Functional diversification during evolution of the murine α1-proteinase inhibitor family: Role of the hypervariable reactive center loop

Abstract

α1-Proteinase inhibitor (α1-PI) is a member of the serpin superfamily of serine proteinase inhibitors that are involved in the regulation of a number of proteolytic processes. α1-PI, like most serpins, functions by covalent binding to, and inhibition of, target proteinases. The interaction between α1-PI and its target is directed by the so-called reactive center loop (RCL), an ∼20 residue domain that extends out from the body of the α1-PI polypeptide and determines the inhibitor's specificity. Mice express at least seven closely related α1-PI isoforms, encoded by a family of genes clustered at the Spil locus on chromosome 12. The amino acid sequence of the RCL region is hypervariable among α1-PIs, a phenomenon that has been attributed to high rates of evolution driven by positive Darwinian selection. This suggests that the various isoforms are functionally diverse. To test this notion, we have compared the proteinase specificities of individual α1-PIs from each of the two mouse species. As predicted from the positive Darwinian selection hypothesis, the various α1-PIs differ in their ability to form covalent complexes with serine proteinases, such as elastase, trypsin, chymotrypsin, and cathepsin G. In addition, they differ in their binding ability to proteinases in crude snake venoms. Importantly, the RCL region of the α1-PI polypeptide is the primary determinant of isoform-specific differences in proteinase recognition, indicating that hypervariability within this region drives the functional diversification of α1-PIs during evolution. The possible physiological benefits of α1-PI diversity are discussed.