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Rho-binding kinase (LET-502) and myosin phosphatase (MEL-11) regulate cytokinesis in the early Caenorhabditis elegans embryo

Genes and Development Research Group, Department of Biochemistry and Molecular Biology, University of Calgary, 3330 Hospital Drive NW, Calgary, Alberta, Canada, T2N 4N1

^* Author for correspondence (e-mail: mains{at}ucalgary.ca )

Accepted 26 March 2002

Rho-binding kinase and myosin phosphatase regulate the contraction of actomyosin filaments in non-muscle and smooth muscle cells. Previously, we described the role of C. elegans genes encoding Rho-binding kinase (let-502) and myosin phosphatase targeting subunit (mel-11) in epidermal cell-shape changes that drive morphogenesis and in spermathecal contraction. Here we analyze their roles in a third contractile event, cytokinesis within early embryos. We demonstrate that these genes function together to regulate the rate of cleavage furrow contraction, with Rho-binding kinase/LET-502 mediating contraction, whereas myosin phosphatase/MEL-11 acts as a brake to contraction: early embryonic cleavage often fails or is slowed when let-502 is mutated, whereas mel-11 mutations result in ectopic furrowing and faster furrow ingression. These phenotypes correspond to changes in the levels of phosphorylated regulatory non-muscle myosin light chain (rMLC). The gene products of let-502 and mel-11 colocalize at cleavage furrows, and their mutations alleviate one another's defects. rMLC is phosphorylated in let-502; mel-11 double mutants, indicating that a kinase is able to phosphorylate rMLC in the absence of both LET-502 and MEL-11. Genetic and molecular epistasis experiments place LET-502 and MEL-11 in a cytokinetic pathway. LET-502 and MEL-11 regulate the activity of non-muscle myosin after actin, non-muscle myosin heavy chain/NMY-2, regulatory non-muscle myosin light chain/MLC-4 and early formin/CYK-1 have formed a contractile ring. Proteins including Rho GTPase activating protein/CYK-4 and late CYK-1, which are required for late stages of cytokinesis, function downstream of LET-502 and MEL-11.

Key words: Rho-binding kinase, Myosin phosphatase, Cytokinesis, Contraction, C. elegans

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