During hermaphrodite vulval development, the inductive LIN-3 EGF signal from the gonadal anchor cell (AC) activates the LET-23 EGFR/LET-60 RAS/MPK-1 MAPK signaling pathway in the neighboring vulval precursor cells (VPCs) to specify the vulval cell fates. Since LIN-3 EGF is synthesized as a transmembrane precursor protein, we are interested in identifying the protease(s) that are required for the secretion of LIN-3 EGF during vulval development. In Drosophila , the Rhomboid-1 protein acts as an intra-membrane serine protease that cleaves a membrane-bound Spitz EGF cell-autonomously to activate the RTK/RAS/MAPK signaling pathway in the adjacent cells. The C. elegans genome encodes four putative homologs of Drosophila Rhomboid, termed
rom-1 (F26F4.3) to
rom-4 . Only ROM-1 contains all the conserved residues that form the catalytic triad necessary for the proteolytic activity. To study the function of ROM-1, we isolated the
zh18 deletion allele that is predicted to inactivate the
rom-1 gene.
rom-1(lf) animals exhibit no phenotype as single mutants. However,
rom-1(lf) partially suppresses the multivulva (Muv) phenotype caused by the hyperactivation of the ras pathway, indicating that ROM-1 positively regulates vulval induction. Around the time of vulval induction, a
rom-1::gfp transcriptional fusion is expressed in the VPCs, but not in the AC. Moreover, the
rom-1(lf) phenotype is rescued by expression of
rom-1(+) under control of the Pn.p cell-specific
lin-31 promoter, suggesting that ROM-1 acts in the signal receiving VPCs rather than in the signal sending AC. Furthermore, loss of
rom-1 function almost completely suppresses the AC-independent induction of vulval cell fates that is observed in gonad-ablated
let-60 ras
(n1046gf) or hs::
mpk-1 animals, suggesting that
rom-1 acts predominantly outside of the AC. Using a similar approach for
lin-3, we uncover an AC-independent activity of LIN-3 EGF; reducing
lin-3activity in gonad-ablated hs::
mpk-1animals through a chromosomal mutation or RNAi-mediated suppression of
lin-3 in the VPCs suppresses vulval induction analogous to loss of
rom-1 function. Finally, we observe that the longer of the two LIN-3 isoforms (LIN-3L) that are generated by differential splicing depends on ROM-1 activity, while the shorter isoform (LIN-3S) acts independently of ROM-1. Taken together, our results suggest that ROM-1 functions in the VPCs to produce a secondary LIN-3 EGF signal that amplifies the inductive AC signal cell-autonomously in the VPCs.This model is reminiscent of the role Drosophila Rhomboid plays during oogenesis in the Follicle cells.