STRUCTURE OF HUMAN α2-PLASMIN INHIBITOR DEDUCED FROM THAT OF cDNA
The complete amino acid sequence of α2-plasmin inhibitor (α2PI) was determined by cDNA cloning. A Agt 10 cDNA library was prepared from poly(A)+mRNA isolated from cultured human liver cells. The labeled oligonucleotides, corresponding to the reported partial amino acid sequences of α2PI, were used as probes to screen the library. One of the positive clones was subcloned into plasmid pUC8. A 2.2 kilobase cDNA clone thus isolated contains a region coding for a portion of a leader sequence, the mature protein, a stop codon (TGA), a 3' noncoding region (733 nucleotides), and a poly(A)tail (37 nucleotides). The amino acid sequence deduced from the cDNA is composed of 452 amino acids starting with an amino-terminal sequence of Asn-Gln-Glu-Gln and ending with a carboxyl-terminal sequence of Gly-Ser-Pro-Lys. The sequence shows approximately 30% homology with those of other plasma serine protease inhibitors. However, α2PI extends 50-52 amino acids beyond the carboxyl-terminal ends of the other inhibitors. This 50-52 carboxyl-terminal amino acid sequence is therefore specific to α2PI, and contains the sequence that is exactly the same as that of the peptide containing the plasminogen binding site. There are three lysine residues possibly involved in the binding to plasminogen in this region. From the homology with the other inhibitors, the inhibitor's reactive-site peptide bond was suggested to be Met-Ser and the same as that of ai-antitrypsin. The Met residue is located at the 362 position from the amino-terminal end.