Molecular cloning and sequence analysis of smooth muscle calponin

cDNA clones encoding two isoforms of chicken gizzard calponin, a recently identified actin- and tropomyosin-binding protein, have been isolated and sequenced. The deduced polypeptides, 292 (Mr 32,333) and 252 (Mr 28,127) amino acids, contain sequences homologous to: a smooth muscle protein SM22 alpha, the Drosophila melanogaster mp20 gene product, troponin T, troponin I, and caldesmon. Calponin mRNAs of approximately 1.3 kilobases, encoding both isoforms, were expressed in all chicken smooth muscle tissues examined. These data, coupled with the inhibition of actomyosin ATPase by calponin (Winder, S. J., and Walsh, M. P. (1990) J. Biol. Chem. 265, 10148-10155; Abe, M., Takahashi, K., and Hiwada, K. (1990) J. Biochem. (Tokyo) 108, 835-838), suggest that calponin may function as a troponin homolog involved in the regulation of thin filament activity.