Influence of actin binding on the conformation of the central segment of the heavy chain of skeletal myosin subfragment 1.

Chymotryptic subfragment 1 (S1) of fast skeletal muscle myosin was digested with trypsin in a low ionic strength buffer in the presence of actin. Under these conditions, leading to S1-induced polymerization of actin (Cooke, R. and Morales, M.F. (1971) J. Mol. Biol. 60, 249-261), the S1 heavy chain was cleaved between Lys-561 and Ser-562, generating the C-terminal fragment with apparent mass of 31 kDa. In the absence of actin, this peptide bond was inaccessible to trypsin. The yield of the 31 kDa fragment decreased with the increase in the ionic strength of the medium. The cleavage was also partially inhibited by magnesium or calcium chloride at millimolar concentrations. The data suggest that in low salt conditions and at low concentrations of divalent cations, actin induces a conformational change in the C-terminal portion of the 50 kDa central segment of the S1 heavy chain.