Nonlinear Cross-Bridge Elasticity and Post-Power-Stroke Events in Fast Skeletal Muscle Actomyosin

Abstract

Generation of force and movement by actomyosin cross-bridges is the molecular basis of muscle contraction, but generally accepted ideas about cross-bridge properties have recently been questioned. Of the utmost significance, evidence for nonlinear cross-bridge elasticity has been presented. We here investigate how this and other newly discovered or postulated phenomena would modify cross-bridge operation, with focus on post-power-stroke events. First, as an experimental basis, we present evidence for a hyperbolic [MgATP]-velocity relationship of heavy-meromyosin-propelled actin filaments in the in vitro motility assay using fast rabbit skeletal muscle myosin (28-29°C). As the hyperbolic [MgATP]-velocity relationship was not consistent with interhead cooperativity, we developed a cross-bridge model with independent myosin heads and strain-dependent interstate transition rates. The model, implemented with inclusion of MgATP-independent detachment from the rigor state, as suggested by previous single-molecule mechanics experiments, accounts well for the [MgATP]-velocity relationship if nonlinear cross-bridge elasticity is assumed, but not if linear cross-bridge elasticity is assumed. In addition, a better fit is obtained with load-independent than with load-dependent MgATP-induced detachment rate. We discuss our results in relation to previous data showing a nonhyperbolic [MgATP]-velocity relationship when actin filaments are propelled by myosin subfragment 1 or full-length myosin. We also consider the implications of our results for characterization of the cross-bridge elasticity in the filament lattice of muscle.

Maximum limit to the number of myosin II motors participating in processive sliding of actin

“…determined the minimum length of filament required to slide filament at maximum velocity. From their length and density estimates we calculate that ~90 HMM molecules are required to sustain processive sliding at saturating ATP; c. Persson et al 23,. determined 161 heads as the minimum number of myosin molecules required for maximum sliding velocity.…”

“…Using this duty ratio value, we can calculate the number of strongly bound molecules N a to be ~1 at N min and ~2 at N max at saturating ATP concentrations. There are recent reports1223 suggesting that N a could be as high as 6–8 during unloaded shortening. The number of simultaneously attached heads during unloaded shortening raises many other questions such as how τ on depends only on ATP concentration.…”

Actomyosin based contraction: one mechanokinetic model from single molecules to muscle?

“…For instance, like several previous large-ensemble muscle models (e.g. (Duke 1999; Eisenberg et al 1980; Månsson 2010; Pate and Cooke 1989; Persson et al 2013), the weakly bound actomyosin state has not been explicitly included. Neither has the phosphate release step been separated from the main force-generating event (“the power-stroke”).…”

Ensemble velocity of non-processive molecular motors with multiple chemical states

“…Finally, neglecting reverse transitions in the cycle is valid if there is no ADP and phosphate in solution (which would lead to reversal of product release steps) and, again, if the forces are too low for significant mechanically induced reverse transitions. In a recent paper, Persson et al [27] discuss the role of the aforementioned effects in a motility assay for rather stiffly anchored heads (2.8 pN nm…”

“…It is frequently assumed to follow a Michaelis-Menten-like dependence, as would be the case with processive motors, even though there is no reason why it should have that form. Some experimental studies on myosins show no deviation from the Michaelis-Menten shape [22][23][24][25][26][27], whereas others show minor, but significant, deviation [28]. Axonemal dyneins also largely follow the Michaelis-Menten dependence [29,30].…”