(Received for publication, January 17, 1997, and in revised form, May 17, 1997)
From the Program in Molecular Biology, Human replication factor C (hRFC) is a
multi-subunit protein complex capable of supporting proliferating cell
nuclear antigen (PCNA)-dependent DNA synthesis by DNA
polymerases Replication factor C (RFC1; also known
as activator 1) and PCNA are the two accessory factors required for
processive DNA synthesis catalyzed by the eukaryotic DNA polymerases
Upon completion of DNA synthesis, the polymerase most likely rapidly
dissociates from the tethered complex, leaving the stable PCNA sliding
clamp associated with the newly synthesized DNA (12, 13). RFC has been
shown to efficiently remove PCNA clamps from DNA (9), an activity of
particular importance in lagging strand replication which involves the
synthesis of a large number of Okazaki DNA fragments. In human cells,
the number of Okazaki fragments formed during one round of replication
has been estimated to be 100 times greater than the molar amount of
cellular PCNA (9). Thus, it is likely that in addition to its role as a
clamp loader, RFC also catalyzes the removal of PCNA from DNA to
fulfill the requirements for a constant supply of PCNA for further DNA
synthesis and other PCNA-dependent reactions.
RFC contains multiple enzymatic activities including the ability to
hydrolyze ATP to catalytically load PCNA onto DNA and subsequently
recruit pol We have previously demonstrated that the hRFC holoenzyme can be
reconstituted from its five subunits expressed in either
baculovirus-infected insect cells (19) or in a reticulocyte in
vitro transcription-translation system (20). Furthermore, it was
shown that the hRFC p40, p37, and p36 subunits synthesized in the
reticulocyte system formed a stable core complex to which the p140 and
p38 subunits bind only when both are present (20).
In this paper, we report that the purified p40·p37·p36 complex
formed in a baculovirus overexpression system contains
DNA-dependent ATPase activity that is stimulated by PCNA.
This three-subunit complex binds preferentially to primed DNA,
interacts with PCNA, and inhibits RFC/PCNA-dependent
DNA elongation catalyzed by pol Poly(dA)300
and oligo(dT)30 were obtained from Pharmacia Biotech Inc.
To prepare annealed poly(dA)300:oligo(dT)30,
poly(dA)300 (33.3 fmol/µl) and oligo(dT)30
were mixed at a molar ratio of 5:1, 1:1, or 1:5 in buffer containing 10 mM Tris-HCl, pH 8.0, and 0.1 M NaCl. The
mixture was heated at 75 °C for 5 min and then slowly cooled to room
temperature and chilled on ice. pET16ap140 and 19bHisp38 DNAs were
prepared as described previously (20), Coupled in vitro transcription-translation
reactions (12 or 24 µl) containing pET16ap140 (37.5 ng/µl) and/or
pET19bHisp38 DNAs (21 ng/µl) were carried out in the presence or
absence of the p40·p37·p36 complex (83 fmol/µl) at 30 °C for
90 min as described (20).
To precipitate
in vitro translated products formed after incubation, the
reaction mixture (12 µl) was incubated with polyclonal antiserum (0.5 µl) against the RFC p37 subunit at 0 °C for 60 min.
Immunocomplexes were then adsorbed to protein A-agarose beads (5 µl)
at 0 °C for 30 min with frequent shaking. The beads were washed four
times with 1 ml of RIPA buffer (50 mM Tris-HCl, pH 8.0, 250 mM NaCl, 5 mM EDTA, 1 mM DTT, 0.5%
Nonidet P-40, and 0.4% BSA) and twice with cold phosphate-buffered
saline. Proteins adsorbed to the beads were eluted with 20 µl of
2 × SDS gel loading buffer (100 mM Tris-HCl, pH 6.8, 4% SDS, 0.2% bromphenol blue, 20% glycerol, 100 mM DTT),
and aliquots (10 µl) were electrophoresed through a 9%
SDS-polyacrylamide gel (SDS-PAGE). After separation, the gel was fixed
in 25% 2-propyl alcohol, 10% acetic acid, soaked in luminographic
enhancer (Amplifer, Amersham Corp.), dried, and autoradiographed.
Quantitation was performed using a phosphorimager (Fuji).
RFC activity was assayed by its ability
to support the elongation of singly primed M13 DNA in the presence of
pol ATPase activity was measured in reaction
mixture (20 µl) containing 25 mM Tris-HCl, pH 7.5, 1 mM DTT, 3 mM MgCl2, 50 µg/ml BSA,
50 µM [ Nitrocellulose filter binding assays were
carried out in reaction mixtures (25 µl) containing binding buffer
(25 mM Hepes-NaOH, pH 7.5, 2 mM
MgCl2, 1 mM DTT, 100 µg/ml BSA, and 20 mM NaCl), the p40·p37·p36 complex (in amounts as
indicated), and 40 fmol of 5 Protein-protein interactions
were examined using surface plasmon resonance. The immobilization of
PCNA on sensor chips was carried out using the carbodiimide covalent
linkage protocol specified in the manufacturer's instructions
(Pharmacia Biosensor). The interaction between immobilized PCNA and
bRFC or the p40·p37·p36 complex in solution was followed by
monitoring changes in the surface concentration of proteins on sensor
chips using the BIAcore 2000 at room temperature.
The loading of
32P-labeled PCNA onto DNA was carried out in reaction
mixtures (50 µl), containing 0.5 pmol of singly nicked pBluescript
DNA, 2.6 pmol of 32P-labeled PCNA trimers (~1500
cpm/fmol) in 50 µl of incubation buffer (20 mM Tris-HCl,
pH 7.5, 0.1 M NaCl, 8 mM MgCl2, 0.5 mM ATP, 4% glycerol, 5 mM DTT, and 40 µg/ml
BSA) and 200 fmol of bRFC. Reactions were incubated for 10 min at
37 °C, stopped on ice, and then applied at 4 °C to a 5-ml gel
filtration column (Bio-Gel A15m, Bio-Rad) equilibrated with incubation
buffer. Fractions of 170 µl were collected, and 32P was
quantitated by Cerenkov counting. The release of 32P-PCNA
complexed to DNA (the unloading reaction) was carried out in reaction
mixtures (50 µl) containing 83 fmol of PCNA (as trimer) loaded onto
singly nicked DNA (isolated by gel filtration), incubation buffer (as
described above), and bRFC or the p40·p37·p36 complex (in amounts
as indicated). Mixtures were incubated at 37 °C for 10 min, and the
reaction was halted by placing tubes on ice. Reactions were then
filtered at 4 °C through a 5-ml gel filtration column (Bio-Gel,
A15m, Bio-Rad) to resolve the 32P-PCNA (eluting in the
included volume) from 32P-PCNA bound to DNA (eluting in the
excluded volume). Fractions of 170 µl were collected and the
32P quantitated by Cerenkov counting.
