The role of valence on the high-affinity binding of Griffonia simplicifolia isolectins to type A human erythrocytes

The Griffonia simplicifolia-I (GS-I) isolectins have been used to probe the effect of lectin valence on their high-affinity binding to human erythrocy tes. These tetrameric lectins are composed of A and B subunits and constitu te a series of five isolectins (A(4), A(3)B, A(2)B(2), AB(3), B-4) The A su bunit is specific for alpha-D-GalNAc end groups and binds to the blood type A determinant GalNAc alpha 1, as well as to terminal alpha-D-Gal groups fo und on type B cells. The B subunit is specific for alpha-D-Gal end groups, and binds very specifically to type B erythrocytes. This series of isolecti ns is tetravalent (A4), trivalent (A3B), divalent (A2B2), and monovalent (A B3) for type A erythrocytes; thus, this system provides the opportunity to examine the effect of lectin valency on the association constants of these GS-I isolectins binding to cells. Cell binding experiments carried out usin g I-125-labeled GS-I isolectins and type A human erythrocytes allowed us to demonstrate that (1) the association constant of the isolectin monovalent for alpha-D-GalNAc (AB3) is virtually identical to its association constant for the haptenic sugar methyl-N-acetyl-alpha-D-galactosaminide, reported p reviously, and (2) the association constant of the GS-I isolectins for huma n type A erythrocytes increases with increasing valency of the isolectin. T hese results indicate that the increased affinity displayed by the GS-I iso lectins for human type A erythrocytes is dependent on their multivalency, a nd not on an extended binding site nor on nonspecific, or noncarbohydrate, interactions of the lectin with the cell surface. These findings should be of general relevance to understanding the high-affinity interactions observ ed between other multivalent proteins and multivalent ligands (e.g., cell s urfaces).