Molecular Cloning and Characterization of Rat Contrapsin-Like Protease Inhibitor and Related Proteins

A glycoprotein with Mr63,000 purified from rat serum was found to inhibit trypsin activity but not chymotrypsin or elastase activity, resembling contrapsin purified from mouse serum. To obtain further information on the molecular structure, a cDNA clone (λCPi-21) for this contrapsin-like protease inhibitor was isolated from a rat liver cDNA library. The 1.6-kb cDNA insert contained an open reading frame that encodes a 416-residue polypeptide (CPi-21), in which the first 29 residues were suggested to comprise a signal peptide by comparison with the NH2-terminal sequence of the purified protein. The predicted structure also contained other peptide sequences determined by Edman degradation. Four potential N-linked glycosylation sites were found in the molecule, presumably accounting for the larger molecular mass of the mature form. Further screening of the cDNA library with a Pst-Xbal fragment (302 bp) of λCPi-21 as a probe yielded two other cDNA clones (λCPi-23 and λCPi-26), which encode 413-residue and 418-residue polypeptides, respectively. A comparison of their amino acid sequences revealed that CPi-21 has 89 and 71% homology with CPi-23 and CPi-26, respectively. The primary structure of each of the three proteins has about 70% homology with that of mouse contrapsin, in contrast to 43–46% homology with that of rat α1-protease inhibitor. These results indicate that all the CPi proteins presented here belong to a subfamily of “serpins” of which mouse contrapsin was the first member to be identified.