Electron Microscopic Recording of the Power and Recovery Strokes of Individual Myosin Heads Coupled with ATP Hydrolysis: Facts and Implications

Abstract

The most straightforward way to get information on the performance of individual myosin heads producing muscle contraction may be to record their movement, coupled with ATP hydrolysis, electron-microscopically using the gas environmental chamber (EC). The EC enables us to visualize and record ATP-induced myosin head movement in hydrated skeletal muscle myosin filaments. When actin filaments are absent, myosin heads fluctuate around a definite neutral position, so that their time-averaged mean position remains unchanged. On application of ATP, myosin heads are found to move away from, but not towards, the bare region, indicating that myosin heads perform a recovery stroke (average amplitude, 6 nm). After exhaustion of ATP, myosin heads return to their neutral position. In the actin–myosin filament mixture, myosin heads form rigor actin myosin linkages, and on application of ATP, they perform a power stroke by stretching adjacent elastic structures because of a limited amount of applied ATP ≤ 10 µM. The average amplitude of the power stroke is 3.3 nm and 2.5 nm at the distal and the proximal regions of the myosin head catalytic domain (CAD), respectively. The power stroke amplitude increases appreciably at low ionic strength, which is known to enhance Ca2+-activated force in muscle. In both the power and recovery strokes, myosin heads return to their neutral position after exhaustion of ATP.

Special Issue: The Actin-Myosin Interaction in Muscle: Background and Overview

“…Sugi and his colleagues have been studying the movements of various parts of myosin heads labelled with specific antibodies carrying gold particles, which are visible in the microscope. We will discuss some of their results later, but full details are given in the Special Issue Review [126] by Sugi et al…”

“…ATP is applied to the specimen iontophoretically by passing a current through the ATP electrode, containing 100 mM ATP. Reproduced from Sugi et al [126] with permission.…”

“…Sites 1 and 2 in ( d , e ) are the distal and proximal labelling positions discussed in ( a , b ), and sites 3 and 3′ are the light chain labelling positions discussed in ( c ). Adapted from ref [126] with permission.…”