Rod phosphorylation favors folding in a catch muscle myosin.

Myosin from a molluscan catch muscle is unusual in being phosphorylated in the rod by an endogenous heavy chain kinase. The overall structure of the molecule resembles that of other muscle myosins, although the tail is somewhat longer (approximately equal to 1700 A). At low ionic strength the unphosphorylated molecules associate in filaments that display a striking axial repeat of 145 A. Phosphorylation of the rod enhances myosin solubility in the range of NaCl between 0.05 and 0.15 M. Depending on the ionic strength and the counterions present, the soluble species corresponds to an antiparallel folded dimer (15 S) or to a folded monomer (10 S). Unphosphorylated myosin can also be partially solubilized into folded monomers by addition of ATP in 0.15 M NaCl. A similar molecular folding has also been observed in smooth muscle and nonmuscle myosins that depends, however, on the state of phosphorylation of the light chains in the myosin head. We discuss these results in relation to possible mechanisms for control of catch contraction.