Co-localization of immunoreactive forms of calponin with actin cytoskeleton in platelets, fibroblasts, and vascular smooth muscle.

Calponin is an actin binding protein found in the smooth muscle cells of chicken gizzard. The localization of the protein was examined in bovine platelets, mouse fibroblasts, and the smooth muscle cells of the bovine aorta. Immunoblotting of whole platelet lysates revealed that the antibody to chicken gizzard calponin recognized two proteins with apparent molecular masses of 37 and 23 kDa in the resting state and an additional high-molecular-weight component (approximately 40 kDa) in the activated state. The localizations of calponin and caldesmon, and the correlation of their localizations with that of the actin cytoskeleton were analyzed by immunofluorescence microscopy using appropriate antibodies and rhodamine-phalloidin. In resting bovine platelets, calponin exhibited the same distribution as actin filaments, which are organized in a characteristic wheel-like structure. A similar distribution was observed with the anti-caldesmon antibody. Colocalization of calponin and actin were shown in activated platelets and along stress fibers of both fibroblasts and smooth muscle cells. These results suggest not only a cytoskeletal role associated with microfilaments but also a regulatory role of these proteins for actin-myosin interaction.