Effect of stabilizing versus destabilizing interactions on plasminogen activator inhibitor-1

Plasminogen activator inhibitor-1 (PAI-1) is a unique member of the serpin family, as it spontaneously converts into a latent conformation. However, the exact mechanism of this conversion is not known. Previous studies reported that neutralizing monoclonal antibodies as well as reversal or removal of charges on the s3C-s4C turn results in a destabilization of PAI-1 leading to an accelerated conversion to its latent form. In this study the effect of the reversal or removal of charges in this "gate region" (R186E/R187E, H190E/K191E, H190L/K191L and R356E) on a stable PAI-l-variant (PAI-l-stab) was investigated. Whereas PAI-l-stab has a half-life of 150 +/- 66 h, PAI-1-stab-R186E-R187E, PAI-1-stab-H190E-K191E, PAI-1-srab-H190L-K191L and PAI-1-stab-R356E have a strongly decreased half-life (p < 0.005 versus PAI-1-stab) of 175 +/- 48 min, 75 +/- 34 min, 68 +/- 38 min and 79 +/- 16 min, respectively. Wild-type PAI-1 (wtPAI-1) had a half-life of 55 +/- 19 min. These data indicate that the stabilization induced by the mutated residues in PAI-1-stab is counteracted by the additional mutations, resulting in half-lives similar to that of wtPAI-1, thereby suggesting that the stabilizing and destabilizing forces act mainly independently in these mutants. Extrapolation of these data to other (stable) serpins leads to the hypothesis that the s3C-s4C turn and the distal hinge region of the reactive site loop plays a role for the stability of serpins in general.