The Structural Basis of the Myosin ATPase Activity*

Myosin is an ATPase that converts chemical energy into directed movement and can be viewed as a molecular motor. This protein comes in many shapes and sizes. Over 11 classes of myosin have been identified, and it is anticipated that more will be found as the search continues (2). Indeed it has been recognized in one form or another in every eukaryotic cell examined. The common feature of all of these molecules is a section close to the N terminus that can be identified as a motor domain. Over the years considerable effort has been devoted to determining the chemical and physical basis of the energy conversion by myosin. All of the isoforms examined so far exhibit similar kinetic strategies and share common features of the cycle that converts chemical energy into directed movement. The key features of this process were identified 25 years ago by Lymn and Taylor (3). Contrary to initial expectations, ATP hydrolysis in myosin is not coincident with the force-generating step (3, 4). Instead, ATP binding initially reduces the affinity of myosin for actin, after which hydrolysis of ATP occurs rapidly and results in a metastable ternary complex between myosin, ADP, and inorganic phosphate (Pi). In this state, the equilibrium complex between ATP and ADP•Pi is between 1 and 10 for skeletal muscle (5–7). During this time there is rapid interconversion between substrate and products (5, 8). Release of the hydrolysis products from myosin is catalyzed by the rebinding of myosin to actin. The energy transduction step occurs during product release (4). Thus myosin is an unusual enzyme in that the chemical step occurs at a different point in the contractile cycle from the energy transduction event. This most likely arose because myosin spends a very small time attached to actin (9). This feature presents several interesting biochemical problems. How is the hydrolysis of ATP coupled to energy transduction, how does myosin catalyze the hydrolysis of ATP, and what is the physical basis of the metastable state? Insight into the structural foundation of these questions has been provided recently by the structure determinations of the motor domain of Dictyostelium discoideum myosin II complexed with several nucleotides together with the earlier studies on chicken skeletal myosin subfragment-1 (10–14).