A key step in morphogenesis of the C. elegans epidermis is ventral closure.
vab-1 and
vab-2 mutants display similar defects in epidermal ventral closure and morphogenesis.
vab-1 encodes a receptor tyrosine kinase (RTK) of the EPH family (see abstract by George and Chisholm).
vab-2 is defined by six alleles which cause morphogenesis defects similar to those caused by hypomorphic
vab-1 alleles. We have found that
vab-2 encodes a member of the family of ligands for EPH RTKs, and that the molecular changes associated with different alleles of
vab-2 are consistent with partial or complete loss of gene function. Vertebrate EPH ligands are either transmembrane proteins or membrane-anchored via a glycosylphosphatidylinositol (GPI) motif. At present, we are unsure whether VAB-2 is transmembrane or membrane-anchored; however the VAB-2 predicted receptor binding domain is 40% identical to those of transmembrane EPH ligands, and ~30% identical to GPI-anchored ligands. EPH RTKs and their ligands function in vertebrates to control the growth of axons, and are also implicated in morphogenetic pathways. Given the similarity of the
vab-1 and
vab-2 phenotypes, VAB-1 and VAB-2 might interact as receptor and ligand in a signalling pathway involved in the control of epidermal morphogenesis.