The crystal structure of plasminogen activator inhibitor 2 at 2.0 angstromresolution: implications for serpin function

Background: Plasminogen activator inhibitor 2 (PAI-2) is a member of the se rpin family of protease inhibitors that function via a dramatic structural change from a native, stressed state to a relaxed form. This transition is mediated by a segment of the serpin termed the reactive centre loop (RCL); the RCL is cleaved on interaction with the protease and becomes inserted in to beta sheet A of the serpin. Major questions remain as to what factors fa cilitate this transition and how they relate to protease inhibition. Results: The crystal structure of a mutant form of human PAI-2 in the stres sed state has been determined at 2.0 Angstrom resolution. The RCL is comple tely disordered in the structure, An examination of polar residues that are highly conserved across all serpins identifies functionally important regi ons. A buried polar cluster beneath beta sheet A (the so-called 'shutter' r egion) is found to stabilise both the stressed and relaxed forms via a rear rangement of hydrogen bonds, Conclusions: A statistical analysis of interstrand interactions indicated t hat the shutter region can be used to discriminate between inhibitory and n on-inhibitory serpins, This analysis implied that insertion of the RCL into beta sheet A up to residue P8 is important for protease inhibition and hen ce the structure of the complex formed between the serpin and the target pr otease.