Definite Difference Between In Vitro Actin-Myosin Sliding and Muscle Contraction Revealed by the Effect of Antibody to Myosin Head Converter Domain

Abstract

Myofilament sliding in muscle is believed to result from rotation of the myosin head catalytic domain (CAD) around the converter domain (CD). To explore the validity of this mechanism, we compared the effect of antibody to myosin head converter domain (IgG, anti-CD antibody) between in vitro actin-myosin sliding and muscle fiber contraction. In agreement with the expectation that binding of massive antibody to the CD impairs rotation of the CAD around the CD, the ATP-dependent sliding of actin filaments over myosin heads on a glass surface was inhibited by the antibody (0.14mg/ml). Meanwhile, the antibody (up to 1.5mg/ml) showed no appreciable effect on the actin-activated myosin head ATPase activity, indicating that the antibody has no effect on the ATPase activity in the CAD. Unexpectedly, the antibody (up to 3mg/ml) showed no appreciable effect on the maximum Ca2+-activated isometric force, the maximum shortening velocity, and the Mg-activated ATPase activity in glycerol-extracted rabbit psoas muscle fibers. The possibility that the antibody does not diffuse into muscle fibers can be excluded by our published results that other antibodies readily inhibit muscle fiber contraction. These findings therefore suggest that the antibody binding to the myosin head CD does not impair performance of myosin heads producing force and motion in muscle fibers.