The molecular effects of skeletal muscle myosin regulatory light chain phosphorylation

1 Department of Physiology and Biophysics, Boston University School of Medicine, Boston, Massachusetts; and 2 Department of Molecular and Cellular Pharmacology, University of Miami Miller School of Medicine, Miami, Florida Submitted 25 March 2009 ; accepted in final form 19 May 2009 Phosphorylation of the myosin regulatory light chain (RLC) in skeletal muscle has been proposed to act as a molecular memory of recent activation by increasing the rate of force development, ATPase activity, and isometric force at submaximal activation in fibers. It has been proposed that these effects stem from phosphorylation-induced movement of myosin heads away from the thick filament backbone. In this study, we examined the molecular effects of skeletal muscle myosin RLC phosphorylation using in vitro motility assays. We showed that, independently of the thick filament backbone, the velocity of skeletal muscle myosin is decreased upon phosphorylation due to an increase in the myosin duty cycle. Furthermore, we did not observe a phosphorylation-dependent shift in calcium sensitivity in the absence of the myosin thick filament. These data suggest that phosphorylation-induced movement of myosin heads away from the thick filament backbone explains only part of the observed phosphorylation-induced changes in myosin mechanics. Last, we showed that the duty cycle of skeletal muscle myosin is strain dependent, consistent with the notion that strain slows the rate of ADP release in striated muscle. in vitro motility assay; mechanics Address for reprint requests and other correspondence: J. R. Moore, Dept. of Physiology and Biophysics, Boston Univ. School of Medicine, L-720, 72 E. Concord St., Boston, MA 02118 (e-mail: jxmoore{at}bu.edu )