Biological tubes, such as our airways and vasculature, rely on a balance of contraction and relaxation to either maintain homeostatic tone, or to constrict or dilate in response to a stimulus. We use the spermatheca, a single layer tube of smooth muscle-like cells and the site of fertilization of Caenorhabditis elegans, as a model biological tube to study how constriction is controlled. Smooth muscle contractility depends on the pool of phosphorylated myosin, which can form bipolar filaments, bind actin, and has ATPase activity. Previous work from our lab shows that contraction requires PLC-1 to generate IP3, which in turn mediates Ca2+ release from the endoplasmic reticulum. We therefore hypothesized that a calmodulin-regulated myosin light chain kinase may be phosphorylating myosin within the spermatheca. We performed an RNAi screen of candidate genes and identified a new myosin light-chain kinase, ZC373.4/mlck-1, that is necessary for constriction in the C. elegans spermatheca and shares significant homology with human MYLK within the kinase domain. DIC time lapse imaging shows that depletion of
mlck-1, either with a hypomorphic allele
mlck-1(
tm4159) or with RNAi, slows or stalls oocyte transit through the spermatheca.
mlck-1 worms have a small brood size, which seems to be the result of maternal damage to oocytes as they pass through the spermatheca. In addition to its kinase domain, MLCK-1 has two putative calmodulin binding domains. Calmodulin is activated upon calcium binding and can disinhibit myosin light-chain kinases, leading to contraction. We find that CMD-1, a calmodulin, is required for transit through the spermatheca. This work identifies and characterizes a new myosin light-chain kinase in C. elegans.