The Form and Function of Actin
Even if actin were only found in muscle, it would be an interesting protein, but it is found in abundance in all cells of the body. For example, it is implicated in pseudopod and filopod formation, in ruffling, in the production of the acrosomal process in invertebrate sperm, in cytokinesis, in the construction of the cytoskeleton, and in specialized extensions such as microvilli and sterocilia. These systems can be quite difficult to attack experimentally and consequently much of our knowledge of actin and motility has come from the study of muscle, a highly periodic structure. In striated muscle, actin, together with troponin and tropomyosin, regulates the attachment of myosin cross-bridges, activates the myosin ATPase, and provides part of the mechanical framework necessary to couple forces from neighboring filaments. Myosin is also found in nonmuscle cells but in much lesser amounts than in muscle. Some but not all of the motile events in nonmuscle cells require myosin and, for these events, it is assumed that muscle serves as a model system. Pseudo-pod formation and cytokinesis are two motile events thought to be driven by an actomyosin contraction.* For other motile events the involvement of myosin has either been ruled out or at least seems improbable: the extension of the acrosomal process in the sperm of Thyone and Limulus, for example, involves the polymerization of actin (Thyone; Tilney, 1975a) and a change in twist of the actin filaments (Limulus; DeRosier et al.,1982). There is no convincing evidence that myosin is present in these sperm and, moreover, the direction of motion is opposite to that produced by myosin. Thus, the function of actin in muscle contraction does not fully describe its role in nonmuscle motility.
KeywordsHair Cell Actin Filament Thin Filament Layer Line Actin Bundle
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