During the development of hermaphrodite vulva three out of six equipotent vulval precursor cells (VPCs, P3.p to P8.p) in the ventral hypodermis are induced by the gonadal anchor cell (AC) to adopt vulval cell fates. The anchor cell signal activates in P6.p the conserved EGFR/RAS/MAPK pathway to specify the primary cell fate. A lateral signal from P6.p then activates the LIN-12/NOTCH signaling pathway in P5.p and P7.p to induce the secondary cell fate. Hyperactivation of the EGFR/RAS/MAPK pathway leads to a multivulva (Muv) phenotype, while inactivation of the EGFR/RAS/MAPK pathway results in a vulvaless (Vul) phenotype. We have previously identified the
gap-1 gene, which encodes a Ras GTP-activating protein, as an inhibitor of vulval development.
gap-1(0) single mutants exhibit wild-type vulval development, but in combination with mutations in other negative regulators of the EGFR/RAS/MAPK pathway a
gap-1(0) mutation cause excess vulval induction and a Muv phenotype. In order to identify inhibitors of the RAS/MAPK pathway that act in parallel with GAP-1, we screened in a
gap-1(0) background for mutations that cause a recessive Muv phenotype. One of the mutations isolated in this screen (
ga139) shows an 86% penetrant synthetic Muv phenotype. Furthermore,
ga139 animals are clear, weakly dumpy and partially sterile. The
ga139 mutation was mapped to the left arm of LG I and RNAi against the predicted gene Y20F4.3 phenocopied the different
ga139 phenotypes. By performing Northern-blot and RT-PCR analysis we found that the mRNA transcript from this region does not match the predicted gene structure. The mRNA contains additional exons not recognized by genefinder and the 3 region corresponds in part to the predicted gene Y51F10.9. We named the gene
gap-3 because it encodes a protein containing two SH2 domains, an SH3, a PH and a C-terminal RasGAP domain. Sequence comparison with other RasGAP proteins indicates that GAP-3 is the C. elegans ortholog of mammalian
p120 RasGAP, while the previously described C. elegans GAP-1 and GAP-2 are members of the more distant RasGAP-like and neuronal synRasGAP families of GAP proteins.
ga139 animals carry a G to A transition in exon 14, resulting in a premature stop codon (Q844stop) in the C-terminal RasGAP domain. Thus, the
ga139 mutation is likely to impair the catalytic activity of GAP-3.