Both lysine-clusters of the NH2-terminal prion-protein fragment PrP23-110 are essential for t-PA mediated plasminogen activation

We have recently shown that the NH2-terminal fragment (PrP23-110) of the human cellular prion protein (PrPc) stimulates t-PA mediated plasminogen activation. PrP23-110 contains an N-terminal lysine cluster (LCI; K-23, K-24, K-27) and a C-terminal one (LC2; K-101, K-104, K-106, K-110). To study their biological function we have substituted all lysine residues of each cluster by alanine and generated the recombinant PrP proteins PrP23-110sLCI and, PrP23-110sLC2. The ability of the mutant proteins to stimulate plasminogen activation was assayed. We found that both lysine clusters are essential for t-PA mediated plasminogen activation. We further studied the binding of soluble PrP23-110 to immobilized t-PA or plasminogen using surface plasmon resonance. The recorded binding curves could not be modeled by classical 1:1 binding kinetics suggesting oligomerisation of PrP23-110. Further plasmon resonance studies show that indeed PrP23-110 binds to itself and that glycosaminoglycans modify this interaction. Binding of t-PA or plasminogen to PrP23-110 was no longer influenced by glycosaminoglycans when PrP23-110 was immobilized on the chip surface. Thus a possible role of heparin as a cofactor in the stimulation of plasminogen activation by t-PA could be the generation of a PrP23-110 form with both lysine clusters accessible for binding of t-PA and plasminogen.