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 ␤-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 ϳ1000 s ؊1, a very similar rate constant to MHC-2, but 10 -15% of the complex must go through a slow isomerization (ϳ20 s ؊1 ) before ATP can dissociate it. Similar slow isomerizations were seen in the displacement of ADP from actinmyosin⅐ADP and provide evidence of three closely related actinmyosin⅐ADP 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 (ϳ20 s ؊1 ), and a third complex that releases ADP too slowly (ϳ6 s ؊1 ) to be on the main ATPase pathway. The role of these actin-myosin⅐ADP complexes in the mechanochemistry of slow muscle contraction is discussed in relation to the load dependence of ADP release.Myosins comprise a family of ATP-dependent motor proteins and are best known for their role in muscle contraction and their involvement in a wide range of other eukaryotic motility processes (1-6). During the myosin ATPase cycle, the myosin motor domain (or cross-bridge) undergoes a series of conformational changes coupled to the binding of nucleotide and actin, which results in a translocation of the myosin cargo domain relative to the actin track (7). The sequence of molecular events in the actin-myosin cross-bridge cycle appears essentially the same for all myosins so far studied. The different mechanochemical properties of each myosin are attributed to a modulation of the rates and equilibrium constants of individual molecular events to match each myosin to its physiological role.The most widely studied myosins are the vertebrate, striatedmuscle myosins, which define the class II myosins. This class of myosins have a dimerization domain that forms a long coiled-coil and which can assemble further to form the backbone of the bipolar thick myosin filament. Of the class II myosin, the myosin heavy chain 2 (MHC-2) 4 isoform found primarily in white, anaerobic, fast-contracting, skeletal muscle has been most thoroughly described at both the molecular and physiological levels. In adult, mammalian, muscle tissue the MHC-2 is found as various isoforms (e.g. 2a, 2b, 2x) and it has been established that the essential mechanical properties of the muscle fiber contraction (e.g. maximum velocity of shortening, force per cross-bridge) are properties of the MHC present in the t...

Myosin MgADP Release Rate Decreases as Sarcomere Length Increases in Skinned Rat Soleus Muscle Fibers

“…Estimates of MgADP dissociation rates range from 48 s À1 in pig soleus to 94 s À1 in bovine masseter at 20 C (39,40), which is roughly four-to sixfold faster than our estimates. These findings suggest that myosin crossbridge kinetics and nucleotide handling rates are influenced by organizational structure of the myofilament lattice, which may slow cross-bridge cycling kinetics as force develops throughout the sarcomere in muscle fibers (or with reduced thick-to-thin-filament spacing) (6,8).…”

Calcium sensitivity of the cross-bridge cycle of Myo1c, the adaptation motor in the inner ear

“…Biphasic ATP-induced dissociation of actin from myosin has been also observed for Myo1b (20,21) and non-muscle myosin II (22,23) and more recently for slow skeletal muscle myosin II (24,25). A distinction among the data for these other myosins and that of Myo1c is the dependence of the fast and slow phases on ATP concentration.…”

Fiber Types in Mammalian Skeletal Muscles

“…The kinetic parameters of the above cycle, and possibly also the size of the power stroke, i.e., the unitary force or displacement step (130), differ among myosin isoforms, and this represents a major determinant of the diversity in contractile properties of muscle fibers (see below). For example, the rate of the ATP hydrolysis step can vary sixfold between 22 s Ϫ1 in slow soleus myosin and 131 s Ϫ1 in fast 2X psoas myosin (368). Under conditions of low or zero load, the rate of ADP release is the rate-limiting step of the cycle and thus a major determinant of the speed of filament sliding (735,854) (see FIG.…”