Activated Ras initiates a cascade of sequential phosphorylation events, including the protein kinases Raf, MEK, and MAP kinase. The Let-60 Ras-mediated signal transduction pathway controls vulval induction in Caenorhabditis elegans. Both Lin-45 Raf and Sur-1 MAP kinase have been determined to be essential factors during vulval induction; however, the C. elegans mek gene has not been identified. In this paper, we have cloned a C. elegans mek gene,
mek-2, and demonstrated that the MEK-2 protein possesses the biochemical properties of MAP kinase kinases: The C. elegans MEK-2 protein can phosphorylate and activate a human MAP kinase (ERK1), and MEK-2 itself can be phosphorylated and activated by immunoprecipitated mammalian Raf. The
mek-2 gene plays a key role in the
let-60 ras-mediated vulval induction pathway, as loss-of-function mutations in the gene (
ku114 and
h294) significantly reduce the signal transmitted through Ras.
mek-2(
ku114) completely suppressed the Multivulva (Muv) phenotype of a hyperactive
let-60 ras mutation, and animals homozygous for
mek-2(
ku114) also displayed a partial larval lethal phenotype. Animals homozygous for
mek-2(
h294) exhibited a highly penetrant sterile and Vulvaless phenotype. Microinjection of a gain-of-function
mek-2 mutation resulted in Muv and other mutant phenotypes, whereas microinjection of a dominant-negative mutation not only suppressed the Muv phenotype of an activated
let-60 ras mutation but also caused an egg-laying defective phenotype in otherwise wild type animals. Our results demonstrate that
mek-2 acts between
lin-45 raf and
sur-1/mpk-1 in a signal transduction pathway used in the control of vulval differentiation and other developmental events.