Siglecs: A family of sialic-acid binding lectins

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Abstract
It used to be thought that immunoglobulin superfamily members (other than antibodies) are primarily involved in protein : protein interactions, and do not recognize carbohydrates. In recent years, several immunoglobulin-like proteins have been described that can recognize biologically relevant glycans. These have now been given the generic name of "I-type lectins" (Powell and Varki, 1995, J. Biol. Chem., 270, 1423–1426). Amongst the I-type lectins, there is a very distinctive subfamily of sialic-acid binding cell surface receptors, that share clear-cut structural and functional similarities. This group of molecules currently comprises sialoadhesin, CD22, CD33, myelin associated glycoprotein (MAG) and Schwann cell myelin protein (SMP) (Crocker et al., 1996, Biochem. Soc. Trans. 24, 150–156). They are all integral membrane proteins with extracellular domains consisting of unusual N-terminal V-set Ig domains, followed by variable numbers of C2-set Ig domains ranging from 16 in sialoadhesin to 1 in CD33. Where known, the gene structures are similar and the genes are likely to have arisen by gene duplication. Each protein is expressed in a highly restricted fashion and is therefore likely to be involved in discrete functions. Thus, sialoadhesin is restricted to macrophages, CD22 to B cells, MAG and SMP to glial cells, and CD33 to myelomonocytic cells.

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