at 50 µg/ml for 30 min at 30 °C. A two-phase cell-free translation protocol was used to test the function of eIF4G-1DM. During phase I, in vitro translation was performed to synthesize the eIF4G-1 isoform of interest in the absence of radioactivity in CVB4 2APro-pretreated MDL. During phase II, in vitro translation of globin mRNA was used to test whether the eIF4G-1 isoforms synthesized in phase I were functional for cap-dependent translation. Reaction mixtures of 25 µl in phase I contained 10 µl of either mock-treated or 2APro-pretreated MDL and the following components: 150 mM potassium acetate, 1 mM MgCl2, 1 mM ATP, 0.2 mM GTP, 15 mM creatine phosphate, 40 µM of each aa with the exception of Met, 2 µM Met, 50 mM HEPES (pH 8.0), and 20 µg/ml eIF4G-1 mRNA. The latter was transcribed from pCITE4G-1aDM and pCITE4G-1eDM plasmids linearized by XhoI as described previously (43). Reactions were carried out at 30 °C for 1 h. Phase II reaction mixtures (25 µl) contained 12.5 µl of phase I reaction mixtures and 12.5 µl of fresh reaction mixtures consisting of 5 µl of either mock-treated or 2APro-pretreated MDL, 160 mM KCl, 1 mM MgCl2, 1 mM ATP, 0.2 mM GTP, 15 mM creatine phosphate, 40 µM of each aa with the exception of Met, 1 mCi/ml [35S]Met, 50 mM HEPES (pH 8.0), and 40 µg/ml globin mRNA. The phase II reactions were incubated at 30 °C for 30 min, and aliquots were subjected to SDS-PAGE. eIF4G-1 synthesis was measured by immunoblotting and globin synthesis by incorporation of [35S]Met into protein.
Deadenylation of Globin mRNA-- Rabbit globin mRNA was labeled at 4 °C overnight in a 100-µl total reaction mixture containing 50 mM Tris-HCl (pH 7.8), 10 mM MgCl2, 10 mM dithiothreitol, 1 mM ATP, 50 µg/ml bovine serum albumin, 1 unit/µl RNasin, 10% Me2SO, 500 µCi/ml [5'-32P]pCp, 200 µg/ml globin mRNA, and 100 units/ml T4 RNA ligase. After purification, an aliquot was mixed with nonradioactive globin mRNA (400 µg/ml) and hybridized to oligo(dT)15 (20 µg/ml) by heating in a 50-µl reaction volume containing 40 mM Tris-HCl (pH 7.5), 10 mM MgCl2, 1 mM dithiothreitol, and 30 µg/ml bovine serum albumin at 65 °C for 5 min and slowly cooling to 37 °C. RNase H (5 units) was added, and the reaction was incubated at 37 °C for 20 min. DNase (1 unit) was added, and the reaction was continued at 37 °C for 15 min. The RNA was isolated by phenol extraction and ethanol precipitation.
Establishment of Stable Cell Lines Expressing Tetracycline-regulated eIF4G-1aDM and eIF4G-1eDM-- The Tet-On HeLa cell line (Clontech) was transfected by electroporation with pTRE2pur4G-1aDM or pTRE2pur4G-1eDM linearized with XhoI. After 24 h, the cells were selected in medium containing 2 µg/ml puromycin. The medium was changed every 3-4 days. Colonies were transferred to 6-well plates, and the culture was continued. Expression of eIF4G-1DM was detected by adding doxycycline to the culture medium (see figure legends). Cell lysates were treated with recombinant CVB4 2APro at 50 µg/ml for 2 h on ice. eIF4G-1 that was detected by immunoblotting with anti-eIF4G-1 antibodies but was resistant to 2APro digestion was scored as eIF4G-1DM. Colonies were selected that had the lowest expression of eIF4G-1DM in the absence of doxycycline but highest expression in its presence.
