Regulatory Light Chain-a Myosin Kinase (aMK) Catalyzes Phosphorylation of Smooth Muscle Myosin Heavy Chains of Scallop, Patinopecten yessoensis

Abstract

Regulatory light chain-a myosin kinase (aMK), which phosphorylates one of the myosin regulatory light chains, RLC-a, contained in the catch muscle of scallop, was also found to phosphorylate heavy chains of scallop myosin. After incubation of myosin isolated from the opaque portion of scallop smooth muscle (opaque myosin) with aMK in the presence of [γ-³²P] ATP, about 2 mol of ³²P was incorporated per mol of the myosin. The radioactivity was mostly found in the heavy chain at 0.26 M KC1. The pH-activity curve and MgCl₂ requirement for the heavy chain phosphorylation were similar to those for RLC-a phosphorylation. In contrast, the dependency of activity on KC1 concentration was different from that for RLC-a. The heavy chain phosphorylation activity decreased with increase in KC1 concentration up to 0.06M, and then increased at concentrations over 0.06 M to a maximum at around 0.26 M KC1. This complicated profile probably reflects the solubility of myosin, and the phosphorylation site may be located in the rod portion insoluble at low KC1 concentrations. Phosphorylation of heavy chain did not change the solubility of the opaque myosin molecule at all. The acto-opaque myosin ATPase activity in the presence of Ca²⁺ was found to be decreased to less than one-fourth by the heavy chain phosphorylation.