Myosin physiological properties

Nonmuscle/smooth muscle myosins-Il are structurally similar to striated muscle myosin-II, but they have slower rates of ATP hydrolysis than do their striated muscle counterparts. Nonmuscle/smooth muscle myosin-II is also regulated differently than striated muscle myosin-II. Nonmuscle myosin-II is divided into the invertebrate and vertebrate branches (Cheney et al., 1993). This group is ubiquitous because it is present in most lower organisms, such as slime molds, amoeba, sea urchins, etc., and in virtually all mammalian nonmuscle cells. Smooth muscle myosin-II is also somewhat heterogeneous in that at least three separate forms of smooth muscle heavy chains, with molecular weights of 196,000, 200,000, and 204,000 have been identified (Kawamoto and Adelstein, 1987). The physiological properties of these separate myosin heavy chains are not yet known. 

The recent explosion in the discovery of new myosin genes has led to the idea that myosins from different classes probably co-exist in cells. This has raised the obvious question as to what functions these myosins subserve within cells. Up to now, only the genes have been cloned for many of the 35 unique myosins. But this is not a question that can be answered solely by cloning rather, it is absolutely imperative to biochemically characterize these proteins if we are to understand their physiological properties. One way to do this is to express the entire protein or parts of the proteins in bacteria, yeast, or insect cells, and to then purify and characterize... 

MLCK plays a central role in the initiation of smooth muscle contraction and many nonmuscle motile processes owing to its Ca2+/calmodulin-dependent phosphorylation of myosin RLC. Physiological experiments with rapid, synchronous activation of smooth muscle cells demonstrate that the significant 500-ms latency for RLC phosphorylation and force development is due to the time required for increases in cytosolic Ca + concentration and activation of MLCK. Once activated, MLCK rapidly (1 s i) phosphorylates RLC in a random mechanism leading to the rapid attachment of cross-bridges and subsequent force development. Although the general properties of this cellu-... 

The specific system that we use to explore these questions is the conventional myosin, also termed myosin 11, which plays key physiological functions in muscle contraction and cell division. It is an ideal system for in-depth theoretical and computational analysis because its structural and kinetic properties have been characterized by a large body of diverse experimental techniques." For example, at the time our research was initiated, myosin 11 was one of the few motor systems for which high-resolution x-ray structures are available for multiple functional states " since then, multiple high-resolution x-ray structures have also been obtained for myosin V and VI, two other widely studied members of the myosin superfamily that are more processive in nature compared to myosin 11. The functional cycle of myosin 11 is best described by the celebrated Lymn-Taylor schane (Figure 2.1a)," in which... 

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