Recognition factors of Ricinus communis agglutinin 1 (RCA(1))

Ricinus communis agglutinin (RCA1) is one of the most important applied lectins that has been widely used as a tool to study cell surfaces and to purify glycans. Although the carbohydrate specificity of RCA1 has been described, the information obtained was mainly focused on inhibition of simple Galbeta1-related oligosaccharides and simple clusters. Here, all possible recognition factors of RCA1 of glycan binding were examined by enzyme-linked lectinosorbent (ELLSA) and inhibition assays, using known mammalian Gal/GalNAc carbohydrate structural units and natural polyvalent glycans. Among the glycoproteins (gps) tested and expressed as 50% nanogram inhibition, the high-density polyvalent Galbeta1-4GlcNAc (II) glycotopes occurring in natural gps, such as Pneumococcus type 14 capsular polysaccharide which is composed of repeating poly II residues, resulted in 9.0 x 10(4), 1.5 x 10(5), 2.3 x 10(4) and 2.1 x 10(4)-fold higher affinities to RCA1 than the monomeric Gal, linear I/II and Tri-antennary-II (Tri-II). Of the ligands tested and expressed as nanomoles of 50% inhibition, Tri-II was the best, being about 2, 4, 25.6 and 33.3 times better inhibitor than Di-II, II, I (Galbeta1-3GlcNAc) and Gal, respectively. From the results of this study, it is concluded that: (a) Galbeta1-4GlcNAc and other Galbeta1-related oligosaccharides are essential for lectin binding and their polyvalent form in macromolecules should be the most important recognition factor for RCA1; (b) the combining site of RCA1 may be a groove type, recognizing Galbeta1-4GlcNAc (II) as the major binding site; (c) its combining size may be large enough to accommodate a tetrasaccharide of beta-anomeric Gal at the non-reducing end and most complementary to human blood group I Ma active trisaccharide (Galbeta1-4GlcNAcbeta1-6Gal) and lacto-N-neotetraose (Galbeta1-4GlcNAcbeta1-3Galbeta1-4Glc); (d) RCA1 has a preference for the beta-anomer of Gal oligosaccharides with a Galbeta1-4 linkage > Galbeta1-6 > or = Galbeta1-3; (e) configuration of carbon-2, -3 -4 and -6 in Gal are essential for binding; (f) hydrophobic interaction in the vicinity of the binding site useful for sugar accommodation increases affinity. These results should be helpful for understanding the functional role of RCA1 and for characterizing glycotopes of mammalian complex carbohydrates.