Recombinant
baculoviruses that produced the p40, p37, and p36 subunits of hRFC were
as described previously (19).
Sf9 cells
(Invitrogen) were grown at 27 °C to a cell density of 2 × 106 cells/ml in Grace's medium supplemented with 10%
fetal bovine serum. Sf9 cells (2 × 108, 200 ml) were
infected simultaneously with recombinant viruses that produced the p40,
p37, and p36 subunits of hRFC at a multiplicity of infection of 5 for
each virus and were maintained in a 2-liter glass flask at 27 °C for
48 h with constant shaking (100 rpm). The cells were then
harvested by centrifugation at 300 × g for 15 min. The
cell pellet was washed with ice-cold phosphate-buffered saline,
resuspended in 2 volumes of hypotonic buffer (50 mM
Tris-HCl, pH 8.0, 10 mM KCl, 1.5 mM
MgCl2, 20 mM sodium phosphate buffer, pH 8.0, 0.5 mM phenylmethanesulfonyl fluoride, 0.2 µg/ml
aprotinin, 0.2 µg/ml leupeptin, and 0.1 µg/ml antipain) per volume
of packed cells, and lysed with 10 strokes of a Dounce homogenizer.
After centrifugation at 2,400 × g for 30 min at
4 °C, the supernatant (cytosolic extract, 95 mg, 9 ml) was saved,
and the nuclear pellet was resuspended in 2 volumes of extraction
buffer (hypotonic buffer without 10 mM KCl) per volume of
packed cells and the mixture adjusted to a final concentration of 0.42 M NaCl. After centrifugation at 43,500 × g
for 30 min at 4 °C, the supernatant (nuclear extract, 35 mg, 5 ml)
was combined with the cytosolic extract, and the mixture was
centrifuged at 44,000 × g for 30 min at 4 °C. The supernatant was used for the purification of the p40·p37·p36
complex as described below.
SDS-PAGE (9%)
followed by Coomassie staining and Western blot analysis using
antibodies specific for the p40, p37, and p36 subunits of RFC were
employed to monitor all purification steps which were carried out at
4 °C.
Extracts (120 mg of protein, 13 ml), prepared as described above, were
adjusted to 0.1 M NaCl and chromatographed on a
SP-Sepharose column (1.5 × 2.9 cm) equilibrated with buffer A (25 mM Tris-HCl, pH 7.5, 1 mM EDTA, 0.01% Nonidet
P-40, 1 mM DTT, 0.2 mM phenylmethanesulfonyl fluoride, 0.1 µg/ml antipain, 0.2 µg/ml leupeptin, 0.2 µg/ml
aprotinin, and 10% glycerol) plus 0.1 M NaCl. After
washing the column with 3-bed volumes of equilibration buffer, bound
proteins were eluted using a 50-ml gradient from 0.1 to 0.4 M NaCl in buffer A. Fractions containing the p40, p37, and
p36 subunits eluted at 0.25 M NaCl and were pooled (14 mg
protein, 15 ml). After adjusting the NaCl concentration to 0.05 M, the pooled fractions were loaded onto a Q-Sepharose
column (1.0 × 1.5 cm) that was developed with a gradient (12 ml)
of 0.05 to 0.4 M NaCl in buffer A. The p40·p37·p36 complex, which eluted at 0.3 M NaCl, was identified by
SDS-PAGE analysis and the DNA-dependent ATPase activity
assay. Prior to this step, the DNA-independent ATPase activity was too
high for reliable measurement of DNA-dependent ATPase
activity. The pooled fractions containing the p40·p37·p36 complex
(3.7 mg of protein, 4.5 ml) contained 1672 units of
DNA-dependent ATPase activity with a specific activity of
452 units/mg. After adjustment to 0.1 M NaCl, these
fractions were loaded onto a heparin-Sepharose column (1.0 × 1.3 cm) equilibrated with 0.1 M NaCl in buffer A. A gradient
solution (10 ml) of 0.1 to 1 M NaCl in buffer A was used to
elute bound proteins. Fractions containing the three RFC subunits
eluted at 0.25 M NaCl and were pooled (2.45 mg, 3.5 ml, 1575 units, 643 units/mg), adjusted to 0.05 M NaCl, and
chromatographed through an ATP-agarose (Sigma, N-6 attachment, 11 atom
spacer) column (1.0 × 1.8 cm) that was developed with a 14-ml
0.05 to 0.5 M NaCl gradient in buffer A. The
p40·p37·p36 complex eluted as a broad peak between 0.1 and 0.5 M NaCl, whereas most of the contaminating proteins eluted
between 0.05 and 0.2 M NaCl. Thus, fractions eluting
between 0.2 and 0.5 M NaCl were pooled (0.59 mg, 6 ml, 520 units, 880 units/mg) and concentrated using the Ultrafree-15 centrifuge
filter membrane (10K, Millipore). An aliquot (200 µg, 150 µl, 131 units, 657 units/mg) of the concentrated material was then sedimented
through a 15-35% glycerol gradient (5 ml) in buffer A plus 250 mM NaCl at 250,000 × g for 24 h at 4 °C. Pooled glycerol gradient fractions (100 µg, 0.66 ml, 114 units, 1140 units/mg) were stored at The p40·p37·p36 complex was assembled in
vivo by coexpressing the p40, p37, and p36 subunits in
baculovirus-infected insect cells. After harvesting the cells,
cytosolic and nuclear extracts were prepared as described under
"Materials and Methods." The combined cytosolic and nuclear
extracts before and after viral infection were analyzed by SDS-PAGE
followed by Coomassie staining (Fig. 1A, lanes
1 and 2, respectively). The overproduced p40, p37, and
p36 subunits were not visible on SDS-PAGE after Coomassie staining
(lane 2). All three subunits were detected by Western blot
analyses using antibodies specific for each subunit, and the amount of
p36, p37, and p40 protein formed was estimated to be 0.5-1% total
protein (data not shown). The p40·p37·p36 complex was purified by a
number of chromatographic steps and glycerol gradient centrifugation as
