Myosin regulatory proteins

In addition to the major proteins of striated muscle (myosin, actin, tropomyosin, and the troponins), numerous other proteins play important roles in the maintenance of muscle structure and the regulation of muscle contraction. Myosin and actin together account for 65% of the total muscle protein, and tropomyosin and the troponins each contribute an additional 5% (Table 17.1). The other regulatory and structural proteins thus comprise approximately 25% of the myofibrillar protein. The regulatory proteins can be classified as either myosin-associated proteins or actin-associated proteins. 

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. 

The muscle sarcomere contains the principal contractile proteins myosin and actin (Fig. 3A to C), which on their own can produce force and movement, together with a number of cytoskeletal and regulatory proteins. The latter include titin, C-protein (MyBP-C), tropomyosin, troponin, a-actinin, myomesin, M-protein, and so on. Some of these help to organize the myosin and actin filaments in the sarcomere, some to define the filament lengths and structure, some to regulate activity, and some to modulate the actin-myosin interaction when the muscle is active. 

Bornhauser BC, Olsson P-A, Lindholm D. 2003. MSAP is a novel MIR-interacting protein that enhances neurite outgrowth and increases myosin regulatory light chain. J Biol Chem 278 35412-35420. 

Olsson P-A, Korhonen L, Mercer EA, Lindholm D. 1999. MIR is a novel ERM-like protein that interacts with myosin regulatory light chain and inhibits neurite outgrowth. J Biol Chem 274 36288-36292. 

Several of the identified proteins were known to be involved in signal transduction and platelet function, e.g., cortactin, myosin regulatory Hght chain (myosin RLG), and protein disulfide isomerase (PDI). 

Figure 9.7 Effects of cAMP contributing to relaxation. AC, adenylate cyclase DG, diacyl glycerol G, guanosine nucleotide regulatory protein IP3, Inositol trisphosphate, MLCK, myosin light chain kinase.

The myocardium contains bundles of striated muscle fibers, each of which is typically 10 to 15 pm in diameter and 30 to 60 pm in length. Work of the heart is generated by the alternating of the contraction and relaxation of these fibers. The fibers are composed of cardiac-specific contractile proteins actin and myosin, and regulatory proteins caEed troponins. They also contain a variety of enzymes that are vital for energy use, such as myoglobin, creatine kinase (CK), and lactate dehydrogenase (LD), some of which can be used as markers of cardiac injury. 

The major regulatory proteins located on an actin filament are troponin and tropomyosin, each occupying 5% of the total myofibrillar proteins. Both proteins confer calcium sensitivity on the ATP-actin-myosin interactions (see Section II). There are minor regulatory proteins that modify the fine structures of myosin and actin filaments and also of Z lines. 

The contraction of ascidian smooth muscle was found to be regulated through the troponin-tropomyosin system. But the action of troponin components was different from that of troponin of vertebrate striated muscles (Endo and Obinata, 1981). In this system, the inhibitory action of troponin I (MW 24,000) is less remarkable compared with vertebrate skeletal troponin I, and troponin C (MW 18,000) does not neutralize the inhibition by troponin I. But upon further addition of troponin T (MW 33,000) in the concomitant presence of all three components and tropomyosin, the contractile interaction of myosin and actin is activated. In this case, the action of troponin T has some similarity with that of the above-mentioned cardiac troponin T hybridized with skeletal troponin C-I. Since actomyosin, without these regulatory proteins, is inhibited regardless of Ca concentration, Ca " and troponin-tropomyosin are activators for contraction of actomyosin in ascidian smooth muscle. In this respect, the type of Ca + regulation of ascidian smooth muscle is the same as that for vertebrate smooth muscles which do not contain troponin (Ebashi, 1980). 

Controlled transformation of the chemical energy of nucleoside triphosphates into mechanical energy is called chemomechanical transduction. In addition to the actin filaments and microtubules, the motor proteins myosin and dynein or kinesin are needed for chemomechanical transduction. Several other proteins are associated with these, including regulatory proteins that control contractile activity and enzymes involved in maintaining the supply of high-energy phosphate. 

G-actin is very highly conserved, both across actin genes within a species and across species. Apparently, the need for so many functional binding sites in a molecule of that size leaves few options for nonlethal mutations. Among the actins sequenced from 30 widely divergent species, there were only 32 amino acid substitutions. One implication of this is that when differences in contractile properties are observed between various types of muscle, those differences must be due to the motor protein (myosin) or to the various regulatory proteins. 

FIGURE 5 Comparison of amino acid sequence for myosin regulatory light chain proteins in different species. Cce, chicken cellular LC20-A, chicken adult smooth muscle LC20-B1, chicken embryonic smooth muscle Ram, rat aortic muscle Hsm, putative human smooth muscle. Amino acid residues are numbered starting at the Ser residue next to the initiator Met. A dot indicates identity of an amino acid residue with that in the top row. Reproduced with permission from Zavodny et al. (1990, Fig. 3, p. 937), Copyright 1994, American Heart Association. 

It is useful to first consider the structure of muscle s mechanochemical transduction elements, actin and the subfragment-1 (S-1) portion of myosin, before analyzing the effects of regulatory proteins that modulate interactions of the two and have the potential therefore to control force generation. Molecular details of the structure of skeletal muscle actin (Kabsch et al, 1990 Holmes and Kabsch, 1991 Lorenz et al., 1993) and S-1 (Rayment etal., 1993a) are known at the atomic level (Fig. 1), and given the general similarity of F-actin and S-1 in smooth and in skeletal muscle, it is reasonable to assume that they are closely related in the two... 

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