Quantal Motor Action in Muscle Contraction

Abstract

AbstractIt is becoming clear that quantal behavior is a central feature of contractile systems. Steplike behavior has been demonstrated in the kinesin - microtubule system and in the myosin - actin filament system-both on molecular scale. We show here that step-like features appear also in the single intact sarcomere. We studied single sarcomeres of single bumblebee myofibrils. Motorimposed ramp length changes on activated myofibrils resulted in sarcomere-length changes that were stepwise. Computer analysis of the stepwise shortening patterns revealed a step-size distribution containing multiple peaks. The peaks were separated by 2.7 nm per half-sarcomere which is the linear actin-subunit spacing. Thus, translation steps are an integer multiple of the actin-subunit spacing. This result parallels the one observed in the kinesin-tubulin spacing where step size is a multiple of the tubulin-subunit spacing. In the muscle system, however, the steps are preserved on a macroscopic scale, implying high synchrony. The quantal steps are easily explained by a model in which the actin filament propels itself over stationary cross-bridges: if actin binds to the cross-bridges between steps, then the observed quantal result is inevitable.