Plant SERine Proteinase INhibitors (serpins) are potent inhibitors of a range of mammalian serine proteases in vitro, and at least seven serpin genes are expressed in Arabidopsis. Serpins from plants display a wide range of functions including protection of storage protein degradation by exogenous proteases and seed survival within the herbivore digestive tract. Comparison between Arabidopsis AtSerpin1 and other serpins reveals several distinguishing features including a plant-specific insertion between s2B and s3B, with a plant-specific motif YXXGXDXRXF and the presence of a beta-bulge in strand s2C. The conserved Asp-230 and Arg-232 in the motif form a network of hydrogen bonds stabilize a loop region, which is otherwise disordered in many other serpin structures. AtSerpin1 is targeted to the secretory pathway and was shown to interact with cysteine protease RD21 (RESPONSIVE TO DESICCATION-21). RD21 accepts peptides and ligates them to the N termini of acceptor proteins so it has been proposed that AtSerpin1 functions to curb this activity. This subgroup corresponds to clade P of the serpin superfamily. In general, serpins exhibit conformational polymorphism shifting from native to cleaved, latent, delta, or polymorphic forms. Many serpins, such as antitrypsin and antichymotrypsin, function as serine protease inhibitors which regulate blood coagulation cascades. Non-inhibitory serpins perform many diverse functions such as chaperoning proteins or transporting hormones. Serpins are of medical interest because mutants have been associated with blood clotting disorders, emphysema, cirrhosis, and dementia. A classification based on evolutionary relatedness has resulted in the assignment of serpins to 16 clades designated A-P along with some orphans.
Comment:depending on the conformational state, the RC loop is surface accessible in the active form or buried and inserted as the central beta strand in the inactive form.
Structure:3LE2_A: Arabidopsis thaliana Serpin 1, native stressed conformation showing an unusually open structure
Jiyao W et al.(2023). "The conserved domain database in 2023.",