Sialic acid recognition by sialoadhesin and related lectins

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.