Molecular and genetic analysis of the sex-determining proteins FEM-1 and FEM-3

Male development in Caenorhabditis elegans depends on the activities of the proteins FEM-1, FEM-2 and FEM-3. Extensive genetic analysis indicates that in XO animals the FEMs inhibit the activity of the female-determining transcription factor TRA-1A to promote male somatic development. In the germline, the FEMs act on TRA-1A and additional targets to promote spermatogenesis. Little is known about the molecular and biochemical mechanisms through which the FEMs act, especially the ankyrin (ANK) repeat protein FEM-1. I molecularly and genetically characterized a collection of fem-1 loss-of-function alleles and found that the N-terminal ANK repeats of FEM-1 are critical for its sex-determining activity. ANK repeats are evolutionarily conserved protein-protein interaction motifs suggesting that a FEM-1-ANK repeat mediated protein interaction is required for male development. Observations of a dominant-negative version of FEM-1 that carries a missense mutation in the sixth ANK repeat support this hypothesis. FEM-1 is unusual among the sex-determining proteins in that it has been well conserved through evolution: orthologues exist in insects and vertebrates. Others have suggested that the conservation of FEM-1 may be due to a role in apoptosis. However, my work demonstrates that FEM-1 is not essential for programmed cell death in C. elegans other than through its characterized role as a regulator of TRA-1A activity.;FEM-3, a protein with no recognizable functional motifs, acts as a developmental switch in sex determination. FEM-3 activity is both necessary and limiting for male development. Previous work showed that FEM-3 physically interacts with FEM-2, a type 2C protein phosphatase, but did not establish the biological significance of the interaction. I tested the hypothesis that the FEM-3/FEM-2 interaction is required for male development. Using a modified version of the yeast 2-hybrid system I isolated FEM-3(D313N), a mutant that is specifically defective in its ability to bind to FEM-2. Low-copy fem-3 transgenes that direct expression of the D313N variant under the control of native fem-3 regulatory sequences show significantly lower masculinizing activity than otherwise identical wild-type transgenes. These data suggest that the ability of FEM-3 to bind to FEM-2 is required for the masculinizing activity of FEM-3.