Expression and Detection of Green Fluorescent Protein (GFP) in Cell Lines Expressing eIF4G-1DM-- The stably transfected HeLa cell lines expressing eIF4G-1aDM and eIF4G-1eDM were cultured in 6-well plates. Cells were either induced by doxycycline to express exogenous eIF4G-1DM for 24 h or mock-induced. Cells were then co-transfected with pVP12A and either pEGFP-C1 or pIRES2-EGFP using Polyfectin (Qiagen). Culture was continued for 24 h, and then cells were observed with a Nikon Eclipase TE 300 fluorescence microscope (Melville, NY). In each well, five random sites were selected for quantitation of the number of GFP-expressing cells. In each site, cells were quantitated in 16 adjacent microscopic fields using the IPLab Scientific Imaging Software from Scanalytics, Inc. (Fairfax, VA).
Quantitation of GFP mRNA by Real Time PCR--
RNA was
isolated from the cells in each well and purified using the RNeasy Mini
Kit (Qiagen, Valencia, CA). mRNA was transcribed into cDNA
using TaqMan reverse transcription reagents (Applied Biosystems, Foster
City, CA). Real time PCR was performed to detect GFP mRNA levels
using an ABI PRISM® 7700 Sequence Detection System. The antisense
primer for GFP was 5'-GTACAGCTCGTCCATGCCGA-3', the sense primer,
5'-CAACGAGAAGCGCGATCACAT-3', and the probe
6-FAM-CTGCTGGAGTTCGTGACCGCCGC-TAMRA. GFP mRNA levels were
normalized by taking the ratio to endogenous glyceraldehyde-3-phosphate
dehydrogenase (GAPDH) mRNA using reagents from Applied Biosystems.
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RESULTS |
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The Cleavage Site for CVB4 2APro in
eIF4G-1aSM Is Different from the Site Previously Identified
in eIF4G-1aWT--
The fact that the
eIF4G-1aSM variant is cleaved at sufficiently high CVB4
2APro concentrations (38) suggests one of two
possibilities: either the single aa substitution is insufficient to
completely prevent cleavage at the original location, or the protease
cleaves eIF4G-1 at an alternate site close to the primary site.
To distinguish between these possibilities, we determined the cleavage
site in eIF4G-1SM, i.e. the site that is cleaved
when the primary site in eIF4G-1 is altered. We expressed in E. coli truncated versions of eIF4G-1WT and
eIF4G-1SM containing the S-peptide of RNase A fused at the
N terminus, designated S-eIF4G-1(590-1601)WT and
S-eIF4G-1(590-1601)SM, respectively. This allowed
immobilization of the products on S-Protein-agarose. Digestion of the
bound proteins with CVB4 2APro released soluble
cpC (Fig. 2A,
lane 4). The eluates were subjected to reverse phase HPLC to
separate cpC from CVB4 2APro and other
contaminating proteins (Fig. 2B). The cpC
fragments from both S-eIF4G-1(590-1601)WT and
S-eIF4G-1(590-1601)SM eluted at 40 min, similar to the
elution position of natural rabbit cpC (10). N-terminal
sequencing of cpC from S-eIF4G-1(590-1601)WT
confirmed the primary cleavage site between Arg682 and
Gly683 (10) (Table I).
N-terminal sequencing of cpC from
S-eIF4G-1(590-1601)SM demonstrated that cleavage of
S-eIF4G-1(590-1601)SM occurred at a site 8-aa residues
upstream of the original site, between Leu674 and
Gly675 (Table I). Cleavage at this alternate site
was not because of the fact that the substrate was made in bacteria,
because S-eIF4G-1(590-1601)WT was cleaved at the site
determined previously (10) in natural rabbit eIF4G-1. These results
indicate that alteration of the primary 2APro cleavage site
in eIF4G-1 results in cleavage at a secondary site (labeled
2° in Fig. 1).
