Myosin properties

Alpha helices are sufficiently versatile to produce many very different classes of structures. In membrane-bound proteins, the regions inside the membranes are frequently a helices whose surfaces are covered by hydrophobic side chains suitable for the hydrophobic environment inside the membranes. Membrane-bound proteins are described in Chapter 12. Alpha helices are also frequently used to produce structural and motile proteins with various different properties and functions. These can be typical fibrous proteins such as keratin, which is present in skin, hair, and feathers, or parts of the cellular machinery such as fibrinogen or the muscle proteins myosin and dystrophin. These a-helical proteins will be discussed in Chapter 14. 

Fibrous proteins can serve as structural materials for the same reason that other polymers do they are long-chain molecules. By cross-linking, interleaving and intertwining the proper combination of individual long-chain molecules, bulk properties are obtained that can serve many different functions. Fibrous proteins are usually divided in three different groups dependent on the secondary structure of the individual molecules coiled-coil a helices present in keratin and myosin, the triple helix in collagen, and P sheets in amyloid fibers and silks. 

Certain proteins endow cells with unique capabilities for movement. Cell division, muscle contraction, and cell motility represent some of the ways in which cells execute motion. The contractile and motile proteins underlying these motions share a common property they are filamentous or polymerize to form filaments. Examples include actin and myosin, the filamentous proteins forming the contractile systems of cells, and tubulin, the major component of microtubules (the filaments involved in the mitotic spindle of cell division as well as in flagella and cilia). Another class of proteins involved in movement includes dynein and kinesin, so-called motor proteins that drive the movement of vesicles, granules, and organelles along microtubules serving as established cytoskeletal tracks. ... 

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... 

Inside the typical smooth muscle cell, the cytoplasmic filaments course around the nuclei filling most of the cytoplasm between the nuclei and the plasma membrane. There are two filamentous systems in the smooth muscle cell which run lengthwise through the cell. The first is the more intensively studied actin-myosin sliding filament system. This is the system to which a consensus of investigators attribute most of the active mechanical properties of smooth muscle. It will be discussed in detail below. The second system is the intermediate filament system which to an unknown degree runs in parallel to the actin-myosin system and whose functional role has not yet been completely agreed upon. The intermediate filaments are so named because their diameters are intermediate between those of myosin and actin. These very stable filaments are functionally associated with various protein cytoarchitectural structures, microtubular systems, and desmosomes. Various proteins may participate in the formation of intermediate filaments, e.g., vimentin. 

The analytic validity of an abstract parallel elastic component rests on an assumption. On the basis of its presumed separate physical basis, it is ordinarily taken that the resistance to stretch present at rest is still there during activation. In short, it is in parallel with the filaments which generate active force. This assumption is especially attractive since the actin-myosin system has no demonstrable resistance to stretch in skeletal muscle. However, one should keep in mind, for example, that in smooth muscle cells there is an intracellular filament system which runs in parallel with the actin-myosin system, the intermediate filament system composed of an entirely different set of proteins, (vimentin, desmin, etc.), whose mechanical properties are essentially unknown. Moreover, as already mentioned, different smooth muscles have different extracellular volumes and different kinds of filaments between the cells. 

The fifth was a molecular biologist, who smiled sweetly and pointed out that all the others had missed the point. The frog jumps because of the biochemical properties of its muscles. The muscles are largely composed of two interdigitated filamentous proteins, actin and myosin, and they contract because the protein filaments slide past each other. This property of the actin and myosin is dependent on the amino acid composition of the two proteins, and hence on chemical, and thus on physical properties. In the last analysis, the molecular biologist insisted, following James Watson, we are all nothing but subatomic particles. 

A basic structural property of protein filaments is polarity, that is, directionality. Almost all naturally occurring filaments are polar (e.g., F-actin, microtubules, TMV, and so on). The few exceptions are either bipolar, like myosin (Huxley, 1963 Squire, 1981), or nonpolar, like intermediate filaments (Herrmann and Aebi, 2004). One method of determining... 

Myocardial infarction, 3 710-711 and blood coagulation, 4 81 Myocardial pacemaker cells, 5 81 Myocardium, 5 79—80 Myoglobin, properties of standard, 3 836t Myosin, role in heart excitation and contraction coupling, 5 81 Myrac aldehyde, 2 278 24 485 Myrascone, 24 571... 

