We now examine the function of different myosin motor proteins in nonmuscle cells and muscle. As discussed in Chapter 3, interactions between myosin II and actin filaments are responsible for muscle contraction. At first, scientists thought that most cell movements were caused by a contrac tile mechanism similar to the sliding of actin and myosin filaments in muscle cells. This idea was based on several properties of at least some nonmuscle cells: the ability of cytosolic extracts to undergo contractile-like movements, the presence of actin and myosin II, and the existence of structures similar to muscle sarcomeres. However, the results of later biochemical studies led to the extraction of "unusual" forms of myosin that differed from myosin II in structure, location, and enzymatic properties.
As biologists investigated various types of cell movements, it became clear that myosin II mediates only a few types, such as cytokinesis and muscle contraction. Other types of cell movements, including vesicle transport, membrane extension, and the movement of chromosomes, require either other myosin isoforms, other motor proteins such as kinesin or dynein, or actin polymerization. In this section, we first consider the properties of various myosins and some of their functions in nonmuscle cells. Contraction, the special form of movement resulting from the interaction of actin and myosin II, is most highly evolved in skeletal muscle cells. However, somewhat similar contractile events entailing less organized systems are found in nonmuscle cells. After reviewing the highly ordered structure of actin and myosin filaments in the sarcomere of skeletal muscle, we describe the primary mechanisms for regulating contraction.
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