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MEK-2, a Caenorhabditis elegans MAP kinase kinase, functions in Ras-mediated vulval induction and other developmental
1Department of Molecular, Cellular, and Developmental Biology, University of Colorado at Boulder 80309, USA.
Abstract:
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.
Insights
Researchers identified the C. elegans mek-2 gene, crucial for the Ras-mediated vulval induction pathway. Loss-of-function mutations disrupted signaling, affecting development and survival, highlighting MEK-2
Area of Science:
- Molecular Biology
- Developmental Biology
- Genetics
Background:
- The Ras signaling pathway, involving sequential phosphorylation of Raf, MEK, and MAP kinase, is vital for cellular processes.
- In Caenorhabditis elegans, the let-60 Ras pathway regulates vulval induction, with Lin-45 Raf and Sur-1 MAP kinase being essential.
- The C. elegans MEK gene homolog had not been previously identified.
Purpose of the Study:
- To clone and characterize the C. elegans MEK gene involved in the let-60 Ras pathway.
- To elucidate the role of the identified MEK gene in vulval induction and other developmental events.
Main Methods:
- Cloning of the C. elegans mek-2 gene.
- Biochemical assays to determine MEK-2 protein's enzymatic activity and regulation by Raf.
- Analysis of mek-2 loss-of-function and gain-of-function mutations in C. elegans.
- Phenotypic analysis of mutant animals, including vulval development, fertility, and lethality.
- Microinjection experiments with mek-2 gain-of-function and dominant-negative constructs.
Main Results:
- The C. elegans mek-2 gene was cloned, and its protein product, MEK-2, demonstrated MAP kinase kinase activity.
- MEK-2 phosphorylated and activated human ERK1 and was activated by mammalian Raf, confirming its biochemical function.
- Loss-of-function mutations in mek-2 (ku114, h294) severely impaired Ras signaling, leading to vulvaless, sterile, and lethal phenotypes.
- Gain-of-function mutations caused Multivulva (Muv) phenotypes, while dominant-negative mutations suppressed activated Ras phenotypes.
Conclusions:
- The C. elegans mek-2 gene is essential for the let-60 Ras-mediated vulval induction pathway.
- MEK-2 functions downstream of Lin-45 Raf and upstream of Sur-1/MPK-1 in this signaling cascade.
- MEK-2 plays a critical role in controlling vulval differentiation and other developmental processes in C. elegans.