Myosin structure and function

Adelstein RS, Sellers JR (1996) Myosin structure and function. In Barany M (ed) Biochemistry of Smooth Muscle Contraction. Academic Press Inc. San Diego, California, pp 3-19... 

Warrick HM, Spudich JA (1987) Myosin structure and function in cell motility. Annu Rev Cell Biol 3 379421... 

Citi S. and Kendrick-Jones J. 1987. Regulation of non-muscle myosin structure and function. Bioessays 1 155-159. 

In striated muscle, there are two other proteins that are minor in terms of their mass but important in terms of their function. Tropomyosin is a fibrous molecule that consists of two chains, alpha and beta, that attach to F-actin in the groove between its filaments (Figure 49-3). Tropomyosin is present in all muscular and muscle-fike structures. The troponin complex is unique to striated muscle and consists of three polypeptides. Troponin T (TpT) binds to tropomyosin as well as to the other two troponin components. Troponin I (Tpl) inhibits the F-actin-myosin interaction and also binds to the other components of troponin. Troponin C (TpC) is a calcium-binding polypeptide that is structurally and functionally analogous to calmodulin, an important calcium-binding protein widely distributed in nature. Four molecules of calcium ion are bound per molecule of troponin C or calmodulin, and both molecules have a molecular mass of 17 kDa. 

Different classes of myosin are important for neuronal function. Myosins are remarkably diverse in structure and function. To date, 15 subfamilies of myosin have been defined by sequence homologies [41]. The brain is an abundant source of nonmuscle myosins and one of the earliest studied. Despite their abundance and variety, the roles of myosins in neural tissues have only recently begun to be defined [40]. 

Bailin, G. Structure and function of a calmodulin-dependent smooth muscle myosin light chain kinase. Experientia, 40, 1185-1188 (1984)... 

G. J. Lutz and R. L. Lieber Skeletal muscle myosin II structure and function. Exercise Sport Science Review 27,63 (1999). 

To further illustrate the properties of motor proteins, we consider myosin II, which moves along actln filaments in muscle cells during contraction. Other types of myosin can transport vesicles along actin filaments in the cytoskeleton. Myosin II and other members of the myosin superfamily are composed of one or two heavy chains and several light chains. The heavy chains are organized into three structurally and functionally different types of domains (Figure 3-24a). 

Stossel TP (1993) On the crawling of animal cells. Science 260 10861094 Suzuki H, Onishi H, Takahashi K, Watanabe S (1978) Structure and function of chicken gizzard myosin. J Biochem 84 15291542... 

Proteins constitute a universally essential class of macromolecules which perform a wide range of specialized functions in living systems. Examples of these functions include the enzymatic catalysis of metabolic pathways, hormonal signaling in the endocrine system, and antibody mediated defense in the immune system. Proteins also perform critical structural roles, for example as the muscle proteins actin and myosin. The study of protein structure and function is therefore essential to our understanding of life and the advancement of medicine [1]. 

The above expectations that describe the structure and function of the myosin II motor suggest a dominant role for the hydrophobic and elastic consilient mechanisms in the function of this representative motor for producing motion in living organisms. 

Whole muscles are composed of groups of muscle fibers, which vary from 1 to 400 mm in length and from 10 to 60 tm in diameter. Muscle fibers, in turn, are composed of groups of myofibrils (Fig. 6.2b), and each myofibril is a series of sarcomeres added end to end (Fig. 6.2c). The sarcomere is both the structural and functional unit of skeletal muscle. During contraction, the sarcomeres are shortened to about 70 percent of their uncontracted, resting length. Electron microscopy and biochemical analysis have shown that each sarcomere contains two types of filaments thick filaments, composed of myosin, and thin filaments, containing actin (Fig. 6.2d). Near the center of the sarcomere, thin filaments overlap with thick filaments to form the AI zone (Fig. 6.2c). 

R26. Applications of phosphorus-31 nuclear magnetic resonance to studies of protein structure and function Sykes, B. D. Can. J. Biochem. Cell Biol. 1983, 61, 155-164. A review with 32 references, emphasizing enzyme mechanistic studies and including alkaline phosphatase, glycogen phosphorylase, actin, and myosin. 

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. 

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. 

Organization into macromolecular structures. There are no apparent templates necessary for the assembly of muscle filaments. The association of the component proteins in vitro is spontaneous, stable, and relatively quick. Filaments will form in vitro from the myosins or actins from all three kinds of muscle. Yet in vitro smooth muscle myosin filaments are found to be stable only in solutions somewhat different from in vivo conditions. The organizing principles which govern the assembly of myosin filaments in smooth muscle are not well understood. It is clear, however, a filament is a sturdy structure and that individual myosin molecules go in and out of filaments whose structure remains in a functional steady-state. As described above, the crossbridges sticking out of one side of a smooth muscle myosin filament are all oriented and presumably all pull on the actin filament in one direction along the filament axis, while on the other side the crossbridges all point and pull in the opposite direction. The complement of minor proteins involved in the structure of the smooth muscle myosin filament is unknown, albeit not the same as that of skeletal muscle since C-protein and M-protein are absent. 

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