described under "Materials and Methods." Following glycerol
gradient centrifugation, three protein bands were observed that
migrated through SDS-polyacrylamide gels at positions corresponding to
those of the hRFC p40, p37, and p36 subunits (lane 3). The
identity of each subunit was confirmed by Western blot analysis using
polyclonal antibodies specific for each subunit (lanes
4-6). Densitometry analysis of the Coomassie-stained gels
indicated that the p40, p37, and p36 subunits were present at a molar
ratio of 1.0:1.1:1.0. The RFC p40, p37, and p36 subunits cosedimented
through the glycerol gradient (Fig. 2A),
peaking in fraction 13, between aldolase (158 kDa, fractions 9 and 10) and BSA (66 kDa, fraction 14) with a sedimentation coefficient of 4.8, indicating that these subunits exist as a stable complex. When
individual subunits were subjected to glycerol gradient centrifugation, the p40 and p37 subunits peaked at fraction 16, whereas the p36 subunit
was detected in lower fractions of the gradient most likely due to
protein aggregation (data not shown). The cosedimentation of the three
subunits is consistent with our previous finding that in
vitro transcribed-translated hRFC p40, p37, and p36 subunits form
a stable complex consisting of equimolar amounts of each subunit
(20).
Purification of the RFC p40·p37·p36
complex from baculovirus-infected Sf9 cells. A, expression
and purification of the RFC p40·p37·p36 complex. The
p40·p37·p36 complex expressed and purified from Sf9 cells was
analyzed by 9% SDS-PAGE followed by staining with Coomassie Brilliant
Blue (lanes 1-3) or by Western blotting (lanes
4-6). The additions to each lane were as follows: lane
1, 30 µg of uninfected Sf9 cell extract; lanes 2, 30 µg of extract from cells infected with three baculoviruses expressing the p40, p37, and p36 subunits; lane 3, 0.8 µg of the
pooled glycerol gradient isolated p40·p37·p36 complex, prepared as
described under "Materials and Methods"; lanes 4-6,
immunoblots of the glycerol gradient purified complex used in
lane 3 probed with antibodies specific for the p40 (lane 4), p37
(lane 5), or p36 (lane 6) subunit. Molecular mass
markers are indicated at the left of the figure, and the
position of each RFC subunit is indicated at the right. B, SDS-PAGE analysis of glycerol gradient fractions. The
p40·p37·p36 complex, eluted from the ATP-agarose column (see
"Materials and Methods"), was subjected to a 15-35% glycerol
gradient centrifugation. Aliquots (10 µl) of glycerol gradient
fractions were analyzed by SDS-PAGE followed by Coomassie staining. The
numbers at the top of the figure represent the
glycerol gradient fraction analyzed. C,
DNA-dependent ATPase activity of glycerol gradient
fractions. Each glycerol gradient fraction (0.5 µl) was assayed for
its ability to hydrolyze ATP in the presence or absence of
We have also constructed a baculovirus vector that produces the RFC p36
subunit with an additional 6-histidine residue at the N terminus. The
p40·p37·p36 complex was assembled by coexpressing the His-tagged
p36 subunit with the p40 and p37 subunits in baculovirus-infected insect cells, and the complex was purified using a Ni2+
affinity column followed by glycerol gradient centrifugation. The p40·p37·p36 complex that eluted from the Ni2+
column with imidazole contained excess levels of the uncomplexed His-tagged p36 subunit in addition to the p40·p37·p36 complex. The
excess p36 subunit was not totally removed by the glycerol gradient
centrifugation. The purified p40·p37·p36 complex containing either
the untagged or His-tagged p36 subunit had essentially identical
properties (data not shown).
ATP hydrolysis is required in order for RFC to
load PCNA onto primed DNA templates and recruit pol The effects of various DNA effectors on the ATPase activity of the
complex were examined (Fig. 2A). The p40·p37·p36 complex possessed weak ATPase activity that was stimulated maximally (34-fold) by We also examined the effects of PCNA and HSSB, previously shown to
enhance hRFC DNA-dependent ATPase activity (3, 19). As
shown in Fig. 2B, PCNA stimulated the ATPase activity of the three-subunit complex 2- to 3-fold in the presence of
poly(dA)300:oligo(dT)30 but not
poly(dA)300 . This property is identical to that of bRFC and hRFC, suggesting that optimal ATP hydrolyzing activity is achieved
when the p40·p37·p36 complex is associated with PCNA on primed DNA.
However, the ATPase activity of the p40·p37·p36 complex was
unaffected by HSSB (data not shown) in contrast to results observed
with hRFC and bRFC. It is possible that the stimulatory effects
of HSSB depend on its interaction with the p140 and/or p38
subunits.
Previous experiments indicated that the interaction between
RFC and DNA is mediated by the p140 subunit (16). However, the observation that the p40·p37·p36 complex contained
DNA-dependent ATPase activity suggests that it should also
interact with DNA. We examined the DNA-binding properties of this
complex using a nitrocellulose filter binding assay. High ionic
strength (175 mM NaCl) was required to observe selective
binding of bRFC to primed DNA and not to single-stranded DNA (19).
