Calcium binding proteins actin interactions

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. 

Cadherins are calcium-dependent cell adhesion proteins that mainly participate in interactions with cadherins, located on other cells. Cadherins have five similar extracellular domains some of which have calcium-binding sites, and an intracellular C-terminal domain that interacts with the actin cytoskeleton. Different cadherins are named according to the tissues were they were first found. E-cadherins were found in many epithelial cells. N-cadherins are present in nerves and muscle and P-cadherin are present in placental cells. 

Cytoplasmic calcium concentration phosphorylation of myosin light chains and (less important) other proteins interactions with actin-binding proteins. 

Schematic diagram of the organization of skeletal muscle thin filament, showing the position of tropo-myosin and the troponin complex on the actin filament. The binding of Ca " to TnC, the calcium-binding subunit of the troponin complex, removes Tnl, the inhibitory subunit, from actin and thus permits an interaction with a specialized protein, myosin, on neighboring thick muscle filaments (not shown). An ATP-driven conformation change in the myosin head group makes the thick and thin filaments move relative to one another, so that muscle contraction occurs.

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