Requirements of multiple domains of SLI-1, a Caenorhabditis elegans homologue of c-Cbl, and an inhibitory tyrosine in LET-23 in regulating vulval differentiation

SLI-1, a Caenorhabditis elegans homologue of the proto-oncogene product c-C bl, is a negative regulator of LET-23-mediated vulval differentiation. Lack of SLI-1 activity can compensate for decreased function of the LET-23 epid ermal growth factor receptor, the SEM-5 adaptor, but not the LET-60 MS, sug gesting that SLI-1 acts before RAS activation. SLI-1 and c-Cbl comprise an N-terminal region (termed SLI-1:N/Cbl-N, containing a four-helix bundle, an EF hand calcium binding domain, and a divergent SH2 domain) followed by a RING finger domain and a proline-rich C-terminus. In a transgenic functiona l assay, the proline-rich C-terminal domain is not essential for sli-1(+) f unction. A protein lacking the SH2 and RING finger domains has no activity, but a chimeric protein with the SH2 and RING finger domains of SLI-1 repla ced by the equivalent domains of c-Cbl has activity. The ICING finger domai n of c-Cbl has been shown recently to enhance ubiquitination of active RTKs by acting as an E3 ubiquitin-protein ligase. We find that the RING finger domain of SLI-1 is partially dispensable. Further, we identify an inhibitor y tyrosine of LET-23 requiring sli-1(+) for its effects: removal of this ty rosine closely mimics the loss of sli-1 but not of another negative regulat or, ark-1. Thus, we suggest that this inhibitory tyrosine mediates its effe cts through SLI-1, which in turn inhibits signaling upstream of LET-60 RAS in a manner not wholly dependent on the ubiquitin-ligase domain.