MutLα and Proliferating Cell Nuclear Antigen Share Binding Sites on MutSβ

MutS beta (MSH2-MSH3) mediates repair of insertion-deletion heterologies but also triggers triplet repeat expansions that cause neurological diseases. Like other DNA metabolic activities, MutS beta interacts with proliferating cell nuclear antigen (PCNA) via a conserved motif (QXX(L/I)XXFF). We demonstrate that MutS beta-PCNA complex formation occurs with an affinity of similar to 0.1 mu M and a preferred stoichiometry of 1:1. However, up to 20% of complexes are multivalent under conditions where MutS beta is in molar excess over PCNA. Conformational studies indicate that the two proteins associate in an end-to-end fashion in solution. Surprisingly, mutation of the PCNA-binding motif of MutS beta not only abolishes PCNA binding, but unlike MutS beta, also dramatically attenuates MutS beta-MutL alpha interaction, MutL alpha endonuclease activation, and bidirectional mismatch repair. As predicted by these findings, PCNA competes with MutL alpha for binding to MutS beta, an effect that is blocked by the cell cycle regulator p21(CIP1). We propose that MutS beta-MutL alpha interaction is mediated in part by residues ((L/I)SRFF) embedded within the MSH3 PCNA-binding motif. To our knowledge this is the first case where residues important for PCNA binding also mediate interaction with a second protein. These findings also indicate that MutS beta- and MutS alpha-initiated repair events differ in fundamental ways.