Phosphorylatable serine residues are located in a non-helical tailpiece of a catch muscle myosin

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Abstract
Myosin from a molluscan catch muscle displays unusual properties: when phosphorylated in the rod by an endogenous heavy-chain kinase, myosin solubility is enhanced and the molecule folds (Castellani & Cohen,Proc. natn. Acad. Sci. U.S.A.84, (1987) 4058–62). We have now localized the sites of phosphorylation to the carboxy-terminal end of the rod by selective proteolytic cleavage. Two major stretches of sequence, 18 and 21 residues long, have been identified, each containing a single residue of phosphoserine. Analysis of the amino-acid sequence of these two peptides indicates that they form a non-helical tailpiece. We discuss how phosphorylation of this tailpiece might influence enzymatic activity in catch muscle thick filaments.