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Cleavage of eIF4G-1SM and Resistance to Cleavage of
eIF4G-1DM by 2APro in Vitro--
This finding
suggested that alteration of both primary and secondary cleavage sites
might confer even greater resistance to CVB4 2APro. We
therefore performed site-directed mutagenesis to modify further eIF4G-1SM (single mutant) and create a new variant,
eIF4G-1DM (double mutant), in which both of the P1' Gly
residues are substituted (G675A and G683E). To test the effect of the
two substitutions on susceptibility to CVB4 2APro, we
synthesized various isoforms of eIF4G-1 in an MDL system programmed
with mRNAs encoding eIF4G-1aWT, eIF4G-1aSM,
eIF4G-1aDM, or eIF4G-1eDM. The predominant
translation product was either full-length eIF4G-1a or eIF4G-1e (Fig.
3A, lower
panel, lanes with no CVB4 2APro), but there were
also bands of faster mobility, as observed previously (43). At least
some of these are due to alternative initiation at downstream AUG
codons (44). The reaction mixtures were then incubated with increasing
concentrations of CVB4 2APro. Immunoblotting with
anti-eIF4G-1 antibodies detected both endogenous rabbit and exogenous
(in vitro synthesized) human eIF4G-1 (Fig. 3A,
upper panel), whereas autoradiography detected only the
exogenous eIF4G-1 (Fig. 3A, lower panel). Newly
synthesized eIF4G-1WT was completely cleaved at 5 µg/ml
CVB4 2APro, whereas endogenous eIF4G-1WT was
cleaved at 50 µg/ml. Most of the eIF4G-1SM was cleaved at
500 µg/ml, as observed previously (38). However, newly synthesized
eIF4G-1aDM and eIF4G-1eDM were only slightly
cleaved at 500 µg/ml. The degree of cleavage of eIF4G-1DM
isoforms at 500 µg/ml was intermediate between that of
eIF4G-1aSM at 5 and 50 µg/ml. Thus, eIF4G-1DM
was 10-100-fold more resistant than eIF4G-1SM.
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We also tested the resistance of eIF4G-1aWT, eIF4G-1aSM, eIF4G-1aDM, and eIF4G-1eDM to 100 µg/ml CVB4 2APro cleavage as a function of time (Fig. 3B). Newly synthesized eIF4G-1WT (lower panel) was completely cleaved by 5 min, whereas the endogenous eIF4G-1WT (upper panel) was cleaved somewhat more slowly. Most of the eIF4G-1aSM was digested by 60 min (Fig. 3B, lower panel), but the eIF4G-1aDM and eIF4G-1eDM variants were only slightly digested. The cleavage of eIF4G-1aDM and eIF4G-1eDM at 60 min was intermediate between that of eIF4G-1aSM at 0 and 5 min. By this criterion, eIF4G-1DM was at least 12-fold more resistant than eIF4G-1SM.
Restoration of eIF4G-1 Function by eIF4G-1eDM to
2APro-pretreated MDL--
We sought to determine whether
the two substitutions in eIF4G-1DM interfered with its
intrinsic activity as an initiation factor. We therefore conducted a
two-phase in vitro translation assay to test if
eIF4G-1DM could restore activity to MDL that had been
pretreated with CVB4 2APro. In phase I, exogenous
eIF4G-1DM isoforms were synthesized in
2APro-pretreated MDL by IRES-driven translation. In phase
II, the newly synthesized eIF4G-1DM was tested for its
ability to restore translation of globin mRNA. Endogenous eIF4G-1
was cleaved in the 2APro-pretreated MDL (Fig.
