Unfolding of titin domains explains the viscoelastic behavior of skeletal myofibrils

Biophys J. 2001 Mar;80(3):1442-51. doi: 10.1016/S0006-3495(01)76116-4.

Abstract

The elastic section of the giant muscle protein titin contains many immunoglobulin-like domains, which have been shown by single-molecule mechanical studies to unfold and refold upon stretch-release. Here we asked whether the mechanical properties of Ig domains and/or other titin regions could be responsible for the viscoelasticity of nonactivated skeletal-muscle sarcomeres, particularly for stress relaxation and force hysteresis. We show that isolated psoas myofibrils respond to a stretch-hold protocol with a characteristic force decay that becomes more pronounced following stretch to above 2.6-microm sarcomere length. The force decay was readily reproducible by a Monte Carlo simulation taking into account both the kinetics of Ig-domain unfolding and the worm-like-chain model of entropic elasticity used to describe titin's elastic behavior. The modeling indicated that the force decay is explainable by the unfolding of only a very small number of Ig domains per titin molecule. The simulation also predicted that a unique sequence in titin, the PEVK domain, may undergo minor structural changes during sarcomere extension. Myofibrils subjected to 1-Hz cycles of stretch-release exhibited distinct hysteresis that persisted during repetitive measurements. Quick stretch-release protocols, in which variable pauses were introduced after the release, revealed a two-exponential time course of hysteresis recovery. The rate constants of recovery compared well with the refolding rates of Ig-like or fibronectin-like domains measured by single-protein mechanical analysis. These findings suggest that in the sarcomere, titin's Ig-domain regions may act as entropic springs capable of adjusting their contour length in response to a stretch.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Amino Acid Sequence
  • Animals
  • Connectin
  • Elasticity
  • Kinetics
  • Models, Biological
  • Monte Carlo Method
  • Muscle Contraction / physiology
  • Muscle Proteins / chemistry*
  • Muscle Proteins / physiology*
  • Muscle, Skeletal / physiology*
  • Myofibrils / physiology*
  • Myofibrils / ultrastructure
  • Protein Folding
  • Protein Kinases / chemistry*
  • Protein Kinases / physiology*
  • Rats
  • Stress, Mechanical
  • Time Factors
  • Viscosity

Substances

  • Connectin
  • Muscle Proteins
  • Protein Kinases