A putative Ca2+-binding protein: structure of the light subunit of porcine calpain elucidated by molecular cloning and protein sequence analysis.

cDNA clones specific for the light subunit of porcine calpain I have been isolated from a porcine kidney cDNA library. The complete primary structure of the light subunit has been revealed by nucleotide sequence analysis of the cDNA clones isolated and amino acid sequence analysis of peptides isolated from the purified mature protein. We found that the light subunit contains two distinct domains. Domain I, the amino-terminal half, has two unusually long, paired polyglycyl sequences and may serve as a binding site to the heavy subunit. Domain II, the carboxyl-terminal half, is a region highly homologous to the putative Ca2+-binding domain of the heavy subunit of chicken calpain elucidated recently. This region has four potential Ca2+-binding sites, each having the "E-F hand" structure. Our results suggest that the Ca2+-mediated proteolytic activity of calpain is controlled through the cooperative and/or sequential actions of multiple Ca2+-binding sites present in both two-subunit molecules, heavy and light subunits of calpain.