4A, lanes 3-7) but
not in mock-treated MDL (Fig. 4A, lanes 1 and
2). eIF4G-1aDM (lane 6) and
eIF4G-1eDM (lane 7) were shown by immunoblotting
(Fig. 4A) and autoradiography (Fig. 4B) to be
resistant to CVB4 2APro. Globin was efficiently synthesized
in mock-treated MDL (Fig. 4, C and D, lane
2) but not in 2APro-pretreated MDL in the absence of
exogenous eIF4G (lane 4) or in the presence of
eIF4G-1aWT (lane 5). (The band for radioactive
globin is broad because endogenous globin is ~100 mg/ml in MDL and
runs as a broad band on SDS-PAGE.) Globin synthesis was partially
restored by eIF4G-1aDM (lane 6) and completely
restored by eIF4G-1eDM (lane 7). These results
show that both eIF4G-1aDM and eIF4G-1eDM are
functional but that eIF4G-1eDM has higher translational
activity than eIF4G-1aDM. This comparison is even more
striking because more eIF4G-1aDM was produced in the
two-phase translation system than eIF4G-1eDM (Fig. 4,
A and B). Apparently, synthesis of
eIF4G-1eDM is less efficient than eIF4G-1aDM,
because the phase I reactions contained the same amount of
mRNA. In interpreting this and subsequent in vitro
experiments, one must keep in mind that the eIF4G-1a and eIF4G-1e
preparations contain full-length proteins but also smaller amounts of a
variety of truncated forms. Thus, the assignment of activities to
specific isoforms of eIF4G-1 cannot be made with complete
confidence.
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eIF4G-1aDM and eIF4G-1eDM Differ in Their
Ability to Restore Translation of Polyadenylated mRNA in
Vitro--
eIF4G-1aDM and eIF4G-1eDM differ by
156 aa at the N terminus (Fig. 1). The extended N-terminal sequence
contains a PABP-binding site (25). We reasoned that the PABP-binding
site might account for the difference in translational activity between
eIF4G-1aDM and eIF4G-1eDM. To test this, we
deadenylated rabbit globin mRNA with oligo(dT) and RNase H. This
treatment caused the loss of radioactivity from 3'-end-labeled globin
mRNA (Fig. 5A).
Concomitantly, ethidium bromide staining showed that untreated globin
mRNA was heterogeneous but was resolved into distinct bands of
faster mobility after RNase H treatment, representing - and
-globin mRNA (Fig. 5B). These findings indicate that
the poly(A) tract of globin mRNA was removed from the 3'-end.
|
The untreated and deadenylated globin mRNAs were then used to test
the activity of eIF4G-1DM isoforms. Either
eIF4G-1aDM or eIF4G-1eDM was synthesized in
phase I (Fig. 6A). In phase
II, the newly synthesized eIF4G-1aDM and
eIF4G-1eDM were used to translate untreated or deadenylated
globin mRNA (Fig. 6B). Deadenylated globin mRNA was
translated at the same efficiency in the presence of either
eIF4G-1aDM or eIF4G-1eDM (lane 9 versus 10, p = 0.36, n = 3). The polyadenylated globin mRNA, however,
was translated at considerably higher efficiency in the presence of
eIF4G-1eDM compared with eIF4G-1aDM (lane
8 versus 7, p < 0.008, n = 3). Furthermore, polyadenylated and deadenylated
globin mRNAs were translated with the same efficiency with
eIF4G-1aDM (lane 7 versus
9), presumably due to the absence of a PABP-binding site in
eIF4G-1aDM.
|
It was possible that the deadenylated mRNA was less stable in the translation system, accounting for its lower activity with eIF4G-1eDM. To test this, we performed in vitro translation of natural or deadenylated globin mRNA, removed aliquots at 0 and 30 min, and tested both the concentration and integrity of globin mRNA by Northern blotting. The results indicated that untreated and deadenylated mRNA were intact and in the same concentrations at the beginning and end of the reaction (data not shown).
eIF4G-1DM Restores Cap-dependent
Translation in Vivo--
In order to test the translational activity
of eIF4G-1DM variants in vivo, we established
HeLa cell lines stably transfected with plasmids expressing either
eIF4G-1aDM or eIF4G-1eDM under control of a
tetracycline-regulated promoter. To distinguish endogenous eIF4G-1 from
eIF4G-1DM, we treated cell extracts with CVB4
2APro before immunoblotting with anti-eIF4G-1 antibodies.
