Structural Basis for the Recruitment of Ctf18-RFC to the Replisome

Ctf18-RFC is an alternative PCNA loader which plays important but poorly understood roles in multiple DNA replication-associated processes. To fulfill its specialist roles, the Ctf18-RFC clamp loader contains a unique module in which the Dcc1-Ctf8 complex is bound to the C terminus of Ctf18 (the Ctf18-1-8 module). Here, we report the structural and functional characterization of the heterotetrameric complex formed between Ctf18-1-8 and a 63 kDa fragment of DNA polymerase 3. Our data reveal that Ctf18-1-8 binds stably to the polymerase and far from its other functional sites, suggesting that Ctf18-RFC could be associated with Pol 3 throughout normal replication as the leading strand clamp loader. We also show that Pol 3 and double-stranded DNA compete to bind the same winged-helix domain on Dcc1, with Pol 3 being the preferred binding partner, thus suggesting that there are two alternative pathways to recruit Ctf18-RFC to sites of replication.