Abstract
Replication factor C (RF-C) and proliferating cell nuclear antigen (PCNA) assemble a complex, called sliding clamp, onto DNA. The clamp in turn loads DNA polymerases (pol) delta and epsilon to form the corresponding holoenzymes, which play an essential role in replication of eukaryotic chromosomal DNA and in several DNA repair pathways. To determine the fate of RF-C after loading of PCNA onto DNA, we tagged the RF-C subunit p37 with a protein kinase A recognition motif, so that the recombinant five-subunit RF-C complex could be 32P-labeled and quantitatively detected in femtomolar amounts. Nonspecific binding of RF-C to DNA was minimized by replacing the p140 subunit with an N-terminally truncated p140 subunit lacking the previously identified nonspecific DNA binding domain. Neither of these modifications impaired the clamp loading activity of the recombinant RF-C. Using gel filtration techniques, we demonstrated that RF-C dissociated from the DNA after clamp loading or pol delta holoenzyme assembly, while PCNA or PCNA.pol delta complex remained bound to DNA. PCNA catalytically loaded onto the template-primer was sufficient by itself to tether pol delta and stimulate DNA replication. The readdition of RF-C to the isolated PCNA.DNA complex did not further stimulate pol delta DNA synthesis. We conclude that pol delta holoenzyme consists of PCNA and pol delta core and that RF-C serves only to load PCNA clamp.
Highlights
A number of eukaryotic proteins have been found to have a dual role in chromosomal DNA replication and DNA repair
Of Replication factor C (RF-C) with p37-his or p36-his resulted in a minor analytical disadvantage, in that the small RF-C subunits overlapped on SDS-PAGE (Ref. 53 and data not shown)
proliferating cell nuclear antigen (PCNA) is involved in multiple DNA metabolic processes [23,24,25], which require RF-C to load it onto DNA
Summary
A number of eukaryotic proteins have been found to have a dual role in chromosomal DNA replication and DNA repair. Replication protein A (reviewed in Ref. 12) is required for nucleotide excision repair [13] and mismatch repair [14]. An increasing number of replication, repair, and cell cycle regulatory proteins appear to interact with PCNA Biochemical studies on the roles of RF-C and PCNA in DNA replication suggest that PCNA is loaded onto DNA by RF-C in a reaction requiring ATP hydrolysis. Previous studies did not resolve unambiguously whether, after the clamp-loading reaction, PCNA and RF-C form a tertiary complex with DNA or whether these proteins bind independently to the same template (PCNA topologically and RF-C directly through its affinity for DNA). The results presented below demonstrate that RF-C dissociates from PCNA after loading and is not required to tether pol ␦ to the clamp
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