Four types of calpastatin isoforms with distinct amino-terminal sequences are specified by alternative first exons and differentially expressed in mouse tissues.

Calpastatin, a specific inhibitor of calpain, consists of a unique N-terminal domain (domain L) and four repetitive protease-inhibitor domains (domains 1-4). The isolated cDNAs from various mammalian species have conspicuous differences in the regions encoding the N-terminal sequences and can be classified into four types. Mouse and bovine calpastatins (Type I and Type II, respectively), which also differ from each other in the uttermost N-terminal regions, possess longer domain L sequences than those of rabbit, pig, and human inhibitors (Type III). A sequence of a shorter isoform, registered in a DNA data bank, starts from a part of domain 2 with a different N-terminal sequence (Type IV). To clarify the source of this molecular diversity, we investigated the entire exon-intron organization of the mouse calpastatin gene. The previously obtained mouse calpastatin cDNA is encoded by as many as 31 exons including the first exon, designated 1xa. Three additional exons specifying the N-terminal sequences of the other types (designated exons 1xb, 1u, and 14t, respectively) were identified in the mouse genomic DNA sequence. While the mRNAs for Types I and III were expressed at high levels in liver, the Type II mRNA was abundant in heart and skeletal muscle and expressed at lower levels in liver, brain and testis. The Type IV mRNA was specifically expressed in testis among the tissues examined. These results suggest that the calpastatin isoforms possessing different N-terminal sequences are generated by alternative transcription initiation from their own promoters and skipping of the mutually exclusive exons.