CHARACTERIZATION AND CONFOCAL IMAGING OF CALPONIN IN GASTROINTESTINALSMOOTH-MUSCLE

Calponin isolated from chicken gizzard smooth muscle binds in vitro to actin in a Ca2+-independent manner and thereby inhibits the actin-act ivated Mg2+-adenosinetriphosphatase of smooth muscle myosin. This inhi bition is relieved when calponin is phosphorylated by protein kinase C or Ca2+/calmodulin-dependent protein kinase II, suggesting that calpo nin is involved in thin filament-associated regulation of smooth muscl e contraction. To further examine this possibility, calponin was isola ted from toad stomach smooth muscle, characterized biochemically, and localized in intact isolated cells. Toad stomach calponin had the same basic biochemical properties as calponin from other sources. Confocal immunofluorescence microscopy revealed that calponin in intact smooth muscle cells was localized to long filamentous structures that were c olabeled by antibodies to actin or tropomyosin. Preservation of the ba sic biochemical properties of calponin from species to species suggest s that these properties are relevant for its in vivo function. Its col ocalization with actin and tropomyosin indicates that calponin is asso ciated with the thin filament in intact smooth muscle cells.