Roles of mammalian structural units, ligand cluster and polyvalency in the Abrus precatorius agglutinin and glycoprotein recognition process

Abstract

Previous studies on one of the toxic type 2 ribosome inactivating proteins (RIP), Abrus precatorius agglutinin (APA), have shown that the recognition domains of APA are restricted to monomers of Galβ1-3GalNAc ( T, Thomsen-Friedenreich glycotope) and Galβ1-3/4GlcNAc (blood group precursor type I/ II sequences); which are essential but play a minor role in the recognition process. In this study, APA recognition factors were expanded to include ligand clusters and polyvalent glycotopes by enzyme-linked lectinosorbent binding and inhibition assays. Based on the results of molar relative potency, the essential mammalian structural units are Galβ1-3GalNAcα/β1- ( T α/ T β) > Galα1-4Gal ( E) > Galβ1-3/4GlcNAc ( I/ II) and avidity for tri-/di-antennary II β, T, E and II monomers was found to be 7.1 × 10 2, 4.0, 5.5, 3.7 and 2.4 times higher than monomeric Gal. Among natural polyvalent glycotopes or clusters, high-density polyvalent T α and complex multivalent I β/ II β glycotopes greatly enhanced the affinity for APA over 10 4 times. Based on these results, it is concluded that contribution of monomeric T α, II β, I β, E β and their clusters and polyvalency play critical roles in this recognition process. The binding intensities of these factors in decreasing order are: polyvalent T α, II β/ I β and E β > tri-antennary II β ≫ monomeric T α, T β, I and II > Gal ≫ GalNAc (weak). As one of type 2 RIP lectins, these recognition factors of the B chain are likely to be crucial for attachment and endocytosis. A comparison of the differential recognition factors and combining sites of APA with those of other lectins ( Ricinus communis agglutinin, RCA 1 and ricin) is also illustrated.