Calcium regulation of molluscan myosin ATPase in the absence of actin

Abstract

In the myosin-linked regulatory mechanism typified by the molluscan scallop adductor muscle, contraction is controlled by Ca²⁺ binding to sites on the thick filament protein, myosin¹. The regulatory light chains of myosin heads are involved directly in this mechanism and early studies^2,3 suggested that, in the absence of Ca²⁺, these subunits prevent the interaction of a myosin–adenosine nucleotide complex with the actin-containing thin filament. Subsequently, Ashiba et al.⁴ reported that the steady-state ATPase of molluscan myosin exhibits a limited degree of Ca²⁺ activation in the absence of actin. Recently, however, we have shown that steady-state ATPase activity in relaxing conditions is dominated by the unregulated molecules in the myosin preparation⁵. Single-turnover kinetic methods are required to monitor the highly suppressed ATPase activity of the regulated population. Using the latter approach, we report here that scallop myosin ATPase is reduced about 100-fold on removal of Ca²⁺. The regulatory light chains maintain the relaxed state via conformational changes which suppress the product release steps, irrespective of the presence of actin.