THE glp-1 and lin-12 genes encode homologous transmembrane proteins that may act as receptors for cell interactions during development3,4. The glp-1 product is required for induction of germ-line proliferation and for embryogenesis3,5. By contrast, lin-12 mediates somatic cell interactions, including those between the precursor cells that form the vulval hypodermis (VPCs) 6. Here we analyse an unusual allele of glp-1, glp-1(q35), which displays a semidominant multivulva phenotype (Muv), as well as the typical recessive, loss-of-function Glp phenotypes (sterility and embryonic lethality) 3. We find that the effects of glp-l(q35) on VPC develop-ment mimic those of dominant lin-12mutations, even in the absence of lin-12 activity. The glp-l(q35) gene bears a nonsense mutation predicted to eliminate the 122 C-terminal amino acids, including a ProGluSerThr (PEST) sequence thought to destabilize proteins. We suggest that the carboxy terminus bears a negative regulatory domain which normally inactivates glp-1 in the VPCs. We propose that inappropriate glp-l(q35)activity can substitute for lin-12 to determine vulval fate, perhaps by driving the VPCs to proliferate.
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