Skeletal muscle resists stretch by rapid binding of the second motor domain of myosin to actin

doi:10.1073/pnas.0707626104

Skeletal muscle resists stretch by rapid binding of the second motor domain of myosin to actin

*Laboratorio di Fisiologia, Dipartimento di Biologia Animale e Genetica, Università degli Studi di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Italy;
^‡Randall Division of Cell and Molecular Biophysics, King's College London, London SE1 1UL, United Kingdom;
^§European Synchrotron Radiation Facility, 38043 Grenoble Cedex, France; and
^†Centro di Ricerca e Sviluppo SOFT, Istituto Nazionale per la Fisica della Materia–Consiglio Nazionale delle Ricerche, Università di Roma “La Sapienza,” 00185 Rome, Italy

Edited by Hugh E. Huxley, Brandeis University, Waltham, MA, and approved October 24, 2007 (received for review August 13, 2007)

Abstract

A shortening muscle is a machine that converts metabolic energy into mechanical work, but, when a muscle is stretched, it acts as a brake, generating a high resistive force at low metabolic cost. The braking action of muscle can be activated with remarkable speed, as when the leg extensor muscles rapidly decelerate the body at the end of a jump. Here we used time-resolved x-ray and mechanical measurements on isolated muscle cells to elucidate the molecular basis of muscle braking and its rapid control. We show that a stretch of only 5 nm between each overlapping set of myosin and actin filaments in a muscle sarcomere is sufficient to double the number of myosin motors attached to actin within a few milliseconds. Each myosin molecule has two motor domains, only one of which is attached to actin during shortening or activation at constant length. A stretch strains the attached motor domain, and we propose that combined steric and mechanical coupling between the two domains promotes attachment of the second motor domain. This mechanism allows skeletal muscle to resist external stretch without increasing the force per motor and provides an answer to the longstanding question of the functional role of the dimeric structure of muscle myosin.

Footnotes

^¶To whom correspondence should be addressed. E-mail: vincenzo.lombardi{at}unifi.it
Author contributions: M.I. and V.L. designed research; E.B., M.R., R.E., M.L., Y.-B.S., T.N., P.P., and G.P. performed research; E.B., M.R., and G.P. analyzed data; and M.I. and V.L. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/cgi/content/full/0707626104/DC1.