Myosin molecular structure

Smooth muscles have molecular structures similar to those in striated muscle, but the sarcomeres are not aligned so as to generate the striated appearance. Smooth muscles contain a-actinin and tropomyosin molecules, as do skeletal muscles. They do not have the troponin system, and the fight chains of smooth muscle myosin molecules differ from those of striated muscle myosin. Regulation of smooth muscle contraction is myosin-based, unlike striated muscle, which is actin-based. However, like striated muscle, smooth muscle contraction is regulated by Ca. ... 

Obermann, W. M., Gautel, M., Weber, K., and Furst, D. O. (1997). Molecular structure of the sarcomeric M band Mapping of titin and myosin binding domains in myomesin and the identification of a potential regulatory phosphorylation site in myomesin. EMBOJ. 16, 211-220. 

Squire, J. M. (1973). General model of myosin filament structure. 3. Molecular packing arrangements in myosin filaments./ Mol. Biol. 77, 291-323. 

The Ca2+ released from the endoplasmic reticulum by IP3 may perform a number of functions. The metabolically most important action of Ca2+, however, is to combine with a very ubiquitous protein called calmodulin. This protein has a molecular weight of about 17,500 and is in many ways similar to troponin. It has four Ca2+ binding sites and constitutes one of several subunits of several enzyme systems. Thus, as cellular [Ca2+] rises, the calmodulin subunit binds Ca2+. The result is that it changes its conformation to that of an a helix and thereby affects the catalytic activity of other constituent subunits. For instance, calmodulin is a component of myosin kinase, which phosphorylates one of the subunits of myosin. The structure of calmodulin is shown in Figure 16.22. 

Smooth muscle myosin has distinctive characteristics that may form the basis for many of the unique functional properties exhibited by smooth muscle tissues. The following section will first review the molecular structure of smooth muscle myosin and the functional implications of its distinctive characteristics. This will be followed by a discussion of the regulation of the assembly of myosin into thick filaments, and the molecular organization of the thick filaments of smooth muscle tissues. 

Molecular structure of the smooth muscle myosin molecule... 

Actin occurs in multiple isoforms within individual smooth muscle cells. All of these isoforms are capable of forming filamentous actin that can interact with myosin to generate force. Although the functional importance of the different actin isoforms is presently unclear, there is evidence to suggest that they may serve to "customize actin filaments to serve different functional roles within the cell by determining its interactions with different binding proteins. The first part of this section will review the molecular structure of the thin filament. The structure of actin and the relationship to the other protein constituents of the thin filament to actin... 

Function of Myosin Molecular Motor — A crossbridge-cycle model for the action of myosin on actin has been widely accepted since 1957 [19,32,33]. Since the atomic structures of actin monomer... 

Rayment, 1., et al. Three-dimensional structure of myosin subfragment-1 a molecular motor. Science 261 5-58, 1996. 

Fisher, A., Smith, C., Thoden, J., et al., 1995. Structural studies of myosin nncleotide complexes A revised model for die molecular basis of muscle contraction. Biophysical Journal... 

Even though dynein, kinesin, and myosin serve similar ATPase-dependent chemomechanical functions and have structural similarities, they do not appear to be related to each other in molecular terms. Their similarity lies in the overall shape of the molecule, which is composed of a pair of globular heads that bind microtubules and a fan-shaped tail piece (not present in myosin) that is suspected to carry the attachment site for membranous vesicles and other cytoplasmic components transported by MT. The cytoplasmic and axonemal dyneins are similar in structure (Hirokawa et al., 1989 Holzbaur and Vallee, 1994). Current studies on mutant phenotypes are likely to lead to a better understanding of the cellular roles of molecular motor proteins and their mechanisms of action (Endow and Titus, 1992). 

The myosin head has long been shown to induce, even in low ionic strength buffers, polymerization of G-actin into decorated F-actin-S i filaments that exhibit the classical arrowhead structure (Miller et al., 1988 and older references therein). However, to date, the molecular mechanism of this polymerization process remains unknown. 

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. 

Rayment, L, Rypniewshi, W.R., Schmidt-Base, K., Smith, R., Tomchick, D.R., Benning, M.M., Winkleman, D.A., Wesenberg, G Holden, H.M. (1993a). Three-dimensional structure of myosin subfiagment-1 A molecular motor. Science 261, 50-58. 

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. 

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