That myosin, a structural protein, also had enzyme activity as an ATPase, had been shown by Engelhardt and Ljubimova (1939-1941). ATP was now found to dissociate actomyosin producing a marked fall in viscosity the ATP was split to ADP and Pj. Contrasting properties of ATP in muscle systems were also observed. The rigor seen at postmortem occurred as ATP levels fell. The ATPase activity of myosin could be inhibited by mercurials (which block SH groups on cysteine) with ATPase blocked, ATP caused muscle fibers to relax (Weber and Portzehl, 1952). 

Figure 13.14 The effect of a change in the Ccf ion concentration on in vitro activity of myosin ATPase activity. At rest, the cytosolic ion concentration is about 0.1 xmoL/L, at which concentration myosin ATPase activity is low. Nervous stimulation of the fibre increases the cytosolic Ca ion concentration to about 2-10 xmol/L, with the half maximum change at about 0.5 xmol/L. Hence, on the basis of this property of myosin ATPase in the test tube, the in vivo change in Ca ion concentration should result in almost total activation of the enzyme. Approximately 50% activation of the myosin ATPase, when measured in vitro, occurs at about 0.5 xmol/L Ca ion concentration. A similar Ca ion concentration in the cytosol of the fibre results in 50% of the maximal force of contraction (Appendix 13.2).

The work that follows pertains primarily to actin networks. Many proteins within a cell are known to associate with actin. Among these are molecules which can initiate or terminate polymerization, intercalate with and cut chains, crosslink or bundle filaments, or induce network contraction (i.e., myosin) (A,11,12). The central concern of this paper is an exploration of the way that such molecular species interact to form complex networks. Ultimately we wish to elucidate the biophysical linkages between molecular properties and cellular function (like locomotion and shape differentiation) in which cytoskeletal structures are essential attributes. Here, however, we examine the iri vitro formation of cytoplasmic gels, with an emphasis on delineating quantitative assays for network constituents. Specific attention is given to gel volume assays, determinations of gelation times, and elasticity measurements. 

In vitro, the enzyme is able to catalyze crosslinking of whey proteins, soy proteins, wheat proteins, beef myosin, casein, and crude actomyosin (which is refined from mechanically deboned meat), improving functional properties such as the texture of food products [49-53], Bonds formed by transglutaminase exhibit a high resistance to proteolytic degradation [54],... 

These transglutaminase-catalysed reactions can be nsed to modify the functional properties of food proteins. Transglutaminase has been nsed to catalyze the cross-hnking of a nnmber of proteins, such as whey proteins, soy proteins, glnten, myosin and... 

Pires, E.M.V. Perry, S.V. Purification and properties of myosin light-chain kinase from fast skeletal muscle. Biochem. J., 167, 137-146 (1977)... 

Nagamoto, H. Yagi, K. Properties of myosin light chain kinase prepared from rabbit skeletal muscle by an improved method. J. Biochem., 95, 1119-1130 (1984)... 

Conti, M.A. Adelstein, R.S. Purification and properties of myosin light chain kinases. Methods Enzymol., 196, 34-47 (1991)... 

Leachman, S.A. Gallagher, P.J. Herring, B.P. McPhaul, M.J. Stull, J.T. Biochemical properties of chimeric skeletal and smooth muscle myosin light chain kinases. J. Biol. Chem., 267, 4930-4938 (1992)... 

Gallus gallus (two myosin heavy-chain kinases a Ca /calmodulin dependent and a Ca /calmodulin independent enzyme [10,11] the Ca /cal-modulin dependent heavy chain kinase has several properties in common with the family of type II calmodulin-dependent protein kinases [11]) [10,... 

Wang, Z.Y. Brzeska, H. Baines, EC. Korn, E.D. Properties of Acanthamoeba myosin I heavy chain kinase bound to phospholipid vesicles. J. Biol. Chem., 270, 27969-27976 (1995)... 

Under the electron microscope the myosin heads can sometimes be seen to be attached to the nearby thin actin filaments as crossbridges. When skeletal muscle is relaxed (not activated by a nerve impulse), the crossbridges are not attached, and the muscle can be stretched readily. The thin filaments are free to move past the thick filaments, and the muscle has some of the properties of a weak rubber band. However, when the muscle is activated and under tension, the crossbridges form more frequently. When ATP is exhausted (e.g., after death) muscle enters the state of rigor in which the crossbridges can be seen by electron... 

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