Cloning, characterization, and phylogenetic analysis of Siglec-9, a new member of the CD33-related group of Siglecs - evidence for co-evolution with sialic acid synthesis pathways

The Siglecs are a subfamily of I-type lectins (immunoglobulin superfamily p roteins that bind sugars) that specifically recognize sialic acids. We repo rt the cloning and characterization of human Siglec-9. The cDNA encodes a t ype 1 transmembrane protein with three extracellular immunoglobulin-like do mains and a cytosolic tail containing two tyrosines, one within a typical i mmunoreceptor tyrosine-based inhibitory motif (ITIM). The N-terminal V-set Ig domain has most amino acid residues typical of Siglecs. Siglec-9 is expr essed on granulocytes and monocytes, Expression of the full-length cDNA in COS cells induces sialic-acid dependent erythrocyte binding. A recombinant soluble form of the extracellular domain binds to alpha 2-3 and alpha 2-6-l inked sialic acids. Typical of Siglecs, the carboxyl group and side chain o f sialic acid are essential for recognition, and mutation of a critical arg inine residue in domain 1 abrogates binding. The underlying glycan structur e also affects binding, with Gal beta 1-4Glc[NAc] being preferred. Siglec-9 shows closest homology to Siglec-7 and both belong to a Siglec-3/CD33-rela ted subset of Siglecs (with Siglecs-5, -6, and -8), The Siglec-9 gene is on chromosome 19q13.3-13.4, in a cluster with all Siglec-3/CD33-related Sigle c genes, suggesting their origin by gene duplications. A homology search of the Drosophila melanogaster and Caenorhabditis elegans genomes suggests th at Siglec expression may be limited to animals of deuterostome lineage, coi ncident with the appearance of the genes of the sialic acid biosynthetic pa thway.