Effect of Antibodies to Myosin Head Reveals Definite Difference between In Vitro Actin-Myosin Sliding and Muscle Contraction

Abstract

Mechanism of myosin head power stroke, responsible for muscle contraction, still remains to be a matter of debate and speculation. Despite considerable progress in studying actin filament sliding over myosin fixed on a glass surface, it is not clear whether the in vitro actin-myosin sliding takes place by a mechanism similar to contraction in muscle, consisting of three-dimensional myofilament lattice structures. To make this point clear, we prepared two different monoclonal antibodies, one directed to reactive lysine residue close to the myosin head converter region (anti-RLR antibody) while the other directed to two peptides of regulatory light chain in the myosin head lever arm region (anti-LD antibody). We compared the effect of these antibodies on in vitro actin- myosin sliding and contraction of skinned rabbit psoas muscle fibers with the following results: (1) anti-RLR antibody completely inhibited in vitro actin-myosin sliding without changing actin-activated myosin head ATPase activity, while it showed no effect on Ca2+-activated contraction of muscle fibers; (2) anti-LD antibody had no effect on in vitro actin-myosin sliding, but suppressed Ca2+-activated muscle fiber contraction without changing Mg-ATPase activity. These results indicate definite difference between in vitro actin-myosin sliding and muscle contraction.