Nonmuscle caldesmon: its distribution and involvement in various cellular processes. Review article.

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, alpha-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.

Immunodetection and intracellular localization of caldesmon-like proteins in Amoeba proteus.

Caldesmon immunoanalogues were detected in Amoeba proteus cell homogenates by the Western blot technique. Three immunoreactive bands were recognized by polyclonal antibodies against the whole molecule of chicken gizzard caldesmon as well as by a monoclonal antibody against its C-terminal domain: one major and two minor bands corresponding to proteins with apparent molecular masses of 150, 69, and 60 kDa. The presence of caldesmon-like protein(s) in amoebae was revealed as well in single cells after their fixation, staining with the same antibodies, and recording their total fluorescence in a confocal laser scanning microscope. Proteins recognized by the antibodies bind to filamentous actin. This was established by a cosedimentation assay in cell homogenates and by colocalization of the caldesmon-related immunofluorescence with the fluorescence of filamentous actin stained with rhodamine-labelled phalloidin, demonstrated in optical sections of single cells in a confocal microscope. Caldesmon is colocalized with filamentous actin in the withdrawn cell regions where the cortical actomyosin network contracts and actin is depolymerized, in the frontal zone where actin is polymerized again and the cortical cytoskeleton is reconstructed, inside the nucleus and in the perinuclear cytoskeleton, and probably at the cell-to-substratum adhesion sites. The regulatory role of caldesmon in these functionally different regions of locomoting amoebae is discussed.