ANALYSES OF CARBOHYDRATE-RECOGNITION BY LEGUME LECTINS - SIZE OF THE COMBINING SITE LOOPS AND THEIR PRIMARY SPECIFICITY

Recognition of cell-surface carbohydrates by lectins has wide implicat ions in Important biological processes. The ability of plant lectins t o detect subtle variations in carbohydrate structures found on molecul es, cells and organisms have made them a paradigm for protein-carbohyd rate recognition. Legume lectins, one of the most well studied family of plant proteins, display a considerable repertoire of carbohydrate s pecificities owing perhaps to the sequence hypervariability in the loo ps constituting their combining site. However, lack of a rigorous fram ework to explain their carbohydrate binding specificities has preclude d a rational approach to alter their ligand binding activity in a mean ingful manner. This study reports an extensive analysis of sequences a nd structures of several legume lectins and shows that despite the hyp ervariability of their combining regions they exhibit within a signifi cant pattern of uniformity. The results show that the size of the bind ing site loop D is invariant in the Man/Glc specific lectins and is po ssibly a primary determinant of the monosaccharide specificities of th e legume lectins. Analyses of size and sequence variability of loops r eveal the existence of a common theme that subserves to define their b inding specificities. These results thus provide not only a framework for understanding the molecular basis of carbohydrate recognition by l egume lectins but also a rationale for redesign of their ligand bindin g propensities. (C) 1997 Academic Press Limited.