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

<jats:p>In the nematode <jats:italic>Caenorhabditis elegans</jats:italic>, the zinc finger transcriptional regulator TRA-1A directs <jats:italic>XX</jats:italic> somatic cells to adopt female fates. The membrane protein TRA-2A indirectly activates TRA-1A by binding and inhibiting a masculinizing protein, FEM-3. Here we report that a part of the intracellular domain of TRA-2A, distinct from the FEM-3 binding region, directly binds TRA-1A. Overproduction of this TRA-1A-binding region has <jats:italic>tra-1</jats:italic>-dependent feminizing activity in somatic tissues, indicating that the interaction enhances TRA-1A activity. Consistent with this hypothesis, we show that<jats:italic>tra-2(mx)</jats:italic> mutations, which weakly masculinize somatic tissues, disrupt the TRA-2/TRA-1A interaction. Paradoxically,<jats:italic>tra-2(mx)</jats:italic> mutations feminize the <jats:italic>XX</jats:italic> germ line, as do <jats:italic>tra-1</jats:italic> mutations mapping to the TRA-2 binding domain. We propose that these mutations render <jats:italic>tra-2</jats:italic> insensitive to a negative regulator in the <jats:italic>XX</jats:italic> 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 soluble protein in vivo through proteolytic cleavage of TRA-2A or through translation of an<jats:italic>XX</jats:italic> germ line-specific mRNA. We further show that tagged derivatives of the intracellular domain of TRA-2 localize to the nucleus, supporting the hypothesis that this domain is capable of modulating TRA-1A activity in a manner reminiscent of Notch and Su(H).</jats:p>