Abstract
Masticatory (superfast) myosin is specifically expressed in jaw-closing muscles and is found in carnivorous lower vertebrates and several orders of mammals. In humans, this myosin is not expressed in those whose masticatory myosin heavy chain gene is defective due to a frame-shift deletion. This myosin has a very ancient phylogenetic origin, and is considered as a distinct subclass (labeled as II M) of vertebrate striated myosins, its heavy chain exhibiting a less than 70% sequence homology to other known striated muscle myosin heavy chains.ATPase activity of masticatory myosin is higher than that of fast myosin, and fibers expressing masticatory myosin exhibit a higher maximal tension than fast or slow fibers and intermediate shortening velocity between those of fast and slow limb fibers. These characteristics are considered very appropriate in carnivores for catching and grasping prey, and in folivores for the mastication of tough vegetable matter.Regarding the structural aspect, myosin heads of masticatory fibers are reported to be mobile and protrude more from the thick filaments toward actin by X-ray diffraction experiments at the synchrotron facility. Electron micrographs of masticatory fibres showed that the sarcomere structure of fibers is disordered and the M-line is barely discernible. These structural characteristics may explain the unique contracting properties.
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