During vulval development, P6.p is induced to adopt the 1o cell fate by a cell in the somatic gonad, the anchor cell. This signaling event is mediated by a receptor-type tyrosine kinase, LET-23 that functions by activating a conserved Ras/MAP kinase signal transduction pathway. Previous work has suggested that the pathway functions in part by phosphorylating and inactivating LIN-1, an ETS protein 1, 2.
lin-1 loss of function mutations cause a Multivulva phenotype similar to that caused by constitutive activation of
let-60 ras. We have obtained evidence that the role of
lin-1 in the induction of the 1o fate may be more complicated than previously thought. Specifically we find that, contrary to expectation,
lin-1 is required positively for the expression of several markers for the 1o fate. For example whereas in WT, Ras/MAP kinase signaling induces the expression of
egl-17::gfp in P6.p, expression of the marker is abolished in
lin-1(0). The requirement for
lin-1 for expression of the marker is downstream of or in parallel to
let-60 ras since
lin-1(0)
let-60(gf) and
lin-1(0);
lin-15(0) double mutants fail to express the marker in P6.p. Furthermore, the effect of
lin-1 on
egl-17::gfp expression appears not to be caused by inappropriate activation or expression of
lin-12,
lip-1 or
mab-5, three genes that each can inhibit the 1o vulval cell fate. Analysis of
egl-17::gfp expression in the background of
lin-1 mutations that affect a MAP kinase docking site in the protein suggest that LIN-1 might require modification by MAPK-1 Map kinase in order to function positively. A positive role for
lin-1 in the expression of markers for the 1o fate is consistent with the SynVul phenotype seen in
lin-1 double mutants with
eor-1 or
eor-23. In the regulation of
egl-17::gfp
lin-1 appears to function independently of
lin-31(0), another target of the pathway, since expression is not affected in
lin-31(0) but is abolished in
lin31(0);
lin-1(0) double mutants. In order to investigate
lin-1's positive role further we have identified conserved elements in the
egl-17 gene promoter that mediate induction of expression in P6.p. We are presently testing for binding to these elements. We are also carrying out experiments to determine the focus of
lin-1 with respect to the expression of markers for the 1o fate. (1) Beitel, G. J., S. Tuck, I. Greenwald, and H. R. Horvitz (1995). Genes & Dev. 9: 3149-3162; (2) Jacobs, D., G. J. Beitel, S. G. Clark, H. R. Horvitz, and K. Kornfeld (1998). Genetics 149: 1809-1822; (3) Horward, R. and M. V. Sundaram (2002). Genes & Dev. 16: 1815-1827.