lin-3 encodes a transmembrane protein with one EGF motif, which is the vulval inducing signal from the anchor cell during vulval development. Reduction of
lin-3 activity causes a Vulvaless (Vul) phenotype, overexpression of
lin-3 in transgenic animals results in a Multivulva (Muv) phenotype. The EGF motif can induce two different vulval fates in a dose-dependent manner. The timing and position of
lin-3 expression in the somatic gonad is tightly regulated.
lin-3 is also necessary for viability, fertility, and male spicule development. The wild-type function of
lin-3 in different tissues are independently mutable, suggesting that
lin-3 has to generate tissue-specificity through either its expresssion or protein structure. We used two main approaches to study the regulation and function of
lin-3: extragenic suppressor screens to identify genes that regulate
lin-3, and transgenic constructs for structure-function studies. To identify positive and negative regulators of
lin-3, we screened for extragenic suppressors of both
lin-3 Muv phenotype and
lin-3 reduction-of-function Vul phenotype. Two genes,
slm-1 and
slm-2, were obtained from the screen for suppressors of Muv phenotype. By their ability to suppress various
lin-3 transgenic constructs, we find that
slm-1 appears to act on
lin-3 transcription, and
slm-2 functions on Lin-3 protein. Three new genes that negatively regulate the
lin-3 signalling pathway have been obtained in a screen of suppressors of severe loss-of-function of
lin-3. Each of these negative regulators only suppresses a subset of
lin-3 phenotypes and are hence tissue-specific. One gene acts at a site downstream of
let-23, which encodes the receptor of Lin-3; one acts upstream of
let-23. Alternative splicing gives rise to two
lin-3 cDNAs, one slightly longer than the other. It is possible that they encode two slightly different proteins which have different functions in development. To study whether
lin-3 can have different protein forms which function in different tissues and have complementary roles when they function in the same tissue, we constructed transgenic animals that carry a short or a long form of Lin-3 (Lin-3S and Lin-3L). Genetic and laser-ablation experiments suggest that both Lin-3S and Lin-3L can act at a distance when overexpressed. However, at a lower dose, Lin-3S can still act at a distance while Lin-3L cannot. We are testing the hypothesis that
lin-3 functions in various tissues utilize two protein forms, with one acting locally, the other at a distance, and that the cooperation of these two forms gives rise to the invariant vulva pattern in wild type animals.