O-LINKED PROTEIN GLYCOSYLATION STRUCTURE AND FUNCTION

There has been a recent resurgence of interest in the post-translation al modification of serine and threonine hydroxyl groups by glycosylati on, because the resulting O-linked oligosaccharide chains tend to be c lustered over short stretches of peptide and hence they can present mu ltivalent carbohydrate antigenic or functional determinants for antibo dy recognition, mammalian cell adhesion and microorganism binding. Co- operativity can greatly increase the affinity of interactions with ant ibodies or carbohydrate binding proteins. Thus, in addition to their k nown importance in bearing tumour associated antigens in the gastroint estinal and respiratory tracts, glycoproteins with O-linked chains hav e been implicated as ligands or co-receptors for selectins (mammalian carbohydrate binding proteins). Microorganisms may have adopted simila r mechanisms for interactions with mammalian cells in infection, by ha ving relatively low affinity ligands (adhesins) for carbohydrate bindi ng, which may bind with higher affinity due to the multivalency of the host ligand and which are complemented by other virulence factors suc h as interactions with integrin-type molecules. In addition to specifi c adhesion signals from O-linked carbohydrate chains, multivalent O-gl ycosylation is involved in determining protein conformation and formin g conjugate oligosaccharide-protein antigenic, and possible functional determinants.