The slow skeletal muscle isoform of myosin shows kinetic features common to smooth and non-muscle myosins

Abstract

Fast and slow mammalian muscle myosins differ in the heavy chain sequences (MHC-2, MHC-1) and muscles expressing the two isoforms contract at markedly different velocities. One role of slow skeletal muscles is to maintain posture with low ATP turnover, and MHC-1 expressed in these muscles is identical to heavy chain of the β-myosin of cardiac muscle. Few studies have addressed the biochemical kinetic properties of the slow MHC-1 isoform. We report here a detailed analysis of the MHC-1 isoform of the rabbit compared with MHC-2 and focus on the mechanism of ADP release. We show that MHC-1, like some non-muscle myosins, shows a biphasic dissociation of actin-myosin by ATP. Most of the actinmyosin dissociates at up to ∼1000 s-1, a very similar rate constant to MHC-2, but 10-15% of the complex must go through a slow isomerization (∼20 s-1) before ATP can dissociate it. Similar slow isomerizations were seen in the displacement of ADP from actinmyosin·ADP and provide evidence of three closely related actinmyosin·ADP complexes, a complex in rapid equilibrium with free ADP, a complex from which ADP is released at the rate required to define the maximum shortening velocity of slow muscle fibers (∼20 s-1), and a third complex that releases ADP too slowly (∼6 s-1) to be on the main ATPase pathway. The role of these actin-myosin·ADP complexes in the mechanochemistry of slow muscle contraction is discussed in relation to the load dependence of ADP release. © 2007 by The American Society for Biochemistry and Molecular Biology, Inc.