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 crysta l structure of subfragment 1 indicates that both the regulatory light chains (RLCs) and the essential light chains (ELCs) stabilize an exten ded alpha-helical segment of the heavy chain. It has recently been sho wn in a motility assay that removal of either light chain markedly red uces actin filament sliding velocity without a significant loss in act in-activated ATPase activity. Here we demonstrate by single actin fila ment force measurements that RLC removal has little effect on isometri c force, whereas ELC removal reduces isometric force by over 50%. Thes e data are interpreted with a simple mechanical model where subfragmen t 1 behaves as a torque motor whose lever arm length is sensitive to l ight chain removal. Although the effect of removing RLCs fits within t he 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.