Differential Requirement for Proliferating Cell Nuclear Antigen in 5′ and 3′ Nick-directed Excision in Human Mismatch Repair

Proliferating cell nuclear antigen (PCNA) is involved in mammalian mismatch repair at a step prior to or at mismatch excision, but the molecular mechanism of this process is not fully understood. To examine the role of PCNA in mismatch-provoked and nick-directed excision, orientation-specific mismatch removal of hetero-duplexes with a pre-existing nick was monitored in human nuclear extracts supplemented with the PCNA inhibitor protein p21. We show here that, whereas 3′ nick-directed mismatch excision was completely inhibited by low concentrations of p21 or a p21 C-terminal fusion protein, 5′ nick-directed excision was only partially blocked under the same conditions. No further reduction of the 5′ excision was detected when a much higher concentration of p21 C-terminal protein was used. These results suggest the following. (i) There is a differential requirement for PCNA in 3′ and 5′ nick-directed excision; and (ii) 5′ nick-directed excision is conducted by a manner either dependent on or independent of PCNA. Our in vitro reconstitution experiments indeed identified a 5′ nick-directed excision pathway that is dependent on PCNA, hMutSα, and a partially purified fraction from a HeLa nuclear extract.