Direct protein-protein interaction between the intracellular domain of TRA-2 and the transcription factor TRA-1A modulates feminizing activity in C. elegans

In the nematode Caenorhabditis elegans, the zinc finger transcriptional reg ulator TRA-1A directs XX somatic cells to adopt female fates. The membrane protein TRA-2A indirectly activates TRA-1A by binding and inhibiting a masc ulinizing protein, FEM-3. Here rye report that a part of the intracellular domain of TRA-2A, distinct from the FEM-3 binding region, directly binds TR A-1A. Overproduction of this TRA-1A-binding region has tra-1-dependent femi nizing activity in somatic tissues, indicating that the interaction enhance s TRA-1A activity. Consistent with this hypothesis, we show that tra-2(mx) mutations, which weakly masculinize somatic tissues, disrupt the TRA-2/TRA- 1A interaction. Paradoxically, tra-2(mx) mutations feminize the XX germ lin e, as do tra-1 mutations mapping to the TRA-2 binding domain. We propose th at these mutations render tra-2 insensitive to a negative regulator in the XX germ line, and we speculate that this regulator targets the TRA-2/TRA-1 complex. The intracellular domain of TRA-2A is likely to be produced as a s oluble protein in vivo through proteolytic cleavage of TRA-2A or through tr anslation of an XX germ line-specific mRNA. We further show that tagged der ivatives of the intracellular domain of TRA-2 localize to the nucleus, supp orting the hypothesis that this domain is capable of modulating TRA-1A acti vity in a manner reminiscent of Notch and Su(H).