Calcium binding proteins caldesmon

Finally, a 150 kDa calmodulin binding protein, caldesmon is found bound to actin filaments at low calcium concentrations. The ratio of caldesmon to actin molecules seems to vary from 1 20 in the aorta to as much as 1 200 in chicken gizzard. It has been reported and disputed that caldesmon crosslinks actin filaments into large bundles. 

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. 

IP3 diffuses to specialized regions of the endoplasmic reticulum and induces it to release a small amount of stored Ca " (Golovina and Blaustein, 1997). The conse-tfuent increase in cytoplasmic levels stimulates a rtumber of events in the cytoplasm, including the further activation of protein kinase C. Calcium ions directly bind to and activate calmodulin, protein kinase C, phospholipase A2, proteins of muscle fibers (troponin, caldesmon), and proteins of the cytoskeleton (geisolin, villin). One might hesitate to cali these proteins Ca-metalloenzymes. It is more accurate to say that these enzymes are regulated by calcium. 

Caldesmon is a cytoplasmic protein with two isoform classes, one of which is found predominantly in smooth muscle cells and other cell types with partial myogenic differentiation. High-molecular-weight isoforms with molecular weights between 89 and 93 kD are capable of binding to actin, tropomyosin, calmodulin, myosin, and phospholipids, and they function to counteract actin-tropomyosin-activated myosin adenosine triphosphatase (ATPase). As such, they are mediators for the inhibition of calcium-dependent smooth muscle contraction." ... 

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