ROLE OF CARBOHYDRATE AND PROTEIN IN THE BINDING OF TISSUE-TYPE PLASMINOGEN-ACTIVATOR TO THE HUMAN MANNOSE RECEPTOR

The 175-kDa mannose receptor is one of the receptors that mediates the clearance of tissue-type plasminogen activator (t-PA). The affinity o f t-PA for the mannose receptor is much higher than the affinity of ot her high-mannose-type oligosaccharide-containing glycoproteins. In ord er to find an explanation for this high affinity, we studied the bioch emical interaction of various forms of t-PA with the isolated human ma nnose receptor in several in vitro binding assays. t-PA showed a high affinity (K-i = 0.2 nM) for the mannose receptor and the interaction c ould be fully inhibited by mannan or polyclonal antibodies against the mannose receptor. The interaction was not affected by non-glycosylate d t-PA. The high affinity differed slightly between t-PAs synthesized by various cell types (range K-i 0.2-0.7 nM) and between various glyco forms of t-PA. No statistically significant difference in affinity bet ween t-PA and t-PA complexed to inhibitors was observed. In contrast t o intact t-PA, a trypsin digest of t-PA had a low affinity (K-i = 0.5 mu M) for the mannose receptor. Both intact and trypsin digests of the high-mannose-type oligosaccharide-containing glycoproteins ribonuclea se B and ovalbumin had a low affinity (K-i 0.5 - 1.5 mu M) for the man nose receptor. We conclude that neither protein-protein interactions, nor the complex-type oligosaccharides and the fucose residue on t-PA c ontribute significantly to the high-affinity binding of t-PA. We sugge st that the conformation of the high-mannose-type oligosaccharide on t -PA is influenced by the protein moiety of t-PA in such a way that the oligosaccharide has a high affinity for the mannose receptor.