AVIAN UROKINASE-TYPE PLASMINOGEN-ACTIVATOR (U-PA) LACKS THE PUTATIVE BINDING-SITE FOR PLASMINOGEN-ACTIVATOR INHIBITOR (PAI) AND IS RESISTANT TO INHIBITION BY HUMAN PAI-1 AND PAI-2

In mammalian cultures, the activation and catalytic activity of both u rokinase-type plasminogen activator (u-PA) and tissue-type PA (t-PA) i s regulated in part by naturally occurring plasminogen activator inhib itors PAI-1 and PAI-2, members of the serpin family of serine protease inhibitors, Interaction of human PAIs with human u-PA (h-u-PA) and t- PA appears to be mediated by a putative PAI binding sequence in the en zymes, Comparison of the amino acid sequences of human, murine, porcin e, bovine, simian and avian u-PA revealed specific sequences in the ma mmalian enzymes that are homologous to the recently discovered PAI-bin ding site in human u-PA and t-PA which is characterized by conserved b asic amino acids, However, no homology was seen in the same region in chicken u-PA (c-u-PA), which is devoid of the consensus arginine and/o r lysine residues. The chicken enzyme is a close structural and cataly tic homolog of mammalian u-PA, The lack of the putative PAI binding se quence would predict that c-u-PA cannot interact with and be inhibited by human PAIs. To test this hypothesis, the effects of human PAI-1 an d PAI-2 on avian u-PA were examined, using purified enzyme and inhibit ors, Under conditions where h-u-PA activity is inhibited by human reco mbinant PAI-1 (rPAI-1), c-u-PA retains 95% activity, c-u-PA was also r esistant to inhibition by human PAI-2, High molecular weight, SDS-stab le complexes do not form readily when c-u-PA is incubated with PAI-1 o r PAI-2, while human as well as murine u-PA formed complexes with both PAIs under the identical conditions, The c-u-PA molecule, thus, is a naturally occurring u-PA variant which lacks a PAI-binding region. The resistance of this enzyme to inhibition by human PAI-1 and PAI-2 supp orts the functional relevance of this sequence in mammalian u-PA and t -PA.