The activation-resistant conformation of recombinant human plasminogen is stabilized by basic residues in the amino-terminal hinge region.

Fully activable recombinant human plasminogen (rPlg) was expressed in mammalian cells employing either recombinant vaccinia virus or stable lines coexpressing alpha 2-plasmin inhibitor. A panel of eight variants of rPlg was constructed, in which progressively up to 6 basic amino acid residues in the hinge region of rPlg between the NH2-terminal acidic domain ("proactivation peptide") and kringle 1 were substituted by neutral residues. Analysis of the cleavage rates of these variants by plasmin revealed that the peptide bond at Arg68 is most susceptible, followed by Lys62 and Lys77. A variant with all 6 basic residues substituted was cleaved at Lys20. Three of these variants, PlgB (R68A, R70A), PlgF (R68A, R70A, K77H, K78H), and PlgG (R61A, K62A, R68A, R70A, K77H, K78H), as well as rPlg, were analyzed in more detail. The conformation of these plasminogens was analyzed by monitoring the change in intrinsic fluorescence upon binding of lysine analogs. This revealed that rPlg exhibits the native tight Glu1-plasminogen conformation, whereas PlgB, PlgF, and Plg G display an open conformation similar to Lys78-plasminogen, leading to an increased affinity for lysine analogs. This allowed a direct study of the impact of the activation-resistant conformation on the properties of Glu1-plasminogen. The open conformation of rPlg variants leads to an increased rate of activation by urokinase-type plasminogen activator and streptokinase and increased binding to a fibrin clot. Fibrin clot lysis mediated by tissue-type plasminogen activator was accelerated for the variants as a result of a lower Km for tissue-type plasminogen activator-mediated plasminogen activation, resulting from the increased affinity of rPlg (variants) for intact fibrin. We conclude that the basic residues in the extremely plasmin susceptible hinge region of plasminogen are directly involved in maintaining the activation resistant Glu1-plasminogen conformation.