Regulatory proteins actin-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. 

The calponin homology (CH) domain has been identified in many molecules of differing function. However, its presence usually signifies an interaction of some sort with the actin cytoskeleton via an association with F-actin. The domain was initially identified as a 100-residue motif found at the N-terminus of the smooth muscle regulatory protein calponin and, hence, was termed the CH domain (Castresana and Saraste, 1995). The refinement of algorithms for the identification of distinct protein motifs has allowed the identification of CH domains in proteins that range... 

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. 

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... 

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.,... 

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