Sialic Acid Recognition by Sialoadhesin and Related Lectins.

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Abstract
Sialic acids are abundant on the surface of mammalian cells where, through their negative charge, they assist in the prevention of undesirable cell-cell interactions. The recent discovery of the sialoadhesin subset of the immunoglobulin super-family, which includes sialoadhesin, CD22, myelin associated glycoprotein (MAG) and CD33, has raised the possibility that sialic acids are also instrumental in promoting cell-cell interactions in a variety of physiological systems. Each of these membrane proteins exhibits a distinct specificity for both the type of sialic acid recognised and its linkage to subterminal sugars. They share a high degree of sequence similarity within the NH2-terminal two Ig domains which also display a unique arrangement of conserved disulphide bonds. Domains 1 and 2 (numbering from the NH2-terminus) are likely to be representative of an ancestral gene that gave rise to the current members of the family through gene duplication. By generating truncated recombinant proteins, together with site-directed mutagenesis, the GFCC'C" faces of the NH2-terminal V-set domains of sialoadhesin and CD22 have been shown to contain the sialic acid binding site. Recently, this has been confirmed by X-ray crystallography which has led to the structural determination of the V-set domain of sialoadhesin complexed with a ligand, 3' sialyllactose. The similarities and differences in sialic acid recognition by sialoadhesin and other sialic acid binding proteins are discussed

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