Primary Structure and Carbohydrate Binding Specificity of a Potent Anti-HIV Lectin Isolated from the Filamentous Cyanobacterium Oscillatoria agardhii

The primary structure of a lectin,designated OAA, isolated from thefreshwater cyanobacterium, Oscillatoriaagardhii NIES-204, was determined by thecombination of Edman degradation andESI-mass spectrometry. OAA is apolypeptide (MW 13,925) consisting of twotandem repeats. Interestingly, each repeatsequence of OAA showed a high degree ofsimilarity to those of a myxobacterium,Myxococcus xanthus hemagglutinin(MBHA), and a marine red alga Eucheumaserra lectin (ESA-2). A systematic bindingassay with pyridylaminated oligosaccharidesrevealed that OAA exclusively binds to highmannose (HM) type N-glycans, but not toother N-glycans, including complex types,hybrid types and the pentasaccharide core,or oligosaccharides from glycolipids. OAAdid not interact with any of free mono-andoligomannoses that are constituents of thebranched oligomannosides. These resultssuggest that the core disaccharide, GlcNAc-GlcNAc, is also essential for binding to OAA.The binding activity of OAA to HMtype N-glycanswas dramatically decreased whenα1-2 Man was attached to α1-3 Manbranched from the α1-6 Man of thepentasaccharide core. This specificity ofOAA for HM type oligosaccharides isdistinct from other HM-binding lectins.Kinetic analysis with an HMheptasaccharide revealed that OAApossesses two carbohydrate-binding sitesper molecule, with an association constant of2.41×10^8M^-1. Furthermore, OAA potentlyinhibits HIV replication in MT-4 cells(EC50=44.5 nM). Thus, we have found anovel lectin family sharing similar structureand carbohydrate binding specificity amongbacteria, cyanobacteria, and marine algae