However, under these conditions the p40·p37·p36 complex did not
bind to either type of DNA. When the salt concentration was lowered,
selective binding of the core complex to primed DNA was observed at 20 mM NaCl. Thus the DNA binding assay with the
p40·p37·p36 complex was carried out at this salt concentration. As
shown in Fig. 3, the p40·p37·p36 complex bound
poly(dA)300 or oligo(dT)30 inefficiently. However, the complex bound poly(dA)300 annealed to
oligo(dT)30, and the binding efficiency was markedly
increased as the molar ratio of oligo(dT)30 to
poly(dA)300 increased, suggesting that the p40·p37·p36
complex specifically recognized DNA primer ends. The selective binding
of the p40·p37·p36 complex to primed DNA is similar to that of the
five-subunit RFC. Selective binding of primed DNA templates under low
ionic condition was also observed with each purified subunit of the
three-subunit complex. However, the efficiency of binding was lower
(2-10-fold). The requirement for low ionic strength to observe DNA
binding by the p40·p37·p36 complex suggests that this three-subunit
complex may not be a major factor influencing the binding of RFC to
primed DNA.
The p40 and p140 subunits of hRFC have been previously shown
to interact with PCNA (17, 25). To explore further the interaction between RFC subunits and PCNA, we determined the direct interaction between PCNA and the p40·p37·p36 complex using the surface plasmon resonance technique as described under "Materials and Methods." As
shown in Fig. 4, when solutions of bRFC or the
p40·p37·p36 complex were passed over a sensor surface on which 33 fmol of PCNA (3,000 RU) was immobilized, an increase in mass on the
sensor surface was detected. In this experiment, 27 fmol of bRFC (8,104 RU) was retained on the PCNA-coated chip, corresponding to a
stoichiometry of ~1 molecule of bRFC bound per molecule of PCNA
trimer (Fig. 4A). The p40·p37·p36 complex (4 fmol, 440 RU) also bound a sensor chip to which an equivalent amount of PCNA had
been coupled, corresponding to a stoichiometry of ~1 molecule of the
p40·p37·p36 complex bound per 8 molecules of PCNA trimer (Fig.
4B). Consistent with this result, about 5 times more PCNA
was co-immunoprecipitated with RFC than with the p40·p37·p36
complex using polyclonal antibodies against the p37 subunit (data not
shown).
The p40·p37·p36 complex did not support the
PCNA-dependent DNA elongation reaction catalyzed by pol
Inhibition of the elongation reaction with each of the core subunits
alone was examined. Only the p40 subunit significantly affected the DNA
elongation reaction, and its effect was qualitatively different than
that observed with the three-subunit complex. The p40 subunit alone
markedly decreased the synthesis of full-length M13 DNA without
reduction of nucleotide incorporation, and its effect was evident at
high concentrations of PCNA, similar to those used in Fig. 5,
lane 7 (data not presented).
The
p40·p37·p36 complex did not load PCNA onto singly nicked circular
duplex DNA (data not shown). However, as shown in Fig. 6A, when the three-subunit complex was
incubated with 32P-labeled PCNA complexed with DNA in the
absence of ATP, PCNA was displaced from the DNA as shown by the
decrease in 32P-PCNA isolated from the excluded region and
the concomitant increase of PCNA detected in the included volume. The
amount of PCNA (23, 43, and 55%) displaced from the PCNA·DNA complex
increased with the addition of increasing amounts of the
p40·p37·p36 complex (6, 12, and 24 pmol, respectively) (Fig. 6,
A and B). However, compared with the
ATP-dependent bRFC-catalyzed unloading reactions in which
5, 20, and 50 fmol of bRFC protein removed 23, 68, and 88% of the PCNA
from DNA, respectively, the efficiency of the three-subunit complex was
low (~103-fold less), suggesting that the p38 and/or p140
subunits of hRFC play important roles in the unloading reaction. High
levels (11 pmol) of the RFC p40 subunit also unloaded PCNA from DNA
(35%) in the absence of ATP but neither the p37 nor p36 subunits
unloaded PCNA from DNA under the same conditions (data not
shown). These findings suggest that p40 subunit is responsible for the
unloading activity of the p40·p37·p36 complex. It should be
emphasized that while RFC unloaded PCNA from DNA in an
ATP-dependent manner, the unloading of PCNA by the
p40·p37·p36 complex or the p40 subunit alone did not require ATP,
and the efficiency of the reaction was unaffected by ATP (data not
presented).
Previous studies in which the five cloned
human genes of RFC were expressed in an in vitro coupled
transcription-translation system showed that these gene products formed
a stable five-subunit complex containing approximately equimolar levels
of each subunit (20). An examination of the interactions between
the RFC subunits indicated that a stable three-subunit core complex
containing the p40, p37, and p36 subunits formed that interacted with
p38 and p140 only when both subunits were present. These findings suggested a model in which the p38 and p140 subunits bind cooperatively to the core complex in assembling the holoenzyme (20).
The availability of the homogeneous p40·p37·p36 core complex
permitted us to examine whether the isolated complex acted as an RFC
holoenzyme assembly intermediate. For this purpose we examined whether
RFC activity could be reconstituted upon mixing the isolated baculovirus expressed p40·p37·p36 complex with the p140 and p38 subunits. As shown in Fig. 7A, when the
p40·p37·p36 complex was incubated with in vitro
translated p140 and p38 subunits and immunoprecipitated with antibodies
specific for the p37 subunit, a complex containing both
35S-labeled p140 and p38 subunits was formed (lane
5). Consistent with previous findings, the large subunit did not
interact with the p40·p37·p36 complex in the absence of the p38
subunit (Fig. 7A, lane 3). However, the p38 subunit was
co-immunoprecipitated with the p40·p37·p36 complex in the absence
of the p140 subunit (lane 4), although the amount
precipitated was about eight times lower than that precipitated when
the p140 subunit was also present in the reaction (Fig. 7A,
lane 5). This result suggests that hRFC p38 subunit plays an
important role in the interaction between the p140 subunit and the
p40·p37·p36 core complex.
