Negative regulation of male development in Caenorhabditis elegans by a protein-protein interaction between TRA-2A and FEM-3

The tra-2 gene of the nematode Caenorhabditis elegans encodes a predicted m embrane protein, TRA-2A, that promotes XX hermaphrodite development. Geneti c analysis suggests that tra-2 is a negative regulator of three genes that are required for male development: fem-l, fem-2, and fem-3. We report that the carboxy-terminal region of TRA-2A interacts specifically with FEM-3 in the yeast two-hybrid system and in vitro. Consistent with the idea that FEM -3 is a target of negative regulation, we find that excess FEM-3 can overco me the feminizing effect of tra-2 and cause widespread masculinization of X X somatic tissues. In turn, we show that the masculinizing effects of exces s FEM-3 can be suppressed by overproduction of the carboxy-terminal domain of TRA-2A. A FEM-3 fragment that retains TRA-2A-binding activity can mascul inize fem-3(+) animals, but not fem-3 mutants, suggesting that it is possib le to release and to activate endogenous FEM-3 by titrating TRA-2A. We prop ose that TRA-2A prevents male development by interacting directly with FEM- 3 and that a balance between the opposing activities of TRA-2A and FEM-3 de termines sex-specific cell fates in somatic tissues. When the balance favor s FEM-3, it acts through or with the other FEM proteins to promote male cel l fates.