Calponin and caldesmon are two thin filament-binding proteins found in smooth muscle that have both been attributed a role in modulating the interaction of actin and myosin. Using high-resolution dual-label immunocytochemistry we have determined the distribution of calponin relative to the contractile and cytoskeletal compartments of the smooth muscle cell. We show, using chicken gizzard smooth muscle, that calponin occurs in the cytoskeleton, with beta-cytoplasmic actin, filamin and desmin, as well as in the contractile apparatus, with myosin and caldesmon. According to the observed labelling intensities, calponin was more concentrated in the cytoskeleton and it was additionally localised in the cytoplasmic dense bodies as well as in the adhesion plaques at the cell surface, which both harbour the beta-cytoplasmic isoform of actin. It is probable that these results explain earlier conflicting reports on the composition of smooth muscle thin filaments and suggest that calponin, together with a Ca(2+)-receptor protein, could just as likely serve a role in the cytoskeleton of smooth muscle as in the contractile apparatus.
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