In Section 17.3 we discussed how actin polymerization nucleated by the Arp2/3 complex can be harnessed to do work, as in the movement of vesicles during endocytosis, at the leading edges of motile cells, and the propulsion of the Listeria bacterium across the eukaryotic cell. In addition to this actin polymerization–based motility, cells have a large family of motor proteins called myosins that can move along actin filaments. The first myosin discovered, myosin II, was isolated from skeletal muscle. For a long time, biologists thought that this was the only type of myosin found in nature. However, they then discovered other types of myosins and began to ask how many different functional classes might exist. Today we know that there are several different classes of myosins, in addition to the myosin II of skeletal muscle, that move along actin. Indeed, with the discovery and analysis of all these microfilament-based motors and the corresponding microtubule-based motors described in the next chapter, our former relatively static view of cells has been replaced with the realization that the cytoplasm is incredibly dynamic—like an organized but busy freeway system with motors busily ferrying components around.

Myosins have the amazing ability to convert the energy released by ATP hydrolysis into mechanical work (movement along actin). All myosins convert energy from ATP hydrolysis into work, yet different myosins perform very different types of functions. For example, many molecules of myosin II pull together on actin filaments to bring about muscle contraction, whereas myosin V binds to vesicular cargo to transport it along actin filaments. The other classes of myosin provide a myriad of functions, from moving organelles around cells to contributing to cell migration.

To begin to understand myosins, we first discuss their general organization. Armed with this information, we explore the diversity of myosin classes in different organisms and describe in more detail some of those that are common in eukaryotes. To understand how the diverse functions of these myosin classes can be accommodated by one type of motor mechanism, we investigate the basic mechanism that converts the energy released by ATP hydrolysis into work, and then see how this mechanism is modified to tailor the properties of specific myosin classes to their specific functions.

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