The exc mutations of Caenorhabditis elegans alter the position and shape of the apical cytoskeleton in polarized epithelial cells. Mutants in
exc-7 form small cysts throughout the tubular excretory canals that regulate organismal osmolarity. We have cloned the
exc-7 gene, the closest nematode homologue to the neural RNA-binding protein ELAV. EXC-7 is expressed in the canal for a short time midway through embryogenesis. Cysts in
exc-7 mutants do not develop until several hours later, beginning at the time of hatching. We find that the first larval period is when the canal completes the majority of its outgrowth, and adds new apical cytoskeleton at a rapid rate. Ultrastructural studies show that
exc-7 mutant defects resemble loss of beta(H)-spectrin (encoded by
sma-1) at the distal ends of the excretory canals. In addition,
exc-7 mutants exhibit synergistic excretory canal defects with mutations in
sma-1, and EXC-7 binds
sma-1 mRNA. These data imply that EXC-7 protein may affect expression of
sma-1 and other genes to effect proper development of the excretory