Cardiac high molecular weight calmodulin binding protein contains calpastatin activity

A high molecular weight calmodulin binding protein (HMWCaMBP) was previously identified and purified from bovine heart cytosolic fraction [Sharma, R.K. (1990) J. Biol. Chem. 265, 1152-1157]. In this study, we report the biological function of this protein. HMWCaMBP was subjected to peptide mapping and three peptides were sequenced. Two of the three peptide sequences were shown to be highly homologous to the calpain inhibitor, calpastatin. However, the third peptide did not show homology to any known proteins. The Western blot analysis of HMWCaMBP and purified calpastatin from bovine cardiac muscle showed immunoreactivity with polyclonal antibody raised against HMWCaMBP. Furthermore, HMWCaMBP inhibited calpain II and calpain I activities in a dose dependent fashion. Our data based on sequence homology, amino acid analysis, antibody reactivity and calpain inhibition suggests that HMWCaMBP is homologous to calpastatin and may be a CaM-binding form of calpastatin.