The products formed in the reactions described in Fig. 7A
were examined for their ability to support the elongation of a singly primed M13 DNA in a replication reaction containing HSSB, pol It should also be noted that the above experiments were carried out
using in vitro translated RFC p140 and p38 subunits. We have
also carried out these experiments using purified RFC p140 and p38
subunits isolated from baculovirus-infected insect cells. Incubation of
the purified three-subunit complex with isolated p140 and p38 subunits
did not lead to the assembly of the five-subunit complex or formation
of a biologically active product. This failure was traced to a defect
in the p38 subunit. Incubation of the in vitro translated
p38 subunit with the baculovirus isolated p140 and core complex yielded
biologically active five-subunit RFC (data not presented). However, the
purified baculovirus expressed p38 subunit could not substitute for the
in vitro translated p38 in these experiments (data not
shown). These observations suggest that the p38 subunit, when expressed
alone in baculovirus-infected insect cells, may be in an aberrant
conformation that prevents its interaction with other RFC subunits. The
in vitro translated p38, on the other hand, synthesized in
the presence of other RFC subunits or alone, can be successfully
assembled into the RFC complex. The reasons for this discrepancy are
unclear.
The mechanism of elongation of primed DNA templates during DNA
replication is functionally conserved in prokaryotes and eukaryotes. DNA polymerases use a circular "sliding clamp" and a multi-subunit "clamp loader" as accessory factors to achieve their processivity. The clamp loaders isolated from E. coli, phage T4, and human
all consist of five subunits, capable of hydrolyzing ATP in a
DNA-dependent manner, binding to primed DNA templates, and
interacting with their corresponding clamp proteins and SSBs (26, 27).
The E. coli and human clamp loaders are also clamp unloaders
while the T4 clamp dissociates spontaneously from DNA in the absence of
the T4 DNA polymerase (9, 28, 29) and thus may not require the gene
product 44/62 to function as a clamp unloader. Significant progress has
been made in all three systems in assigning functions to the individual
subunits of the clamp loaders (summarized in Table
I).
Table I.
Properties of individual subunits of clamp loaders from E. coli,
phage T4, and human
and
. The hRFC complex consists of five different
subunits with apparent molecular masses of 140, 40, 38, 37, and 36 kDa.
We have previously reported the expression of a three-subunit core
complex, consisting of the p40, p37, and p36 subunits following coupled
in vitro transcription-translation of the cDNAs
encoding these proteins (Uhlmann, F., Cai, J., Flores-Rozas, H., Dean,
F. B., Finkelstein, J., O'Donnell, M., and Hurwitz, J. (1996)
Proc. Natl. Acad. Sci. U. S. A. 93, 6521-6526). Here we
describe the isolation of a stable complex composed of the p40, p37,
and p36 subunits of hRFC from baculovirus-infected insect cells. The
purified p40·p37·p36 complex, like the five-subunit RFC, contained
DNA-dependent ATPase activity that was stimulated by PCNA,
preferentially bound to primed DNA templates, interacted with PCNA, and
was capable of unloading PCNA from singly-nicked circular DNA. In
contrast to the five-subunit RFC, the three-subunit core complex did
not load PCNA onto DNA. The p40·p37·p36 complex inhibited the
elongation of primed DNA templates catalyzed by the DNA polymerase
holoenzyme. Incubation of the p40·p37·p36 complex with the hRFC
p140 and p38 subunits formed the five-subunit hRFC complex that
supported PCNA-dependent DNA synthesis by DNA polymerase
.
and
(pol
and
) (1-8). Following its association with
DNA at a primer-template junction, RFC catalyzes the transfer of PCNA
(the "clamp") onto DNA in the presence of ATP ("clamp loading")
(9, 10). Pol
is recruited to this protein-DNA complex and tethered
to the DNA primer-template junction through its interaction with PCNA in a reaction that requires ATP hydrolysis. The resulting complex (pol
holoenzyme) is then capable of highly processive DNA chain elongation (1-8, 11).
. Human RFC (hRFC) contains five subunits of 140, 40, 38, 37, and 36 kDa that share conserved regions, referred to as RFC
boxes. Two of the conserved regions that contain
GXXXXGK(S/T) (box III) and DE(V/A)D sequences (box V) are
possible ATP binding sites (14, 15), but no single subunit of hRFC has
been shown to hydrolyze ATP. The p40 subunit was shown to bind ATP but
exhibited no detectable ATP hydrolyzing activity (16, 17). The yeast hRFC p36 homologue (Rfc3) exhibited DNA-dependent ATPase
activity (18), but the purified hRFC p36 subunit isolated from
Escherichia coli or baculovirus overexpression systems
failed to show such activity (data not shown). Thus, the identity of
the hRFC subunit (or subunits) that catalyzes ATP hydrolysis is not
known.
. High levels of the p40·p37·p36
complex can unload PCNA from DNA but cannot load PCNA onto DNA. We also
present evidence that the p40·p37·p36 complex, when incubated with
the p140 and p38 subunits, is converted to the five-subunit RFC active
in supporting DNA elongation. This finding indicates that the
p40·p37·p36 complex is an intermediate in assembly of the RFC
holoenzyme.
Preparation of DNAs and Proteins
X174 single-stranded circular
(ssc) viral DNA was obtained from New England Biolabs. Singly primed
M13 ssc DNA and singly-nicked pBluescript (pBS) DNAs were prepared as
described (21, 22) as were HSSB, pol
, PCNA, and hRFC, purified from
HeLa cytosolic extracts (Refs. 1, 3, 21, and 22, respectively).
32P-Labeled PCNA (1500 cpm/fmol) was prepared using
recombinant PCNA containing a cAMP-dependent protein kinase
consensus sequence at its N terminus as described previously (23).
, PCNA, HSSB, and ATP (20). The standard assay (10 µl)
contained 40 mM Tris-HCl, pH 7.5, 0.5 mM DTT,
0.01% BSA, 7 mM magnesium acetate, 2 mM ATP,
100 µM each of dATP, dGTP, dTTP, 20 µM
[
-32P]dCTP (1-2 × 104 cpm/pmol),
4.4 fmol of singly primed circular M13 DNA, 0.25 µg HSSB, 50 ng of
PCNA (or as indicated), 20 fmol of pol
(or as indicated), and RFC
or subunits in amounts as indicated. Reaction mixtures were incubated
at 37 °C for 30 min, stopped with 10 mM EDTA, and
separated by electrophoresis through an alkaline agarose gel (1.5%)
followed by autoradiography. For quantitation, an aliquot (1 µl) of
the reaction mixture was withdrawn, and the amount of acid-insoluble
material formed was determined. When in vitro translated products were used, hRFC subunits produced in 24-µl reactions were
adsorbed to protein A-agarose beads (5 µl) containing polyclonal antibodies against the hRFC p37 subunit as described in the above immunoprecipitation procedure. The beads were added to DNA elongation reaction mixtures (15 µl) containing 10 fmol of singly primed M13 DNA
and other components as described for the standard assay or as
indicated. After incubation for 30 min at 37 °C with frequent shaking, replication products were analyzed as described above.
