The Serpins Are an Expanding Superfamily of Structurally Similar but Functionally Diverse Proteins

Abstract

squamous cell carcinoma antigen 1 or 2 α1 antitrypsin (α1 proteinase inhibitor) α2 antiplasmin amyloid-β α1 antichymotrypsin antithrombin III monocyte-neutrophil elastase inhibitor ovalbumin plasminogen activator inhibitor type 1 or 2 pigment epithelium-derived factor vascular endothelial growth factor reactive site loop tissue plasminogen activator urokinase plasminogen activator The serpins (serineproteinase inhibitors) are a superfamily of proteins (350–500 amino acids in size) that fold into a conserved structure and employ a unique suicide substrate-like inhibitory mechanism. The serpins were last reviewed in 1994 (1Potempa J. Korzus E. Travis J. J. Biol. Chem. 1994; 269: 15957-15960Abstract Full Text PDF PubMed Google Scholar). More recent studies show: 1) an expanded distribution within the kingdoms of metazoa and plantae, as well as certain viruses, 2) a surprising effect on the covalently bound target proteinase, and 3) novel biochemical and biological functions. Most serpins inhibit serine proteinases of the chymotrypsin family. However, cross-class inhibitors have been identified. The viral serpin CrmA and, to a lesser extent, PI9 (SERPINB9) inhibit the cysteine proteinase, caspase 1 (2Komiyama T. Ray C.A. Pickup D.J. Howard A.D. Thornberry N.A. Peterson E.P. Salvesen G. J. Biol. Chem. 1994; 269: 19331-19337Abstract Full Text PDF PubMed Google Scholar), and SCCA11 (SERPINB3) neutralizes the potent papain-like cysteine proteinases, cathepsins L, K, and S (3Schick C. Pemberton P.A. Shi G.-P. Kamachi Y. Cataltepe S. Bartuski A.J. Gornstein E.R. Bromme D. Chapman H.A. Silverman G.A. Biochemistry. 1998; 37: 5258-5266Crossref PubMed Scopus (259) Google Scholar). In addition, several members no longer function as proteinase inhibitors but perform other roles such as hormone transport (thyroid-binding globulin (SERPINA6), corticosteroid-binding globulin (SERPINA7)), and blood pressure regulation (angiotensinogen (SERPINA8)) (1Potempa J. Korzus E. Travis J. J. Biol. Chem. 1994; 269: 15957-15960Abstract Full Text PDF PubMed Google Scholar). Data base searching provides evidence for ∼500 serpins, with full-length coding sequences known or predicted for about one-half of those (4Irving J.A. Pike R.N. Lesk A.M. Whisstock J.C. Genome Res. 2000; 10: 1845-1864Crossref PubMed Scopus (523) Google Scholar). A phylogenetic analysis divides serpins into 16 clades (see Supplemental Data, Table A) and 10 highly diverged “orphans” (4Irving J.A. Pike R.N. Lesk A.M. Whisstock J.C. Genome Res. 2000; 10: 1845-1864Crossref PubMed Scopus (523) Google Scholar). These data facilitate the construction of a consistent expandable nomenclature (see Supplemental Data for Serpin Nomenclature Guidelines, Table B). The completed DNA sequences of several organisms have yielded insight into the complexity of the family. The Caenorhabditis elegans, Drosophila melanogaster, and Arabidopsis thalianagenomes encode for ∼20,000, 13,000, and 25,000 genes, respectively. However, these three species harbor ∼9, 32, and 13 serpin genes, respectively. The nonlinear relationship among the number of serpin genes, relative to the total gene number, suggests that at least a subset of serpins has evolved divergent functions despite a striking degree of sequence and structural conservation. Serpins adopt a metastable conformation that is required for their inhibitory activity (5Stein P.E. Carrell R.W. Nat. Struct. Biol. 1995; 2: 96-113Crossref PubMed Scopus (398) Google Scholar). This conformation consists of a conserved secondary structure comprised of β-sheets A, B, and C and at least 7 α-helices (most typically have 9, lettered A–I; Fig.1 A). The RSL, which contains the proteinase recognition site, is an exposed, flexible stretch of ∼17 residues tethered between β-sheets A and C. Serpins can undergo major structural rearrangements that involve alternative conformations for the RSL, β-sheet A, and the attached strand 1 of β-sheet C. Considering only intramolecular structural changes, serpins can convert to the more stable latent form (Fig. 1 B). The RSL inserts into the middle of β-sheet A to give a fully antiparallel β-sheet, and s1C is extracted from β-sheet C to provide an exposed “return” from the bottom of the serpin. Serpins in the latent conformation are noninhibitory but can be converted back to the active state by denaturation and refolding. The Tm for unfolding of latent PAI1 (SERPINE1) is 17 °C higher than that for the native state (reviewed in Ref. 6Gettins P.G.W. Patston P.A. Olson S.T. Serpins: Structure, Function and Biology, Molecular Biology Intelligence Unit. R. G. Landes Co., and Chapman & Hall, Austin, TX1996Google Scholar). The most stable state for inhibitory serpins is the RSL-cleaved form, in which the RSL has fully inserted into β-sheet A, as in the latent conformation, but without the need to extract s1C from β-sheet C (Fig. 1 C). Estimates of the Tm for unfolding of such conformations are >120 °C, compared with ∼60 °C for the