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Atomic model of the human CTF18-RFC-PCNA-DNA ternary complex with narrow PCNA opening state I (state 5)
Structural highlights
FunctionCTF18_HUMAN Chromosome cohesion factor involved in sister chromatid cohesion and fidelity of chromosome transmission. Component of one of the cell nuclear antigen loader complexes, CTF18-replication factor C (CTF18-RFC), which consists of CTF18, CTF8, DCC1, RFC2, RFC3, RFC4 and RFC5. The CTF18-RFC complex binds to single-stranded and primed DNAs and has weak ATPase activity that is stimulated by the presence of primed DNA, replication protein A (RPA) and by proliferating cell nuclear antigen (PCNA). The CTF18-RFC complex catalyzes the ATP-dependent loading of PCNA onto primed and gapped DNA. Interacts with and stimulates DNA polymerase POLH. During DNA repair synthesis, involved in loading DNA polymerase POLE at the sites of local damage (PubMed:20227374).[1] [2] [3] [4] Publication Abstract from PubMedThe DNA sliding clamp PCNA is a multipurpose platform for DNA polymerases and many other proteins involved in DNA metabolism. The topologically closed PCNA ring needs to be cracked open and loaded onto DNA by a clamp loader, e.g., the well-studied pentameric ATPase complex RFC (RFC1-5). The CTF18-RFC complex is an alternative clamp loader found recently to bind the leading strand DNA polymerase epsilon and load PCNA onto leading strand DNA, but its structure and the loading mechanism have been unknown. By cryo-EM analysis of in vitro assembled human CTF18-RFC-DNA-PCNA complex, we have captured seven loading intermediates, revealing a detailed PCNA loading mechanism onto a 3'-ss/dsDNA junction by CTF18-RFC. Interestingly, the alternative loader has evolved a highly mobile CTF18 AAA+ module likely to lower the loading activity, perhaps to avoid competition with the RFC and to limit its role to leading strand clamp loading. To compensate for the lost stability due to the mobile AAA+ module, CTF18 has evolved a unique beta-hairpin motif that reaches across RFC2 to interact with RFC5, thereby stabilizing the pentameric complex. Further, we found that CTF18 also contains a separation pin to locally melt DNA from the 3'-end of the primer; this ensures its ability to load PCNA to any 3'-ss/dsDNA junction, facilitated by the binding energy of the E-plug to the major groove. Our study reveals unique structural features of the human CTF18-RFC and contributes to a broader understanding of PCNA loading by the alternative clamp loaders. Cryo-EM reveals a nearly complete PCNA loading process and unique features of the human alternative clamp loader CTF18-RFC.,He Q, Wang F, O'Donnell ME, Li H Proc Natl Acad Sci U S A. 2024 Apr 30;121(18):e2319727121. doi: , 10.1073/pnas.2319727121. Epub 2024 Apr 26. PMID:38669181[5] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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