-32P]ATP (1.5 × 104 cpm/pmol), 12.5 µM DNA (as nucleotides),
and the p40·p37·p36 complex as indicated. After 60 min at 37 °C,
aliquots (1 µl) were spotted on polyethyleneimine-cellulose thin
layer plates which were developed in 1.0 M formic acid, 0.5 M LiCl for 20 min at room temperature. The plates were
dried and exposed for autoradiography. One unit of ATPase activity was
defined as the formation of 1 nmol of Pi under the reaction
conditions used as quantitated by phosphorimager (Fuji) analysis.
32P-labeled
poly(dA)300 (700-1500 cpm/fmol), or 5
32P-labeled oligo(dT)30 (2000-4000 cpm/fmol),
or 5
32P-labeled poly(dA)300 hybridized to
unlabeled oligo(dT)30 at various molar ratios as indicated.
After incubation for 30 min at 0 °C, the mixtures were filtered
through alkaline-washed nitrocellulose filters (Millipore, HA
0.45 µm) which were then washed three times with 0.5 ml of binding
buffer. The radioactivity adsorbed to the filter was measured by liquid
scintillation counting.
80 °C and showed no loss of
activity over a 2-month period with repeated cycles of freezing and
thawing. The yield of the p40·p37·p36 complex obtained from 200 ml
of infected Sf9 cells was ~0.3 mg, representing a total recovery of
10% of the protein (as estimated by Western blot analysis). Densitometry analysis of Coomassie-stained gels was performed using the
Molecular Imager and Imaging Densitometer (Bio-Rad).
Isolation of the p40·p37·p36 Complex from Baculovirus-infected
Insect Cells
Fig. 1.
X174 ssc
DNA as described under "Materials and Methods."
[View Larger Version of this Image (22K GIF file)]
Fig. 2.
Characterization of the ATPase activity of
the p40·p37·p36 complex. A, stimulation of the ATPase by
various DNA effectors. ATPase assays were carried out as described
under "Materials and Methods." Reaction mixtures contained the
p40·p37·p36 complex in amounts as indicated, in the absence or
presence of 12.5 µM (nucleotide concentration) of the
following DNAs: oligo(dT)30, poly(dA)300,
poly(dA)300:oligo(dT)30 (1:5 molar ratio), or
X174 ssc DNA. B, effect of PCNA on the
DNA-dependent ATPase activity of the three-subunit complex.
PCNA was added in amounts as indicated to reaction mixtures containing
50 ng of the p40·p37·p36 complex in the presence of 12.5 µM of poly(dA)300 or
poly(dA)300:oligo(dT)30.
[View Larger Version of this Image (15K GIF file)]
. However, none
of the hRFC subunits isolated from E. coli or baculovirus
overexpression systems has been shown to exhibit ATPase activity (17;
data not shown) suggesting that multiple subunits may be required for
this activity. Therefore, we examined the purified p40·p37·p36
complex for its ability to hydrolyze ATP, and as shown in Fig.
1C this DNA-dependent activity peaked in
fraction 13 of the glycerol gradient, coincidental with the
sedimentation of the p40·p37·p36 protein complex.
X174 ssc DNA, 10-fold by
poly(dA)300:oligo(dT)30, 6-fold by poly(dA)300, and 4-fold by oligo(dT)30. These
properties are similar to those observed for the five-subunit RFC
purified from HeLa cells (3) or from baculovirus-infected insect cells
(19). Each molecule of the p40·p37·p36 complex hydrolyzed 4.5 and
1.4 molecules of ATP/min at 37 °C in the presence of
X174 ssc DNA and poly(dA)300:oligo(dT)30, respectively.
These values are 2-3 times lower than those observed with the
five-subunit hRFC purified from HeLa cells (3) and 4-7 times lower
than that of the bRFC (19). The His-tagged p40·p37·p36 complex
contained DNA-dependent ATPase activity that was 2-fold
higher than the untagged three-subunit complex. It is possible that the
many purification steps required to isolate the untagged
p40·p37·p36 complex leads to some inactivation of this complex.
Fig. 3.
The RFC p40·p37·p36 complex
preferentially binds to primed DNA. A nitrocellulose filter
binding assay was used to examine the DNA-binding properties of the
p40·p37·p36 complex as described under "Materials and Methods."
The complex, in amounts as indicated, was incubated with
32P-labeled poly(dA)300,
oligo(dT)30, or
poly(dA)300:oligo(dT)30 at a molar ratio of
5:1, 1:1, and 1:5, and the interaction monitored as described under
"Materials and Methods." The 100% value represented 40 fmol of
input DNA.
[View Larger Version of this Image (17K GIF file)]
Fig. 4.
The p40·p37·p36 complex interacts with
PCNA. During a 7-min period, a solution of 0.8 pmol of bRFC or the
p40·p37·p36 complex in 35 µl of buffer (10 mM
Hepes-HCl, pH 7.4, 150 mM NaCl, and 3.4 mM
EDTA) was passed over each of two sensor chips to which 33 fmol of PCNA
trimer (3000 RU) had been coupled in the presence of carbodiimide.
Following completion of this 7-min period, buffer without protein was
passed over each chip at the same flow rate (5 µl/min). Background
values obtained following the passage of bRFC or the p40·p37·p36
complex over blank sensor chips have been subtracted from the tracings
presented.