The level of exogenous eIF4G-1aDM (Fig.
7A, + lanes) and
eIF4G-1eDM (Fig. 7B, + lanes)
increased with increasing doxycycline concentration. Unfortunately, the
expression level of exogenous eIF4G-1DM was only a small
fraction of endogenous eIF4G-1 ( lanes),
despite the fact that we surveyed numerous clonal lines.
|
We tested the translational activity of eIF4G-1aDM and eIF4G-1eDM by co-transfecting the stable cell lines with vectors expressing GFP and CVB3 2APro. To distinguish between cap-dependent and cap-independent translation, we used vectors expressing GFP mRNA with either a normal 5'-untranslated region (pEGFP-C1) or an EMCV IRES (pIRES2-EGFP). The plasmid producing CVB3 2APro also contained an EMCV IRES, so the protease was produced regardless of the cleavage status of eIF4G. eIF4G-1DM isoforms were induced with doxycycline for 24 h. Cells were then transiently co-transfected with the GFP- and 2APro-expressing vectors.
It was necessary to control for differences in GFP mRNA levels
under the various conditions tested, because eIF4G strongly enhances
the rate of initiation and because the stability of an mRNA is
affected by its state of translation. We therefore obtained quantitative estimations of GFP mRNA levels by real time PCR, normalizing for RNA yield and reverse transcription efficiency using
GAPDH mRNA as endogenous control (Fig.
8). The threshold cycles
(CT) for all GAPDH cDNA samples fell between 18 and 20 (Fig. 8A), whereas those for GFP cDNA fell
between 15 and 18 (Fig. 8B). Table
II shows the average number of
fluorescent cells in five randomly selected sites for each cell culture
well. The levels of GFP mRNA for each cell type and condition was
measured in duplicate. Rates of GFP synthesis were calculated by
dividing the number of fluorescent cells by the GFP mRNA level. The
results are presented graphically in Fig.
9.
|
|
|
When mediated by cap-dependent translation, GFP synthesis was not significantly different in the presence or absence of doxycycline in eIF4G-1aDM cells not expressing 2APro (Fig. 9A, GFP, open versus filled bars; 0.5 < p < 0.8; eight degrees of freedom). This was also true for eIF4G-1eDM cells (Fig. 9B, GFP; 0.5 < p < 0.8). Expression of 2APro dramatically decreased GFP synthesis in eIF4G-1aDM cells uninduced with doxycycline (Fig. 9A, open bars, GFP + 2A versus GFP; p < 0.05). This was also true for eIF4G-1eDM cells (Fig. 9B, open bars, GFP + 2A versus GFP; p < 0.02). However, induction of eIF4G-1aDM significantly increased GFP expression (Fig. 9A, GFP + 2A, open versus filled bars, p < 0.01). Similarly, induction of eIF4G-1eDM significantly increased GFP expression (Fig. 9B, GFP + 2A, open versus filled bars; p < 0.001).
When GFP was expressed by IRES-dependent translation, the
results with 2APro were quite different. In
eIF4G-1aDM cells not expressing 2APro, GFP
expression was not affected by doxycycline (Fig. 9C, IGFP, 0.2 < p < 0.5). This was also true in
eIF4G-1eDM cells not expressing 2APro (Fig.
9D, IGFP, 0.1 < p < 0.2). Unlike
cap-dependent GFP synthesis, IRES-dependent GFP
synthesis increased dramatically with 2APro expression in
uninduced eIF4G-1aDM cells (Fig. 9C, open
bars, IGFP + 2A versus IGFP; p < 0.01). This was also true for uninduced eIF4G-1eDM cells
(Fig. 9D, open bars, IGFP + 2A versus
IGFP; p < 0.01). This increase in GFP expression was
suppressed by the induction of eIF4G-1eDM by doxycycline
(Fig. 9D, IGFP + 2A, p < 0.01) but not
eIF4G-1aDM (Fig. 9C, IGFP + 2A; 0.5 < p < 0.8).
