The crystal structure of plasminogen activator inhibitor 2 at 2.0 Å resolution: Implications for serpin function

Background: Plasminogen activator inhibitor 2 (PAI-2) is a member of the serpin 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 into β sheet A of the serpin. Major questions remain as to what factors facilitate this transition and how they relate to protease inhibition. Results: The crystal structure of a mutant form of human PAI-2 in the stressed state has been determined at 2.0 Å resolution. The RCL is completely disordered in the structure. An examination of polar residues that are highly conserved across all serpins identifies functionally important regions. A buried polar cluster beneath β sheet A (the so-called 'shutter' region) is found to stabilize both the stressed and relaxed forms via a rearrangement hydrogen bonds. Conclusions: A statistical analysis of interstrand interactions indicated that the shutter region can be used to discriminate between inhibitory and non-inhibitory serpins. This analysis implied that insertion of the RCL into β sheet A up to residue P8 is important for protease inhibition and hence the structure of the complex formed between the serpin and the target protease.