Striated muscle myosin molecules

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. ... 

Thin filaments (8 nm diam.) consist of polymerized Actin (see) (30(M00 actin molecules per 1.0 pm length), each chain being accompanied by threadlike tropomyosin molecules and, in striated muscle, globular molecules of troponin. In smooth muscle troponin is replaced by caldesmon. The actin and myosin are responsible for muscle contraction, while the tropomyosin and troponin or caldesmon are regulatory proteins... 

Of the several kinase activities which are important in smooth muscle, myosin light chain kinase, MLCK, is the one responsible for activation of the actin-myosin system to in vivo levels. MLCK is present in the other nonmuscle cell types which have the actin-myosin contractile system and all of these are probably activated in a manner similar to smooth muscle rather than by way of the Ca -troponin mechanism of striated muscle. MLCK from smooth muscle is about 130 kDa and is rather variable in shape. It is present in smooth muscle in 1-4 pM concentrations and binds with an equally high affinity to both myosin and actin. Thus, most MLCK molecules are bound to actin. Myosin light chain serine-19 is the primary target of smooth muscle myosin light chain kinase. 

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. 

The calcium mediated contraction of smooth muscle, which unlike striated muscle does not contain troponin, is quite different and requires a particular calcium-binding protein called calmodulin. Calmodulin (CM) is a widely distributed regulatory protein able to bind, with high affinity, four Ca2+ per protein molecule. The calcium—calmodulin (CaCM) complex associates with, and activates, regulatory proteins, usually enzymes, in many different cell types in smooth muscle the target regulatory proteins are caldesmon (CDM) and the enzyme myosin light chain kinase (MLCK). As described below, CaCM impacts on both actin and myosin filaments. 

Fig. 28. Classification of crossbridge configurations in myosin filaments in different muscles. In each case, the axial separation is 143-145 A and the lateral separation is 120-150 A. There are three main classes (A) Class I, where the interaction is between heads of the same molecule as in vertebrate striated muscles (B) Class II, where interaction occurs between heads of adjacent myosin molecules in the same crown, as seen in insect (Lethocerus) flight muscles and (C) Glass III, where the interaction appears to be between heads in different crowns, as seen in tarantula and Limulus.

Organization of myosin in striated-muscle thick filaments. Filament formation begins with tail-to-tail (antiparallel) binding of myosin molecules, with subsequent parallel binding of myosin molecules to the ends of the initial nucleus, leaving the central clear zone. There are approximately 500 myosin molecules per striated thick filament. 

Regulation by myosin phosphorylation, which is of particular importance for smooth muscle, is a feature of the less specialized actomyosin contractile systems. In striated muscle it would appear that light chain phosphorylation has been relegated to a modulatory role in the cross-bridge cycle, and response to stimulation has been accelerated by the evolution of the troponin system located in the I filament. The consequence is that the rapid binding of calcium to one molecule of troponin C renders seven actin molecules available to interact with as many heads of myosin molecules as they can accommodate. In smooth muscle, binding of cal-... 

Electron Microscopic Structure of the Muscle Cell. The usual cell components are of little significance in striated muscles. The cells are filled with myofibrils which produce the familiar striped. pattern. Electron optical studies reveal that the A-bands consist of thick, thread-like molecules, presumably myosin. The space between these molecules is occupied by thin filaments which continue into the isotropic area (I-band) and end at the Z-line. The latter consists of actin and tropomyosin (cf. Fig. 62 and 63). According to recent investigations by A. F. Huxley and H. E. Huxley, the thin fibrils slide past the myosin fibrils during contraction so that the I-bands eventually disappear and the Z-Une approaches the end of the myosin fibrils. 

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