C O N T E N T S
Siglecs are a group of adhesion receptors with tyrosine-based inhibitory signaling motifs. They are mainly expressed by cells of the hematopoietic system. Recently new members of the Siglec family were identified both in the human and the mouse. These comprise a CD33-related subgroup. All Siglecs specifically recognize sialic acids that are broadly expressed on many cell surfaces. These properties suggest that Siglecs may be involved in cell-cell interactions resulting in intracellular inhibitory signals.(1)
I-type lectins are a family of carbohydrate-binding proteins within the immunoglobulin superfamily. Included are a group of sialic-acid-binding lectins (the Siglecs) and several non-sialic-acid-binding lectins.
A very significant feature of the Siglecs is their recognition of sialic-acid-containing structures in a linkage-specific manner. CD22 is absolutely specific for α2-6-linked sialic acid residues and MAG, Schwann cell myelin protein, and CD33, for α2-3-linked sialic acid residues; sialoadhesin can recognize α2-3-linked and α2-8-linked residues. The initial analysis of Siglec-5 using synthetic sialosides indicates that it is capable of binding to α2-3- and α2-6-linked sialic acid residues.
Chromosomal localization studies indicate that CD22, CD33, Siglec-5, and MAG are clustered on human chromosome 19q and the syntenic murine chromosome 7. Twenty related genes of the IgSF, including members of the carcinoembryonic antigen and pregnancy-specific glycoprotein families, are also found in close proximity, suggesting their evolutionary origin by gene duplication. Sialoadhesin maps to murine chromosome 2 and human chromosome 20 (determined using the murine sialoadhesin as a probe), and thus is unlinked to the other Siglecs in both species. The presence of other Siglecs in proximity to the sialoadhesin gene has yet to be explore
Compared to the other Siglecs, sialoadhesin binds to the widest array of sialylated structures. Early studies demonstrated binding to α2-3 sialic acid residues, including Siaα2-3Galβ1-3GalNAc and Siaα2-3Galβ1-3(4)GlcNAc. These structures are the products of several different α2-3-specific sialyltransferases and can be found on glycolipids and N-linked and O-linked chains on glycoproteins . More recently, sialoadhesin has been demonstrated to bind to gangliosides carrying terminal α2-8 sialic acid residues such as GD3 and GD1b. Thus, potential ligands for sialoadhesin are likely to be found on virtually all cell types. However, sialoadhesin (expressed on COS cells, macrophages, or as an Sn-Rg [sialoadhesion-receptor globulin] construct) shows marked preference for binding to granulocytes and erythroblasts over other cell types. (2)
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