A SPECTROSCOPIC STUDY OF THE STRUCTURES OF LATENT, ACTIVE AND REACTIVE-CENTER-CLEAVED TYPE-1 PLASMINOGEN-ACTIVATOR INHIBITOR

Type-1 plasminogen-activator inhibitor (PAI-1) was studied by Fourier- transform infrared spectroscopy, far-ultraviolet CD spectroscopy, and fluorescence-emission spectroscopy, with the aim to obtain structural information about its active form. The spectra of latent, active and r eactive-center-cleaved forms of PAI-1 produced by HT-1080 cells were d ifferent. While the cleaved and the latent forms were similar with reg ard to their beta-structure content, comparison of the spectra of thes e forms with the spectra of active PAI-1 suggested a much higher degre e of unordered structure for the active form compared with the latent and reactive-center-cleaved forms than previously assumed. We discuss our results with reference to the known three-dimensional X-ray struct ures of latent PAI-1, of reactive-center-cleaved serpins, including re active-center-cleaved PAI-1, and of intact serpins, and with reference to previous results on the differences in the affinity of mAbs for th e different PAI-1 forms. We interpret our results in favor of a global rearrangement of secondary structure during latency transition and re active-center cleavage in PAI-1, not only involving the reactive-cente r loop and parts of beta-sheets A and C, but also the 'rear' side of t he molecule, such as helices H and G. Thus, we suggest flexibility in serpin structural elements that were previously regarded as rigid.