Cytoplasmic streaming

▲ FIGURE 19-16 Functions of myosin tail domains.

(a) Myosin I and myosin V are localized to cellular membranes by undetermined sites in their tail domains. As a result, these myosins are associated with intracellular membrane vesicles or the cytoplasmic face of the plasma membrane. (b) In contrast, the coiled-coil tail domains of myosin II molecules pack side by side, forming a thick filament from which the heads project. In a skeletal muscle, the thick filament is bipolar. Heads are at the ends of the thick filament and are separated by a bare zone, which consists of the side-by-side tails.

in conjunction with the neck domain couple ATP hydrolysis to movement of a myosin molecule along an actin filament via a common mechanism involving cyclical binding and hydrolysis of ATP and attachment/detachment of myosin and actin (see Figures 3-24 and 3-25).

The role of a particular myosin in vivo is related to its tail domain. For example, the tail domains of myosins I, V, VI, and XI bind the plasma membrane or the membranes of intracellular organelles; as a result, these molecules have membrane-related activities (Figure 19-16a). In contrast, the coiled-coil tail domains of myosin II dimers associate to form bipolar thick filaments in which the heads are located at both ends of the filament and are separated by a central bare zone devoid of heads (Figure 19-16b). The close packing of myosin molecules into thick filaments, which are a critical part of the contractile apparatus in skeletal muscle, allows many myosin head domains to interact simultaneously with actin filaments.

The number and type of light chains bound in the neck region vary among the different myosins (see Table 19-3). The light chains of myosin I and myosin V are calmodulin, a Ca2+-binding regulatory subunit in many intracellular enzymes (see Figure 3-28). Myosin II contains two different light chains called essential and regulatory light chains (see Figure 3-24); both are Ca2+-binding proteins but differ from calmodulin in their Ca2+-binding properties. All myosins are regulated in some way by Ca2 + ; however, because of the differences in their light chains, the different myosins exhibit different responses to Ca2+ signals in the cell.

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