THE AMINO-TERMINAL IMMUNOGLOBULIN-LIKE DOMAIN OF SIALOADHESIN CONTAINS THE SIALIC-ACID BINDING-SITE - COMPARISON WITH CD22

Sialoadhesin and CD22 are members of a recently characterized family o f sialic add dependent adhesion molecules belonging to the immunoglobu lin superfamily. Sialoadhesin is a macrophage-restricted receptor cont aining 17 extracellular Ig-like domains which recognizes oligosacchari des terminating in NeuAc alpha 2-3Gal in N- and O-linked glycans, CD22 is a B cell-restricted receptor with seven Ig-like domains which sele ctively recognizes oligosaccharides terminating in NeuAc alpha 2-6Gal in N-glycans, Sequence similarity between these proteins is highest wi thin their first four amino-terminal Ig-like domains. Here we identify the domain(s) containing the binding sites of both molecules by gener ating a series of extracellular domain deletion mutants fused to the F c portion of human IgG1. Binding activity was analyzed by solid phase cell adhesion assays and also by surface plasmon resonance using purif ied glycophorin and CD45 as Ligands for sialoadhesin and CD22, respect ively. For sialoadhesin, the amino-terminal V-set Ig-like domain was b oth necessary and sufficient to mediate sialic acid-dependent adhesion of the correct specificity. In contrast, for murine CD22, only constr ucts containing both the V-set domain and the adjacent C2-set domain w ere able to mediate sialic acid-dependent binding. These results are c onsistent with the sialic acid binding site for both proteins residing in the membrane distal V-set domain, but for CD22 a direct contributi on in binding from the neighboring C2-set domain cannot be excluded.