MSH4 acts in conjunction with MLH1 during mammalian meiosis

MSH4 is a meiosis-specific MutS homolog, In yeast, it is required for recip rocal recombination and proper segregation of homologous chromosomes at mei osis I. MLH1 (MutL homolog 1) facilitates both mismatch repair and crossing over during meiosis in yeast. Germ-line mutations in the MLH1 human gene a re responsible for hereditary nonpolyposis cancer, but the analysis of MLH1 -deficient mice has revealed that MLH1 is also required for reciprocal reco mbination in mammals. Here we show that hMSH4 interacts with hMLH1. The two proteins are coimmunoprecipitated regardless of the presence of DNA or ATP , suggesting that the interaction does not require the binding of MSH4 to D NA, The domain of hMSH4 responsible for the interaction is in the amino-ter minal part of the protein whereas the region that contains the ATP binding site and helix-turn-helix motif does not bind to hMLH1. Immunolocalization analysis shows that MSH4 is present at sites along the synaptonemal complex as soon as homologous chromosomes synapse. The number of MSH4 foci decreas es gradually as pachynema progresses, During this transition, MLH1 foci beg in to appear and colocalize with MSH4, These results suggest that MSH4 is f irst required for chromosome synapsis and that this MutS homologue is invol ved later with MLH1 in meiotic reciprocal recombination.