| Popis: |
In an in vitro motility assay, actin sliding velocities, V, exhibit a multi-phasic response to increasing myosin densities, N. At low N, V increases with increasing N reaching a peak value before decreasing. The mechanism underlying this biphasic response remains unclear. Here we propose a simple model in which at low myosin numbers, the working step of a myosin head, which occurs upon actin-myosin attachment, is free to displace an actin filament, resulting in a V that is attachment limited. At high myosin numbers, actin-bound myosin heads impose a load against which the working step of a myosin head cannot move, resulting in a detachment limited V. We test this hypothesis by measuring the effects of perturbations to attachment- and detachment-kinetics on the N-dependence of V. The resulting data are accurately fit to our simple model. These results provide insights into the basic mechanism of muscle contraction with implications for how in general multiple motors work together to move cargo. Specifically, these results suggest that V measured in conventional in vitro motility assays is influenced by both attachment and detachment kinetics. |
