Nonmuscle caldesmon: its distribution and involvement in various cellular processes

Summary.

Smooth muscle caldesmon is a thin-filament constituent which takes part in the Ca²⁺-dependent regulation of actomyosin motor activity which converts chemical energy of ATP into force. The molecular anatomy of its counterpart found in a variety of nonmuscle cells is similar. Both contain about 20 nm long terminal domains responsible for functionally important multisite interactions with filamentous actin, tropomyosin, Ca²⁺/calmodulin, and myosin and differ by a 35 nm long central, α-helical fragment which is lacking in nonmuscle caldesmon. The different structural organisation of nonmuscle cells and thus distinct distribution of caldesmon implicates its different physiological functions. Due to direct interaction with globular and filamentous actin as well as with tropomyosin, nonmuscle caldesmon is involved in the assembly, dynamics, or stability of microfilaments, whereas the indirect inhibitory effect on interaction of the microfilaments with myosin causes its participation in the regulation of cell contraction and intracellular motional processes. These functions of nonmuscle caldesmon of vertebrates are controlled by Ca²⁺/calmodulin (or other Ca²⁺-binding proteins) or caldesmon phosphorylation catalysed by various protein kinases. Examples of nonmuscle caldesmon involvement in functions of higher and lower eukaryote, animal and plant cells are presented.