We isolated zygotic mutants of a novel class, Hmp (humpback), specifically defective in embryo elongation. They identify three genes:
hmp-1,
hmp-2, and
hmr-1. Elongation of the embryo is mediated by cell shape changes in the hypodermis, the outermost cellular layer of the embryo. In Hmp mutants, the dorsal and lateral hypodermis do not elongate. The ventral hypodermis elongates slightly and extends toward the dorsal side of the embryo, forcing the dorsal hypodermis into bulges. In wild-type hypodermal cells, actin filament bundles form at the start of elongation and appear to contract to generate the force that elongates the embryo. These bundles are oriented circumferentially around the embryo and are linked to adherens junctions between hypodermal cells. In
hmp-1 mutants, circumferential bundles form normally but then disassociate from adherens junctions in the dorsal hypodermis during contraction. Germline mosaic analyses reveal that
hmp-1 is also required before elongation for migration of the leading hypodermal cells during hypodermal enclosure of the embryo.
hmp-1 encodes a protein homologous to the actin-binding protein a-catenin, a component of vertebrate adherens junctions. Antibodies against HMP-1 show the protein localizes to adherens junctions in all hypodermal cells.
hmp-2 gene activity is required to localize HMP-1 protein to adherens junctions.
hmp-2 can encode a protein homologous to the a-catenin-binding protein b-catenin/Armadillo. The
hmr-1 gene product appears similar to the homotypic cell adhesion protein cadherin, which binds b-catenin. We are investigating whether HMP-1, HMP-2, and HMR-1 form a complex that anchors circumferential actin filament bundles at adherens junctions and transmits contractile force between hypodermal cells.