Biochemistry and Biology of the Siglec Family

For many years evidence has accumulated that sialic acids function in cellular interactions either in masking or as recognition site. However, receptors or adhesion molecules mediating such functions between eukaryotic cells were unknown until about 5 years ago, when it was found that the members of the Selectin family mediate adhesion of leukocytes to specific endothelia through binding to sialylated glycans like sialyl Lewis^x. More recently, the Sialoadhesin family, now called siglecs, of sialic acid-dependent adhesion molecules was defined within the superfamily of immunoglobulin-like molecules. So far, it has been shown that sialoadhesin (Sn), CD22, CD33, the myelin-associated glycoprotein (MAG) and the Schwann cell myelin protein (SMP) belong to this family. In contrast to the selectins, these proteins are associated with diverse biological processes, i.e. hemopoesis, neuronal development and immunity.

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Introduction to the siglec family

myelin-associated glycoprotein (MAG or CD22)

Binding of resialylated human erythrocytes to COS cells expressing sialoadhesin (Sn),

myelin-associated glycoprotein (MAG or CD22).

Human erythrocytes (native) were sialidase treated to remove cell surface sialic acids of all linkages (asialo) followed by resialylation with purified sialyltransferases. These resialylated erythrocytes contained only Neu5Ac α2,3-linked on O-glycans (3-O), α2,3-linked on N-glycans (3-N) or α2,6-linked on N-glycans (6-N). Clusters of bound eryhtrocytes are marked with white arrows.

Methods

Sialidase treatment of cells

Adhesion assay with immobilised Fc-chimeras

Iodination of proteins