Caldesmon.

Caldesmon is a protein that is found in smooth muscle and in non-muscle cells. Two isoform classes produced by alternative splicing of one gene have been characterized. The smooth muscle, high molecular weight (89-93 kDa), caldesmon isoforms are exclusively found in adult and fully differentiated smooth muscle cells. The non-muscle, low molecular weight (59-63 kDa), caldesmon isoforms are found in non-muscle and in de-differentiated smooth muscle cells. The conserved regions of all isoforms contain caldesmon's properties such as binding to actin, tropomyosin, Ca(2+)-calmodulin, myosin and phospholipids. All isoforms are also very potent inhibitors of the actin-tropomyosin activated myosin MgATPase. Non-muscle and smooth muscle isoforms of caldesmon perform different roles in vivo. This may be reflected by the distinct cellular distribution of these isoform classes. Non-muscle caldesmon is a regulatory factor in the microfilament network and is thus involved in the assembly and stabilization of microfilaments. Smooth muscle caldesmon together with tropomyosin is a mediating factor for Ca(2+)-dependent inhibition of smooth muscle contraction.