MOLECULAR CHARACTERIZATION OF MYOPLASMIN-C1 - A CYTOSKELETAL COMPONENT LOCALIZED IN THE MYOPLASM OF THE ASCIDIAN EGG

Myoplasmin-Cl is a polypeptide detected by a monoclonal antibody, whic h is localized in the myoplasm of ascidian eggs. Since microinjection of the antibody blocks larval muscle development, myoplasmin-Cl may pl ay a role in muscle cell differentiation (Nishikata et al. 1987). Isol ation and characterization of myoplasmin-Cl cDNA clones revealed that the predicted amino acid sequence of myoplasmin-Cl had no similarity t o any known protein. However, the deduced protein contains heptad repe ats similar to those in myosin heavy chain, tropomyosin and the Drosop hila Bicaudal D gene product, suggesting that it is a filamentous comp onent of the myoplasmic cytoskeleton. The predicted amino acid sequenc e also showed several possible phosphorylation sites. Consistent with the prediction that myoplasmin-Cl is a cytoskeletal component, the pro tein remained in the myoplasmic cytoskeletal domain after detergent ex traction. These results suggest that myoplasmin-Cl is a cytoskeletal c omponent of the myoplasm and that it plays a role in anchoring and seg regating muscle determinants.