Structural determinants of Ricinus communis agglutinin and toxin specificity for oligosaccharides.

Structural determinants of oligosaccharides affecting interaction with the saccharide-binding sites of two lectins, the agglutinin of M, 120,000 and toxin of 82, 60,000 (RCA* and RCA~I, respectively), isolated from Ricinus communis beans have been examined by performing binding studies with iodinated glycopeptides and their degradation products. Association constants can be determined by Scatchard plot analysis of the saturation curves obtained. Interaction with complex, Nacetylglucosamine-containing glycopeptides and with glycopeptides bearing Gal/?l,3GalNAc moieties is directed predominantly at the P-linked galactose residues for both RCA* and RCArr. The presence of sialic acid substituents on these galactose residues markedly reduces the association constants of both RCA1 and RCA11 for complex glycopeptides and completely abolishes binding of glycopeptides bearing Galj31,3GalNAc oligosaccharides. RCA1 and RCArI yield indistinguishable association constants for binding of tribranched, complex oligosaccharides with 3 galactose residues, whereas RCA1 binds dibranched structures and their degradation products with association constant values some 2to 3-fold higher than those obtained with RCAI1. In contrast, the association constants obtained for RCA11 binding of glycopeptides bearing 1, 2, or 4 Galjl1,3GalNAc disaccharides are all some 2.5to lofold higher than those obtained with RCAI. The ability of RCAIr to bind to Galpl,BGalNAc disaccharides appears to be markedly influenced by the relative locations of the disaccharides along the peptide backbone. The ability of either RCA1 or RCA11 covalently coupled to Sepharose 4B to bind iodinated glycopeptides correlates with association constant values greater than 5 X 10’ M-‘.