Importance of the amino‐acid composition of the shutter region of plasminogen activator inhibitor‐1 for its transitions to latent and substrate forms

The serpins are of general protein chemical interest due to their ability to undergo a large conformational change consisting of the insertion of the reactive centre loop (RCL), which becomes strand 4, into the central β sheet A. To make space for the incoming RCL, the ‘shutter region’ opens by the β strands 3A and 5A sliding apart over the underlying α helix B. Loop insertion occurs during the formation of complexes of serpins with their target serine proteinases and during latency transition. This type of loop insertion is unique to plasminogen activator inhibitor‐1 (PAI‐1). We report here that amino‐acid substitutions in a buried cluster of three residues forming a hydrogen bonding network in the shutter region drastically accelerate PAI‐1 latency transition; that the rate was in all cases normalized by the PAI‐1 binding protein vitronectin; and that substitution of an adjacent β strand 5A Lys residue, believed to anchor β strand 5A to other secondary structural elements, had differential effects on the rates of latency transition in the absence and the presence of vitronectin, respectively. An overlapping, but not identical set of substitutions resulted in an increased tendency to substrate behaviour of PAI‐1 at reaction with its target proteinases. These findings show that vitronectin regulates the movements of the RCL through conformational changes of the shutter region and β strand 5A, are in agreement with RCL insertion proceeding by different routes during latency transition and complex formation, and contribute to the biochemical basis for the potential use of PAI‐1 as a therapeutic target in cancer and cardiovascular diseases.