Force Generation in Single Conventional Actomyosin Complexes Under High Dynamic Load

¹ National Heart, Lung and Blood Institute, NIH
² UCLA School of Medicine
³ University of Pennsylvania School of Medicine

^* To whom correspondence should be addressed. E-mail: shuman{at}mail.med.upenn.edu.

Submitted on June 9, 2005
Revised on August 8, 2005
Accepted on 4 November 2005

	Abstract

The mechanical load borne by a molecular motor affects its force, sliding distance, and its rate of energy transduction. The control of ATPase activity by the mechanical load on a muscle tunes its efficiency to the immediate task, increasing ATP hydrolysis as the power output increases at forces less than isometric (the Fenn effect) and suppressing ATP hydrolysis when the force is greater than isometric. In this work, we used a novel 'isometric' optical clamp to study the mechanics of myosin II molecules, to detect the reaction steps that depend on the dynamic properties of the load. An actin filament, suspended between two beads, held in separate optical traps, is brought close to a surface that is sparsely coated with motor proteins on pedestals of silica beads. A feedback system increases the effective stiffness of the actin by clamping the force on one of the beads and moving the other bead electro-optically. Forces measured during actomyosin interactions are increased at higher effective stiffness. The results indicate that single myosin molecules transduce energy nearly as efficiently as whole muscle and that the mechanical control of the ATP hydrolysis rate is in part exerted by reversal of the force-generating actomyosin transition under high load without net utilization of ATP.

Key Words: Actomyosin mechanics, External load, Force generation, Muscle, Optical trap, Single molecule

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