![]() |
DISCUSSION |
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Comparison of the cleavage sites for 2APro in the
viral polyproteins of 22 entero- and rhinoviruses yields the consensus
sequence Leu-Thr-Thr-XGly-Pro ...
(P4-P3-P2-P1
P1'-P2' ...; see Ref. 45). The most conserved
positions are P1', which is Gly in all cases, and P2, which is Thr in
16 out of 22 cases. The 2APro of HRV2 and CVB4 cleave
eIF4G-1 initially at
Leu679-Ser-Thr-Arg
Gly, a sequence that
contains both of these consensus aa residues (10). Our previous
observation that the G683E variant eIF4G-1aSM was still
cleaved (38), albeit 100-fold more slowly than eIF4G-1aWT,
meant either that CVB4 2APro had a preference but not an
absolute requirement for Gly at P1' or that it cleaved at another site.
Finding a secondary cleavage site between Leu674 and
Gly675, 8-aa residues upstream from the primary site,
supports the latter mechanism. Whereas the P1' position in the
secondary cleavage site is occupied by the highly conserved Gly, the P2
position is occupied by the less conserved Asn, which occurs in only 2 of the 22 viral polyprotein sequences (45). This may account for the
slower rate of cleavage at the secondary site. It is possible that both
sites are cleaved in eIF4G-1WT, but our method of analyzing
cpC by Edman sequencing detects only the downstream site
(10). Even if this occurs, the results of Fig. 3 indicate that the
secondary cleavage is ~100-fold slower. Interestingly, the Lb
protease of foot-and-mouth virus, which is a Papain-like protease,
cleaves eIF4G-1 between Gly675 and Arg676 (46),
only 1-aa residue from the secondary cleavage site of CVB4
2APro. Thus, this region of eIF4G-1 may be particularly
accessible to proteases. Because of this proximity of sites, the
eIF4G-1DM variant may have some resistance to Lb
protease as well.
Careful examination of the immunoblots in Fig. 3 reveals that the newly synthesized eIF4G-1aWT is cleaved more readily than endogenous eIF4G-1 (Fig. 3, A and B, upper versus lower panels). There are several possible explanations for this as follows: human eIF4G-1 may be a better substrate for CVB4 2APro than rabbit eIF4G-1, because human matches the consensus CVB4 2APro-recognition sequence better than does rabbit (10); newly synthesized eIF4G-1 may associate more readily with eIF4E, because eIF4G-1 alone is a poorer substrate for HRV2 2APro than the eIF4G-1·eIF4E complex (47); or eIF4G-1a may be more susceptible to proteolysis than the larger forms.
We found that the ability of eIF4G-1eDM to restore translation to 2APro-pretreated MDL was significantly higher than that of eIF4G-1aDM when the system contained natural globin mRNA. By contrast, the activities of eIF4G-1aDM and eIF4G-1eDM were similar with deadenylated globin mRNA. This is most likely explained by the finding of Imataka et al. (25) that eIF4G-1e but not eIF4G-1a contains a PABP-binding site. It might be argued that 2APro-pretreated MDL lacks both functional eIF4G-1 and PABP, because the latter is also cleaved by 2APro (36, 37). However, we have previously shown that PABP cleavage occurs at only approximately one-tenth the rate of eIF4G-1 cleavage (36). Under the conditions actually used in the present study, we demonstrated, by immunoblotting with anti-PABP antibodies, that pretreatment of MDL with CVB4 2APro resulted in only one-third cleavage of PABP but complete cleavage of eIF4G-1 (data not shown). The fact that eIF4G-1eDM is able to fully restore translation of globin mRNA indicates that (i) any hypothetical changes to the translational system caused by 2APro pretreatment other than eIF4G cleavage do not affect the overall rate of translation, and (ii) the 2-aa substitution in eIF4G-1eDM does not affect its activity for in vitro translation of capped and polyadenylated mRNA.
