lin-2 is required for the induction of P6.p by the anchor cell at the onset of vulval differentiation.
lin-2 encodes a Membrane-Associated Guanylate Kinase (MAGUK) with similarity to the product of the Drosophila tumor suppressor gene lethal(1)discs-large-1.
lin-2 acts cell-autonomously in P6.p to allow the efficient activation of the receptor tyrosine kinase LET-23. LIN-2 localizes LET-23 to the basal side of the Pn.p cell junctions since loss of
lin-2 function causes mislocalization of LET-23 to the apical compartment of the Pn.p cells. LIN-2 may form a complex with LET-23 and LIN-7 (see abstract of S. Kaech and S. K. Kim). We have used the yeast two hybrid system to isolate cDNAs encoding proteins that bind LIN-2 and found a specific interaction between LIN-2 and LIN-36, which is the product of a class B synthetic multivulva gene. Synthetic multivulva genes form two redundant signaling pathways (termed class A and B) that inhibit vulval differentiation.
lin-36 encodes a novel protein that inhibits the induction of the Pn.p cells in a cell-autonomous fashion (Thomas and Horvitz, WBG 13.2, 33-34). In vitro binding studies with recombinant proteins confirmed the specificity of the interaction. Deletion analysis indicated that the C-terminus of LIN-2 encoded by amino acids 903-961 is both necessary and sufficient for binding to LIN-36. Two
lin-2 alleles (
e1453 and
n105) cause C-terminal truncations of LIN-2 at amino acids 897 and 940, respectively. These alleles would be predicted to eliminate or reduce binding to LIN-36 mediated by the C-terminus of LIN-2, but should not affect the localization of LET-23 mediated through the interaction of LIN-2 with LIN-7. Both alleles display a hyperinduced phenotype in which the outer Pn.p cells P3.p, P4.p or P8.p are induced by the anchor cell in addition to P6.p. A failure to interact with LIN-36 could therefore be responsible for the hyperinduced phenotype. In summary, these findings may suggest a role for
lin-2 in the inhibition of vulval development through interaction with
lin-36.