[View Larger Version of this Image (18K GIF file)]
(see below, Fig. 7B). However, the three-subunit complex
inhibited the elongation of singly primed M13 DNA catalyzed by pol
,
PCNA, and bRFC. As shown in Fig. 5, the addition of
increasing amounts of the p40·p37·p36 complex to this DNA
elongation system resulted in an increased inhibition of DNA synthesis,
as evidenced by the accummulation of DNA products shorter than full
length (lanes 1-6). The inhibitory effects of the
three-subunit complex were pronounced at low levels of PCNA (Fig. 5).
As shown in Fig. 5 (lane 7), the addition of high levels of
PCNA reversed the inhibition, whereas high levels of pol
or RFC was
less effective (data not presented). These observations suggest that
the p40·p37·p36 complex inhibited DNA elongation by competing with
the five-subunit RFC for the interaction with PCNA.
Fig. 7.
The p40·p37·p36 complex is an
intermediate in the assembly of the five-subunit RFC. A,
complex formation after incubation of the p40·p37·p36 complex and
the p140 and p38 subunits. Coupled in vitro
transcription-translation reactions (12 µl) with plasmid DNAs
containing either the coding sequence for hRFC p140 or the p38 subunit
were carried out in the presence or absence of the p40·p37·p36
complex (1 pmol) followed by immunoprecipitation (IP) using
a polyclonal antibody against the hRFC p37 subunit. The immunoprecipitated products were analyzed by 9% SDS-PAGE followed by
autoradiography to visualize the 35S-labeled p140 and p38
protein products. Shown in lane 1 is 10% of the reaction
mixture containing the p40·p37·p36 complex and in vitro
translated p140 and p38 subunits prior to immunoprecipitation. Lanes 2-6 represent immunoprecipitation reactions
containing various RFC subunits as follows: lane 2, in
vitro translated 35S-p140 and 35S-p38;
lane 3, the p40·p37·p36 complex and
35S-p140; lane 4, the p40·p37·p36 complex
and 35S-p38; lane 5, the p40·p37·p36 complex
and the 35S-p140 and 35S-p38 subunits.
B, reconstitution of RFC activity. RFC subunits immunoprecipitated on protein A-agarose beads were assayed for their
ability to support the elongation of singly primed M13 DNA as described
under "Materials and Methods." Products were analyzed by alkaline
agarose gel electrophoresis followed by autoradiography. Reactions
shown in lanes 1 and 2 were carried out in the
presence and absence of 50 fmol of bRFC, respectively; reactions shown in lanes 3-8 were carried out after immunoprecipitation of
RFC subunits as follows: lane 3, the p40·p37·p36
complex; lane 4, in vitro translated p140 and p38;
lane 5, the p40·p37·p36 complex and in vitro
translated p140; lane 6, the p40·p37·p36 complex and in vitro translated p38; lane 7, the
p40·p37·p36 complex and in vitro translated p140 and
p38; lane 8, 100 fmol of bRFC. The reaction shown in
lane 9 was carried out with 50 fmol of bRFC and protein
A-agarose beads devoid of any antibodies. Total nucleotide incorporation (pmol), detected following acid precipitation and liquid
scintillation counting, was as follows: lane 1, 16.2;
lane 2, <0.2; lane 3, <0.2; lane 4,
<0.2; lane 5, <0.2; lane 6, <0.2; lane
7, 9.18; lane 8, 8.58; lane 9, 7.92.
[View Larger Version of this Image (18K GIF file)]
Fig. 5.
The p40·p37·p36 complex inhibits the
elongation of singly primed M13 DNA catalyzed by the pol holoenzyme. DNA elongation reactions were carried out as described
under "Materials and Methods" using 4.4 fmol of singly primed M13
DNA. Reactions shown in lanes 1-7 were carried out in the
presence of 10 fmol of pol
, 20 fmol of bRFC, 11.3 fmol of PCNA
trimer, and the p40·p37·p36 complex in amounts as follows:
lane 1, none; lane 2, 1.12 pmol; lane
3, 0.56 pmol; lane 4, 0.28 pmol; lane 5,
0.14 pmol. The reactions shown in lanes 6 and 7 were carried out in the presence of 10 fmol of pol
, 20 fmol of
bRFC, 560 fmol of PCNA trimer in the presence and absence of 0.28 pmol
of the p40·p37·p36 complex, respectively. Total nucleotide
incorporation (pmol), measured following acid precipitation and liquid
scintillation counting, was as follows: lane 1, 16.8;
lane 2, 8.64; lane 3, 12; lane 4, 12;
lane 5, 19.1; lane 6, 30.4; lane 7,
29.6.
[View Larger Version of this Image (52K GIF file)]
Fig. 6.
The p40·p37·p36 complex unloads PCNA from
singly nicked pBS DNA. A, 32P-PCNA was assembled
onto DNA as described under "Materials and Methods" and the product
isolated by separation through a Bio-Gel A15m column. The isolated
32P-PCNA-DNA complex (containing 83 fmol of PCNA) was then
incubated in the presence or absence of the p40·p37·p36 complex
(amounts as indicated) as described under "Materials and Methods"
with the exception that ATP was omitted prior to gel filtration to resolve free 32P-PCNA from the 32P-PCNA·DNA
complex. Fractions from both the included (free 32P-PCNA) and
excluded regions (32P-PCNA·DNA complex) were quantitated
by Cerenkov counting. B, the results shown in A
are presented graphically with the percentage of PCNA unloaded
representing the total amount of PCNA removed from DNA in the presence
of the p40·p37·p36 complex. The unloading of PCNA by bRFC (in the
presence of ATP) is presented for comparison. Note that the levels of
bRFC were in the fmol range, whereas pmol levels of the p40·p37·p36
complex were added.
[View Larger Version of this Image (22K GIF file)]
, and
PCNA. As shown in Fig. 7B, the complex containing all five subunits (lane 7) supported DNA synthesis. No replication
activity was observed upon omission of RFC (lane 2), p140
and p38 (lane 3), the p40·p37·p36 complex (lane
4), the p38 (lane 5), or the p140 subunits (lane
6). These results are in keeping with the requirement for all five
RFC subunits in the DNA elongation reaction. It should be noted that
the adsorption of RFC to protein A beads containing antibodies to the
p37 subunit reduced the activity of RFC by approximately 70% (compare
lanes 1 and 8). Most of this inhibition was due
to the presence of protein A beads which reduced incorporation by
approximately 50% (compare lanes 1 and 9). Since the p40·p37·p36 complex immunoprecipitated on the beads was in molar excess over the reconstituted five-subunit RFC and could potentially inhibit DNA elongation (see Fig. 5), high levels of PCNA
(750 fmol) were used to obviate this problem.
