Duty ratio of cooperative molecular motors

Motor proteins are specialized molecules that convert chemical energy into useful mechanical work and govern many important biological processes. They bind to actin filaments and microtubules and use them as tracks, on which they propagate in order to transport different types of cargo across the cell. Among these molecules we find myosin II motors that interact with elastic actin filaments. Myosin II are non processive motors that exhibit a low duty ratio, which means that in order to work effectively they need to cooperate with each other. The collective work of myosin II motors on elastic actin tracks suggests that these filaments may be subjected to significant tensile stresses. It has been found that the stress applied on the actin may induce an indirect crosstalk between the motors in order to diminish the elastic energy, which is expressed by changes in their binding / unbinding statistics. This type of indirect communication between motors via the elastic track, which has been termed the elasticity mediated crosstalk (EMC) effect, may significantly affect the motors’ effective processivity. In this thesis, we use a statistical mechanical analysis and Monte Carlo computer simulations, to explore the magnitude and role of the EMC effect in two types of systems where the collective action of myosin II motors on actin filament is present: (i) gliding motility assays and (ii) muscle contraction. In motility assays, we find that the EMC effect has a small impact on the collective action of motors, and that the duty ratio of the motors remains effectively unchanged. In muscle contraction, where the actin filament is subjected to an external force opposing the action of the motors, we find that the EMC effect leads to a non uniform attachment probability along the actin track. Such duty ratio variations between motors may have a serious negative influence on the ability of the motors to perform effectively. Nevertheless, we find that this feature becomes significant only when the size of the system is larger than the size of the sarcomere (which is the basic contractile unit of the muscle cell). The EMC effect may, thus, serve as an explanation for the specific dimensions of the sarcomere, which are found to be essentially identical across different vertebrate species.