During C. elegans morphogenesis, the embryo elongates through epidermal cell shape changes. Two processes are required for elongation: actomyosin contractility controlled by LET-502/ROCK activity, and muscle contractions, which trigger a mechanotransduction pathway in the epidermis through hemidesmosomes (HDs). In addition, microtubules (MTs) play a poorly understood role (Priess and Hirsh, 1986). To define how MTs contribute to elongation, we first examined their organization and dynamics in elongating embryos. We find that two minus-end MT regulators, γ-tubulin and NOCA-1, localize to adherens junctions and along HDs. To disrupt MTs, we specifically expressed the MT-severing protein SPASTIN in the epidermis. This does not strongly affect morphogenesis, unless LET-502/ROCK activity or HDs are partially compromised. Cellular and kinetic data suggest that MTs mainly act by promoting the transport of E-cadherin to adherens junctions and myotactin/LET-805 to HDs, which are critical for elongation. Importantly, let-502/ROCK activity potentiates MT transport, independently of myosin-dependent tension. To look for genes involved in MT-mediated transport and elongation, we performed two independent RNAi screens. The first screen targeted genes whose depletion phenocopied the hypo-elongated phenotype of spastin expression in let-502(lf) animals. Tubulin folding and centrosome biogenesis genes were identified, confirming that MT levels are critical when let-502 is lowered. Among traffic candidate genes, the syntaxin SYX-5 stood out as a key player acting with LET-502. Our data suggest that syx-5 could act by promoting vesicle fusion events close to adherens junctions or HDs. In the second screen, we looked for genes acting in parallel to git-1, the earliest acting factor in the mechanotransduction pathway. We identified genes of the minus-end directed MT motor dynein/dynactin, including a novel component of the complex, FARL-11. Several members of the dynein complex, including farl-11 and dlc-1, were found to localize along HDs. Interestingly, dlc-1 localization pattern depends on a critical serine residue, which needs to be unphosphorylated. Altogether, our data suggest that HDs act as a platform where MTs could be nucleated and trapped, to allow the delivery of proteins essential for elongation. Our finding that LET-502/ROCK reinforces this process independently of its role as a myosin II activator implies that it acts through a target controlling trafficking.
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