Co-transfection of stable cell lines expressing eIF4G-1DM isoforms with plasmids that express GFP and 2APro indicated that eIF4G-1DM isoforms substantially protect cap-dependent protein synthesis from inhibition by 2APro in vivo. The response of GFP synthesis to 2APro was opposite depending on how translation of GFP was initiated. Cap-dependent GFP synthesis was dramatically inhibited by 2APro expression, whereas IRES-dependent GFP translation initiation was greatly increased. This latter result agrees with the observation that initiation of both IRES-containing and uncapped mRNAs is either unaffected or even stimulated upon eIF4G cleavage (15-17). This may be due to two factors as follows: (i) cpC is more active than intact eIF4G-1 for initiation of IRES-containing mRNAs, and (ii) the inhibition of cap-dependent translation of exogenous mRNAs by 2APro makes limiting components of the translational machinery available for IRES-driven synthesis. Doxycycline-induced expression of eIF4G-1eDM substantially reversed the effects of 2APro in both cases; inhibition of cap-dependent translation by 2APro was relieved (Fig. 9B), and stimulation of IRES-dependent translation by 2APro was suppressed (Fig. 9D). For reasons we do not understand, eIF4G-1aDM was able to restore cap-dependent translation (Fig. 9A) but not suppress the IRES-dependent stimulation (Fig. 9C).
Initial attempts to protect cells against CVB3 infection were not
successful. This may be because the amount of exogenous eIF4G-1 was low
compared with the endogenous level (Fig. 7). With improved techniques
for producing eIF4G-1DM at higher levels in intact cells,
it should be possible to use the approach demonstrated here to gain
further insight into the cytopathic effect of picornaviruses.
Furthermore, because there are numerous isoforms of eIF4G-1 (24), this
approach may provide a means of testing them for unique activities or
preferential recruitment of mRNA populations.
![]() |
ACKNOWLEDGEMENTS |
---|
We are grateful to Aili Cai for preparation of globin mRNA; Dr. Peter Sarnow for the plasmid encoding the N-terminal extension of eIF4G-1; Dr. Chris Bradley for valuable advice; Clint A. Benoit for help with microscopy; Lijia Yin for help with real time PCR; and the LSUHSC-S Research Core Facility for instrumentation.
![]() |
FOOTNOTES |
---|
* This work was supported by Grants R01 GM20818 and R01 HL57365 from the National Institutes of Health.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.
§ Present address: ProdiGene, 101 Gateway Blvd., Ste. 100, College Station, TX 77845.
To whom correspondence should be addressed: Dept. of
Biochemistry and Molecular Biology, Louisiana State University Health Sciences Center, 1501 Kings Highway, Shreveport, LA 71130-3932. Tel.:
318-675-5161; Fax: 318-675-5180; E-mail: rrhoad@lsuhsc.edu.
Published, JBC Papers in Press, December 9, 2002, DOI 10.1074/jbc.M212393200
2 The nomenclature system recommended by an ad hoc committee appointed by the IUBMB is used here (48).
3 The names of plasmids pCITE4GG486E and pAD4GG486E have not been changed from those used in the original publication (38). The site of the G486E substitution is aa 683 when numbered according to the longest known eIF4G-1 isoform, eIF4G-1f (24).
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ABBREVIATIONS |
---|
The abbreviations used are: eIF4, eukaryotic initiation factor 4; aa, amino acid; nt, nucleotide; EMCV, encephalomyocarditis virus; IRES, internal ribosome entry site; GFP, green fluorescent protein; EGFP, enhanced GFP; PABP, poly(A)-binding protein; MDL, messenger RNA-dependent lysate; HPLC, high pressure liquid chromatography; FAM, 6-carboxyfluorescein; TAMRA, tetramethylrhodamine; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; HRV2, human rhinovirus 2; CVB3, Coxsackievirus B3.
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