Myotactin, a novel hypodermal protein involved in muscle-cell adhesion in Caenorhabditis elegans

In C, elegans, assembly of hypodermal hemidesmosome-like structures called fibrous organelles is temporally and spatially coordinated with the assembl y of the muscle contractile apparatus, suggesting that signals are exchange d between these cell types to position fibrous organelles correctly. Myotac tin, a protein recognized by monoclonal antibody MH46, is a candidate for s uch a signaling molecule. The antigen, although expressed by hypodermis, fi rst reflects the pattern of muscle elements and only later reflects the pat tern of fibrous organelles. Confocal microscopy shows that in adult worms m yotactin and fibrous organelles show coincident localization. Further, cell ablation studies show the bodywall muscle cells are necessary for normal m yotactin distribution. To investigate myotactin's role in muscle-hypodermal signaling, we characterized the myotactin locus molecularry and geneticall y. Myotactin is a novel transmembrane protein of similar to 500 kd, The ext racellular domain contains at least 32 fibronectin type III repeats and the cytoplasmic domain contains unique sequence. In mutants lacking myotactin, muscle cells detach when embryonic muscle contraction begins. Later in dev elopment fibrous organelles become delocalized and are not restricted to re gions of the hypodermis previously contacted by muscle, These results sugge st myotactin helps maintain the association between the muscle contractile apparatus and hypodermal fibrous organelles.