The MutSα-proliferating cell nuclear antigen interaction in human DNA mismatch repair

We have examined the interaction parameters, conformation, and functional significance of the human MutS alpha center dot proliferating cell nuclear antigen ( PCNA) complex in mismatch repair. The two proteins associate with a 1: 1 stoichiometry and a K-D of 0.7 mu M in the absence or presence of heteroduplex DNA. PCNA does not influence the affinity of MutS alpha for a mismatch, and mismatch-bound MutS alpha binds PCNA. Small angle x-ray scattering studies have established the molecular parameters of the complex, which are consistent with an elongated conformation in which the two proteins associate in an end-to-end fashion in a manner that does not involve an extended unstructured tether, as has been proposed for yeast MutS alpha and PCNA ( Shell, S. S., Putnam, C. D., and Kolodner, R. D. (2007) Mol. Cell 26, 565 578). MutS alpha variants lacking the PCNA interaction motif are functional in 3 '- or 5 '- directed mismatch- provoked excision, but display a partial defect in 5 '- directed mismatch repair. This finding is consistent with the modest mutability conferred by inactivation of the MutS alpha PCNA interaction motif and suggests that interaction of the replication clamp with other repair protein(s) accounts for the essential role of PCNA in MutS alpha-dependent mismatch repair.