Regulatory proteins myosin-associated

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. 

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. 

To account for activation of arterial smooth muscle independently of LC20 phosphorylation, attention has been focused on the possible roles of the thin filament-associated regulatory proteins, caldesmon and calponin. Both proteins have been localized in the actomyosin domain of the smooth muscle cell and both have been shown to inhibit actin-activated myosin ATPase by interacting with F-actin, tropomyosin, and/or myosin (Clark et al., 1986 Takahashi et al.,... 

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. 

We are still much in the dark regarding CP function. Does it perform both structural and regulatory roles, and if so what are they Are these roles carried out in the cytoskeleton or in the contractile apparatus, or both Does CP influence or complement CD function And if Ca2+ binding proteins are the primary regulators, which one(s) operate on CP To determine the function of CP in vivo will require new approaches. In part because of the presence of multiple targets for Ca2+ binding proteins in smooth muscle (including MLCK, CD, and CP), it has so far proved difficult to define the relative roles that myosin and the actin-associated proteins play in smooth muscle. Here specifically, we will want to know what roles CD and CP perform in the process of tension maintenance... 

This of course all happens very quickly, and contraction is driven by thousands of heads in each muscle fibre acting in concert. The instruction from the brain, by way of the nerve, to contract is implemented by the release of calcium ions from the membrane that surrounds the muscle fibres. The calcium ions bind to a protein that is part of the regulatory system associated with the actin filaments. Once again a structural distortion ensues, and a second protein component of that system is moved out of the way to allow the myosin heads to bind to the actin and start to work. 

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