CAENORHABDITIS-ELEGANS SEX-DETERMINING PROTEIN FEM-2 IS A PROTEIN PHOSPHATASE THAT PROMOTES MALE DEVELOPMENT AND INTERACTS DIRECTLY WITH FEM-3

Male sexual development in the nematode Caenorhabditis elegans require s the genes fem-1, fem-2 and fem-3. The current model of sex determina tion portrays the FEM proteins as components of a novel signal transdu ction pathway, but the mechanisms involved in signaling through the pa thway are not understood. We report the isolation of fem-2 cDNAs in a yeast two-hybrid screen for clones encoding proteins that interact wit h FEM-3. Association of FEM-3 and FEM-2 in two independent in vitro bi nding assays substantiates the interaction detected in the two-hybrid system. FEM-2 is related in sequence to protein serine/threonine phosp hatases of Type 2C (PP2C). We demonstrate that FEM-2 exhibits magnesiu m-dependent casein phosphatase activity, typical of PP2C, in vitro. Po int mutations that abolish the casein phosphatase activity of FEM-2 wi thout affecting its FEM-3-binding activity reduce severely its ability to rescue male development in fem-2 mutant nematodes. These results s uggest that protein phosphorylation regulates sex determination in C. elegans.