Epidermal elongation requires the coordination of the morphogenesis between epidermis and underlying body wall muscles. This process is mediated in part by hemidesmosome-like structures, called fibrous organelles, found in regions of the epidermis adjacent to muscle. We report here that the
vab-19 gene functions in epidermal elongation and encodes a novel conserved, ankyrin repeat containing protein that appear to be associated with fibrous organelles.
vab-19 is defined by a single cold-sensitive allele,
e1036 (isolated by Jim Lewis). At 15deg C almost all of
e1036 animals arrest during embryogenesis. At 22.5 degC 90% of the
e1036 animals survive to adulthood and show variable notched-head and notched-tail morphological phenotypes (Vab). Four-dimensional Nomarski microscopy analysis shows that
vab-19 mutants are defective in elongation and have no obvious defects in earlier embryogenesis. Preliminary results of
vab-19 temperature shift experiments also suggest that
vab-19 is required in elongation.
vab-19 maps to chromosome II. We rescued the mutant phenotypes with an 11 kb DNA fragment containing a gene that encodes a novel protein with four ankyrin (ANK) repeats in the C-terminus. The
e1036 mutation results in a stop codon, creating a truncated protein lacking the ANK repeats. VAB-19 homologs, of unknown function, exist in flies, mice, and humans. A rescuing VAB-19::GFP transgene is expressed in epidermal cells, within which it is localized to or close to fibrous organelles. The C-terminal ANK repeats are not essential for this sub-cellular localization. The function of these repeats in VAB-19 is thus not yet clear. To understand how
vab-19 acts in embryonic morphogenesis, we have begun to examine genetic interactions with
vab-19 by constructing double mutants with other known elongation defective mutations. We found that loss of function in the b H -spectrin
sma-1 specifically suppresses
vab-19 Vab and cold sensitive phenotypes. The suppression of
vab-19 by
sma-1 suggests that VAB-19 and SMA-1 may play antagonistic roles in the epidermal cytoskeleton.