■ All myosin isoforms can interact with actin filaments through their head domains, but their cellular roles differ, depending on their tail domains (see Table 19-3).
■ Movement of actin filaments by myosin can be directly monitored in the sliding-filament assay (see Figure 19-17).
■ Myosins I, V, and VI power intracellular translocation of some membrane-limited vesicles along actin filaments. A similar process is responsible for cytoplasmic streaming, which is probably mediated by myosin XI, one of the fastest moving myosins (see Figure 19-19).
■ In nonmuscle cells, actin filaments and myosin II form contractile bundles that have a primitive sarcomere-like organization. Common examples are the circumferential belt present in epithelial cells and the stress fibers in cells cultured on plastic or glass surfaces; in the latter case, they may be an artifact. Both structures function in cell adhesion.
■ The contractile ring, a transient bundle of actin and myosin II, forms in a dividing cell and pinches the cell into two halves in cytokinesis.
■ In skeletal muscle cells, actin thin filaments and myosin thick filaments are organized into highly ordered structures, called sarcomeres (see Figure 19-22). The (+) end of the thin filaments is attached to the Z disk, the demarcation between adjacent sarcomeres.
■ During skeletal muscle contraction, myosin heads at each end of a thick filament walk along thin filaments toward the Z disks bounding a sarcomere. The force generated by myosin movement pulls the thin filaments toward the center of the sarcomere, shortening its length (see Figure 19-23).
■ The rapid rise in cytosolic Ca2+ induced by nerve stimulation of a skeletal muscle changes the interaction between actin filaments and tropomyosin, exposing the myosin-binding sites and thus permitting contraction to occur (see Figure 19-24).
■ Contraction of smooth muscle and nonmuscle cells is triggered by phosphorylation of the myosin regulatory light chains either by myosin LC kinase, in response to a rise in cytosolic Ca2 + , or by Rho kinase, in response to external signals (see Figure 19-25).
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