ABSTRACT Glycosylation represents one of the most frequent post translational modifications of proteins. The cell surface oligosaccharide structures mediate many cellular interactions and undergo profound modifications during cell differentiation and neoplastic transformation. Sialic acid is a negatively charged sugar which often terminates the oligosaccharide chains, usually linked α2, 3- or α2, 6-to galactose or α2, 6 to N-acetylgalactosamine. This review summarizes the current knowledge on the biology of the oligosaccharide epitope formed by sialic acid α2, 6-linked to lactosaminic structures and the biochemistry of the cognate sialytransferase; β–galactoside α2, 6-sialytransferase (ST6Gal.I). The expression of this enzyme is regulated by multiple transcriptional and post transcriptional regulatory mechanisms which allow its tissue- and stage- specific modulation. The sialyl-α2, 6-lactosaminyl-linkage is oncodevelopmentally regulated in several tissues and probably plays a role in colon cancer progression, α2, 6-sialylated lactosamic structures form the B cell-specfic antigen CDw75 and are the ligand of the mammalian lectin CD22. In mice, the expression of the sialyl-α2, 6-lactosaminyl-linkage is required for a proper development of the immune system.
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