Non-Linear Elasticity of Skeletal Myosins is Essential for Collective Force Generation in Muscle

Abstract

Muscle contraction occurs through the rotation of the myosin heads pulling actin filaments past myosin filaments. In order to achieve contractions efficiently, it is crucial for myosins to generate force collectively and to minimize the interference between motors. In the previous studies, it was suggested that the elasticity of myosin is linear so that myosins generate the large drag force opposing to muscle contraction if negatively strained. However, none of the previous studies has investigated the elasticity of single myosin explicitly in the negative strain region. Therefore, we investigated the elasticity of single skeletal myosins in both the positive and negative strain directions. In the absence of ATP, single myosins embedded in synthetic myosin-rod filaments were tightly bound to actin filaments and were stretched and shortened repeatedly by oscillating the two trapped beads on the both ends of acting filament manipulated by optical tweezers. The elasticity of myosins was characterized by obtaining the force-displacement curves, where forces on myosin heads were estimated by measuring the displacement of two beads, and displacements of myosin heads were obtained by tracking the position of quantum dots attached to the actin filament. We found that the elasticity is non-linear, in which stiffness is high (∼2.8 pN/nm) and low (∼0.02 pN/nm) in the positive and negative strain regions. The non-linear elasticity ensures high force generation with a small stretching of the elastic portion, and minimizing the drag force of negatively-strained myosins. Furthermore, the estimates of the actin sliding distance by the non-linear elasticity model were more consistent with experimental results observed in 20μM ATP than those by the linear elasticity model. Therefore, the non-linear elasticity might be an essential mechanical property of single myosin designed for the collective force generation in muscle.