Activity
E. coli (
complex)
Phage T4
(gp44/62)
Human (hRFC)
Subunits
,
,
,
,
gp44,62a
p140,p40,p38,p37,p36
DNA-dependent
ATPase
,
,
gp44
p40 · p37 · p36
Clamp
binding
gp62?
p140,p40,p38,p36
SSB binding
?
?
DNA
binding
?
gp44?
p140,p40 · p37 · p36
Clamp
unloading
NA
p40 · p37 · p36,p40
a
The phage T4 clamp loader contains four gp44 subunits
complexed with one gp62 subunit.
The intrinsic DNA-dependent ATPase activity of RFC is
essential for both the catalytic loading of PCNA onto primed DNA and subsequent recruitment of pol . We report here that a sub-complex consisting of hRFC p40, p37, and p36 subunits hydrolyzes ATP in a
DNA-dependent manner with close to 50% of the efficiency
observed with the five-subunit hRFC. The contributions of the p140 and p38 subunits to the hRFC ATPase activity remain to be determined. The
finding that the hRFC ATPase activity resides in a core p40·p37·p36 heterotrimer but not with these subunits alone is surprising since the
Saccharomyces cerevisiae Rfc3 subunit has been reported to contain DNA-dependent ATPase activity (18). Whether this
reflects different functions of these evolutionary related subunits or a unique structural problem with the human p36 subunit is presently unclear.
In E. coli, the subunit alone binds to ATP but exhibits
very little DNA-dependent ATPase activity, whereas the
heterodimers
and
and the heterotrimer
all
contain significant DNA-dependent ATPase activity (30-33).
These observations are similar to those found with hRFC subunits as
reported here. The hRFC p40 subunit can bind ATP (16, 17) and thus by
analogy with the
subunit in E. coli may be the critical
site for the ATPase activity of the clamp loader. It is possible that
p40 when expressed alone folds into an aberrant conformation incapable
of hydrolyzing ATP but when coexpressed with the p36 and p37 subunits
becomes folded into an active conformation capable of hydrolyzing ATP.
In support of this, the stable p40·p37·p36 complex could not be
formed in vitro by mixing purified p40, p37, and p36
subunits (data not shown). At present, we cannot rule out the
possibility that the p36 or p37 may act as the catalytic subunit of
hRFC ATPase, since both contain ATP-binding sequences, although neither
purified p36 nor p37 binds ATP (17, data not shown). RFC complexes
reconstituted with subunits individually mutated in their putative ATP
binding sites should define the subunit(s) essential for ATPase
activity.
It was previously reported that RFC bound to DNA through the large subunit (16) and that the p37 subunit had weak DNA-binding activity (17). In this report, we provide evidence that p37, together with the p36 and p40 subunits, possess DNA-binding activity essential for the DNA dependence of the ATPase activity. Based on these observations, we suggest that the p140 subunit mediates the initial RFC DNA binding step followed by a DNA-p40·p37·p36 interaction required for the stimulation of ATP hydrolysis.
The p40·p37·p36 complex is not a clamp loader nor does it support
PCNA-dependent DNA synthesis by pol , indicating an
essential role for the p140 and/or p38 subunit in the loading reaction. The unloading of PCNA from DNA may be less complicated, since both p40
alone and the three-subunit complex were also capable of unloading
PCNA. This is consistent with the notion that unloading only requires
the PCNA ring to be opened at one or more of the trimer interfaces,
while loading requires PCNA opening, positioning on the primed DNA, and
subsequent closing of the ring around DNA.
In contrast to the hRFC holoenzyme, unloading of PCNA by the p40·p37·p36 complex does not require ATP, suggesting that ATP is not essential for opening the PCNA ring structure. Since the p40·p37·p36 complex can interact with PCNA, it is possible that interactions between the RFC subunits and PCNA are sufficient to open the ring-shaped PCNA, leading to dissociation of PCNA from DNA. Consistent with this, the p40 subunit alone can also unload PCNA in an ATP-independent manner (data not presented). The requirement of ATP in the loading and unloading of PCNA by hRFC may reflect changes in the structure of hRFC induced by ATP binding and hydrolysis that alter the subunit(s) that interact(s) with PCNA contributing to its catalytic activity.
The 103-fold difference in the unloading efficiency between the three-subunit complex and hRFC indicates that the p140 and p38 subunits are essential for efficient unloading. The p140 and p38 subunits may contribute to the unloading reaction in two ways: (i) strong DNA binding of the p140 subunit may bring hRFC into close proximity with PCNA on DNA stabilizing this interaction, and (ii) additional interactions between the p140 or p38 subunits and PCNA may be required for efficient opening of the PCNA ring. Both the p140 and p38 subunits alone interact with PCNA (25, 34, data not shown). Likewise, the loading of PCNA onto DNA may also require multiple interactions between the different RFC subunits and PCNA.
Following incubation with the p38 and p140 subunits, the p40·p37·p36 sub-complex can be converted to the five-subunit complex active in supporting DNA replication. Thus the five-subunit RFC complex may be assembled in two steps: the formation of the p40·p37·p36 complex, and the subsequent recruitment of the p140 and p38 subunits. Since p38 interacts with both the p140 subunit and the p40·p37·p36 complex, it may act as a bridge between the large subunit and the three-subunit core complex. However, the four-subunit complex (p40, p37, p36, and p38) isolated by immunoprecipitation after mixing the p40·p37·p36 complex with p38 could not be converted to the five-subunit complex following incubation with the p140 subunit (data not shown), suggesting that p38 and p140 subunits bind cooperatively to the core complex. Complexes that contain less than five subunits could not support PCNA-dependent DNA synthesis, indicating that all five subunits are required to constitute functional RFC.
We are indebted to Dr. Z.-Q. Pan for helpful discussions.