Structure and function of superfast myosin expressed in carnivoral masticatory muscle.

Abstract

Masticatory (superfast) myosin is expressed in jaw muscles of eutherian and marsupial mammals including carnivores. It develops high maximal force with high ATPase cycling rate and exhibits moderately fast contraction velocity. These characteristics were hypothesized to be due to the protrusion of its head from the thick filament backbone toward thin filaments so as to increase attachment rate to actin. In this study, we carried out an X-ray diffraction experiment at SPring8 (BL45XU) on relaxed skinned fibers of dog masseter expressing the masticatory myosin. Compared with fast type muscle (tibialis anterior), masseter exhibited significantly wider myofilament spacing, roughly comparable 1,1/1,0 ratio, and weaker myosin layer lines. These suggested that the heads of masticatory myosin are reaching close to the thin filaments relieved from restriction of the backbone as expected from the hypothesis. As a functional test of the hypothesis, we performed tension and stiffness measurements. In the absence of calcium, slight decrease in ionic strength induced tension development in the masseter fibers. It suggested that their myosin readily interact to actin consistent with the hypothesis. [Jpn J Physiol 55 Suppl:S121 (2005)]