Structure of Human α2-Plasmin Inhibitor Deduced from the cDNA Sequence1

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Abstract
We have isolated three cDNA clones for human α2-plasmin inhibitor (α2-PI). Two clones are from human hepatoma cell line, Hep G2, and cover the entire protein coding region plus the 3′-flanking region up to the poly(A) sequence, and the other clone is from human liver and contains the carboxyl-terminal half. The total length of the cDNAs is 2.29 kb, corresponding to more than 95 % of the full-length mRNA. α2-PI seems to consist of 452 amino acid residues plus 39 amino acid residues for the signal peptide. The amino acid sequence shows 23 to 28 % homology to those of five other protease inhibitors, plasminogen activator inhibitor (PAI), protein C inhibitor (PCI),α1-antitrypsin (α1-AT), antithrombin m (AT HI), and α1-anti-chymotrypsin (α1-AC). α2-PI seems to be the most distantly related among these inhibitors. Comparison of the phylogenetic trees of proteases and their inhibitors indicates that four proteases, namely elastase (or trypsin), chymotrypsin, plasminogen activator, and thrombin, may have evolved concurrently with the corresponding inhibitors. However, α2-PI and PCI seem to have evolved asynchronously from their substrates. The data suggest that α2-PI may originally have inhibited some protease other than plasmin, and protein C may have had an inhibitor different from the present one early in its evolutionary history.

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