MOLECULAR EVOLUTION OF A SEX DETERMINATION PROTEIN - FEM-2 (PP2C) IN CAENORHABDITIS

Somatic sex determination in Caenorhabditis elegans involves a signal transduction pathway linking a membrane receptor to a transcription fa ctor. The fem-2 gene is central to this pathway, producing a protein p hosphatase (FEM-2) of the type 2C (PP2C). FEM-2 contains a long amino terminus that is absent in canonical PP2C enzymes. The function of thi s domain is difficult to predict, since it shows no sequence similarit y to any other known proteins or motifs. Here we report the cloning of the fem-2 homologue from Caenorhabditis briggsae (Cb-fem-2). The sequ ence identity is much higher than that observed for other C. briggsae homologues of C. elegans sex determination proteins. However, this lev el is not uniform across the entire lengths of the proteins; it is muc h lower in the amino termini. Thus, the two domains of the same protei n are evolving at different rates, suggesting that they have different functional constraints. Consistent with this, Cb-FEM-2 is able to rep lace some, but not all, of the Ce-FEM-2 in vivo function. We show that removal of the amino terminus from Ce-FEM-2 has no effect on its in v itro phosphatase activity, or its ability to replace the in vivo funct ion of a yeast PP2C enzyme, but that it is necessary for proper FEM-2 function in worms. This demonstrates that the amino terminus is not an extended catalytic domain or a direct negative regulator of phosphata se activity.