The essential light chain is required for full force production by skeletal muscle myosin.

Myosin, a molecular motor that is responsible for muscle contraction, is composed of two heavy chains each with two light chains. The crystal structure of subfragment 1 indicates that both the regulatory light chains (RLCs) and the essential light chains (ELCs) stabilize an extended alpha-helical segment of the heavy chain. It has recently been shown in a motility assay that removal of either light chain markedly reduces actin filament sliding velocity without a significant loss in actin-activated ATPase activity. Here we demonstrate by single actin filament force measurements that RLC removal has little effect on isometric force, whereas ELC removal reduces isometric force by over 50%. These data are interpreted with a simple mechanical model where subfragment 1 behaves as a torque motor whose leyer arm length is sensitive to light-chain removal. Although the effect of removing RLCs fits within the confines of this model, altered crossbridge kinetics, as reflected in a reduced unloaded duty cycle, probably contributes to the reduced velocity and force production of ELC-deficient myosins.