GARLIC (ALLIUM-SATIVUM) LECTINS BIND TO HIGH-MANNOSE OLIGOSACCHARIDE CHAINS

Two mannose-binding lectins, Allium sativum agglutinin (ASA) I (25 kDa ) and ASAIII (48 kDa), from garlic bulbs have been purified by affinit y chromatography followed by gel filtration. The subunit structures of these lectins are different, but they display similar sugar specifici ties. Both ASI and ASAIII are made up of 12.5- and 11.5-kDa subunits. In addition, a complex (136 kDa) comprising a polypeptide chain of 54 +/- 4 kDa and the subunits of ASAI and ASAIII elutes earlier than thes e lectins on gel filtration. The 54-kDa subunit is proven to be alliin ase, which is known, to form a complex with garlic lectins. Constituen t subunits of ASAI and ASAIII exhibit the same sequence at their amino termini. ASAI and ASAIII recognize monosaccharides in mannosyl config uration. The potencies of the ligands for ASAs increase in the followi ng order: mannobiose (Man alpha 1-3Man) < mannotriose (Man alpha 1-6Ma n alpha 1-3Man) approximate to mannopentaose much less than man(9)-oli gosaccharide. The addition of two GIcNAc residues at the reducing end of manotriose or mannopentaose enhances their potencies significantly, whereas substitution of both alpha 1-3- and alpha 1-6-mannosyl residu es of mannotriose with GlcNAc at the nonreducing end increases their a ctivity only marginally. The best manno-oligosaccharide ligand is Man( 9)GlcNAc(2)Asn, which bears several alpha 1-2-linked mannose residues. Interaction with glycoproteins suggests that these lectins recognize internal mannose as well as bind to the core pentasaccharide of N-link ed glycans even when it is sialylated. The strongest inhibitors are th e high mannose-containing glycoproteins, which carry larger glycan cha ins. Indeed, invertase, which contains 85% of its mannose residues in species larger than Man(20)GlcNAc, exhibited the highest binding affin ity. No other mannose- or mannose/glucose-binding lectin has been show n to display such a specificity.