Molecular Cloning and Characterization of Human Non-Smooth Muscle Calponin

cDNA clones encoding a calponin isoform with 309 amino acids have been isolated from human heart. The deduced amino acid polypeptide (Mr 33,697) showed a neutral isoelectric point of 7.1. The mRNA, expressed in cultured smooth muscle cells as well as in fibroblasts, vascular endothelial cells, and keratinocytes, contains a 3′ untranslated region of 1.2 kilobases that includes an Alu repetitive sequence in the antisense direction. On the basis of the nucleotide sequence identity to an expressed sequence tag, HUM21ES93 [Cheng, J.-F., Boyartchuk, V., and Zhu, Y. (1994) Genomics 23,75–84], the human neutral calponin gene is assigned to chromosome 21q11.1. The amino acid sequence indicates that this protein is the human equivalent of mouse calponin-h2 (94.8% identity) [Strasser, P., Gimona, M., Moessler, H., Herzog, M., and Small, J.V. (1993) FEBS Lett 330, 13–18]. Three tandem repeats of 29 amino acids, a Vav-homologous region and an actin-binding sequence, originally identified in the basic calponin isoform, are conserved. There are two consensus phosphorylation sites for tyrosine kinase. An immunoreactive form of the neutral calponin appears to be localized with vinculin in the cell-to-cell junctions of cardiomyocytes. Mouse calponin-h2 is also expressed in both embryonic and adult heart. These results indicate that the human neutral calponin is a non-smooth muscle isoform, and may play a physiological role in cytoskeletal organization.