Dissociation Rate of Regulatory Light Chain from Molluscan Myosins

Abstract

The temperature dependence of myosin Mg-ATPase activity and of the heat-dissociation rate of myosin regulatory light chains were studied by using the myosins from striated and smooth adductor muscles of akazara scallop, and from surf-clam foot muscle.
1. Mg-ATPase activity of akazara striated adductor myosin showed a significantly decreased Ca-sensitivity above 20°C, whereas those of akazara smooth adductor and surf-clam foot myosins were to a much less extent affected.
2. The regulatory light chains of akazara striated adductor myosin were readily dissociated by heat-treatment at 30°C, even in the presence of 2mM MgCl₂, unlike those of surf-clam foot myosin
3. The Ca²⁺ concentration for 50% dissociation of regulatory light chains by heat (30°C)- treatment was estimated to be 4μM for akazara striated myosin, 0.8μM for akazara smooth myosin, 0.2μM for surf-clam foot mysoin. The Mg²⁺ concentration for the 50% dissociation was 240, 20, and 16μM, respectively.