The length of the reactive center loop modulates the latency transition of plasminogen activator inhibitor-1.

Plasminogen activator inhibitor-1 (PAI-1) belongs to the serine protease inhibitor (serpin) protein family, which has a common tertiary structure consisting of three beta-sheets and several alpha-helices. Despite the similarity of its structure with those of other serpins, PAI-1 is unique in its conformational lability, which allows the conversion of the metastable active form to a more stable latent conformation under physiological conditions. For the conformational conversion to occur, the reactive center loop (RCL) of PAI-1 must be mobilized and inserted into the major beta-sheet, A sheet. In an effort to understand how the structural conversion is regulated in this conformationally labile serpin, we modulated the length of the RCL of PAI-1. We show that releasing the constraint on the RCL by extension of the loop facilitates a conformational transition of PAI-1 to a stable state. Biochemical data strongly suggest that the stabilization of the transformed conformation is owing to the insertion of the RCL into A beta-sheet, as in the known latent form. In contrast, reducing the loop length drastically retards the conformational change. The results clearly show that the constraint on the RCL is a factor that regulates the conformational transition of PAI-1.