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 t o undergo a large conformational change consisting of the insertion of the reactive centre loop (RCL) as strand 4 of the central beta sheet A. To make space for the incoming RCL, the 'shutter region' opens by the beta strands 3A and 5A sliding apart over the underlying alpha helix B. Loop insertion occurs during the formation of complexes of serpins with their target serin e proteinases and during latency transition. This type of loop insertion is unique to plasminogen activator inhibitor-1 (PAI-1). We report here that a mino-acid substitutions in a buried cluster of three residues forming a hyd rogen bonding network in the shutter region drastically accelerate a 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 beta stra nd 5A Lys residue, believed to anchor beta strand 5A to other secondary str uctural elements, had differential effects on the rates of latency transiti on in the absence and the presence of vitronectin, respectively. An overlap ping, but not identical set of substitutions resulted in an increased tende ncy to substrate behaviour of PAI-1 at reaction with its target proteinases . These findings show that vitronectin regulates the movements of the RCL t hrough conformational changes of the shutter region and beta strand 5A. are in agreement with RCL insertion proceeding by different routes during late ncy transition and complex formation, and contribute to the biochemical bas is for the potential use of PAI-1 as a therapeutic target in cancer and car diovascular diseases.