MRCK-1 activates non-muscle myosin for outgrowth of a unicellular tube inCaenorhabditis elegans

The formation and patterning of unicellular biological tubes is essential for metazoan development. It is well established that vascular tubes and neurons use similar guidance cues to direct their development, but the downstream mechanisms that promote the outgrowth of biological tubes are not well characterized. We show that the conserved kinase MRCK-1 and its substrate the regulatory light chain of non-muscle myosin, MLC-4, are required for outgrowth of the unicellular excretory canal inC. elegans. Structure-function analysis of MRCK-1 indicates that the kinase domain, but not the small GTPase-binding CRIB domain, is required for canal outgrowth. Expression of a phosphomimetic form of MLC-4 rescues canal truncations inmrck-1mutants and shows enrichment at the growing canal tip. Specific depletion of MRCK-1 or MLC-4 in the canal causes severe truncations, with unlumenized projections of the basal membrane. Moreover, our work reveals a novel function for non-muscle myosin downstream of MRCK-1 in excretory canal outgrowth that may be conserved in the development of seamless tubes in other organisms.