A small region of the natural killer cell receptor, Siglec-7, is responsible for its preferred binding to α2,8-disialyl and branched α2,6-sialyl residues -: A comparison with Siglec-9

Siglec-7 is a sialic acid-binding lectin recently identified as an inhibitory receptor on natural killer cells. Here we characterize the sugar-binding specificity of Siglec-7 expressed on Chinese hamster ovary cells using polyvalent streptavidin-based glyco-probes. Glycoprobes carrying unique oligosaccharide structures such as GD3 (NeuAcalpha2,8NeuAcalpha2,3Galbeta1,4Glc) and LSTb (Galbeta1,3[NeuAcalpha2,6]GlcNAcbeta1,3Galbeta1,4Glc) oligosaccharides bound to Siglec-7 better than those carrying LSTc (NeuAcalpha2,6Galbeta1,4GlcNAcbeta1,3Galbeta1,4Glc) or GD1a (NeuAcalpha2,3 Galbeta1,3GalNAcbeta1,4[NeuAcalpha2,3]Gal-beta1,4Glc) oligosaccharides. In contrast, Siglee-9, which is 84% identical to Siglec-7, did not bind to the GD3 and LSTb probes but did bind to the LSTc and GD1a probes. To identify a region(s) responsible for their difference in binding specificity, we prepared a series of V-set domain chimeras between Siglecs-7 and -9. Substitution of a small region, Asn(70)-Lys(75), of Siglec-7 with the equivalent region of Siglec-9 resulted in loss of Siglec-7-like binding specificity and acquisition of Siglec-9-like binding properties. In comparison, a Siglec-9-based chimera, which contains Asn(70)-Lys(75) with additional amino acids derived from Siglec-7, exhibited Siglec-7-like specificity. These results, combined with molecular modeling, suggest that the C-C' loop in the sugar-binding domain plays a major role in determining the binding specificities of Siglecs-7 and -9.