Review TRENDS in Biotechnology Vol 22 No 10 October 2004 Sugar mediated ligand receptor interactions in the immune system Pauline M Rudd Mark R Wormald and Raymond A Dwek Oxford Glycobiology Institute Department of Biochemistry University of Oxford South Parks Road Oxford OX1 3QU UK Most molecules involved in the recognition and elimination of pathogens by the immune system are glycoproteins Oligosaccharides attached to glycoproteins initiate biological functions through mechanisms that involve multiple interactions of the monosaccharide residues with receptors For example calreticulin a quality control lectin like chaperone interacts with glucosylated mannose glycans presented by empty major histocompatibility complex MHC class I molecules retaining them in the endoplasmic reticulum ER until antigenic peptide is loaded Clusters of specific IgG glycoforms present in increased amounts in rheumatoid arthritis bind mannose binding lectin MBL providing a potential route to inflammation through activation of the complement pathway Secretory IgA glycans bind gut bacteria and an unusual cluster of mannose residues on gp120 the surface coat protein of the HIV virus is recognized by the novel domainswapped IgG 2G12 serum antibody Glycosylation is essential for life Almost all organisms including fungi yeast plants insects fish birds and mammals glycosylate most of their secreted and cellsurface proteins In addition viruses which have no glycosylation machinery of their own attach sugars to their envelope proteins by exploiting the biosynthetic pathways of their hosts The essential role of glycosylation in viability has been demonstrated in knockout mice The Mgat1 gene encodes N acetylglucosamine transferase I an enzyme required in the processing of complex type sugars Box 1 When Mgat1 was deleted in mice the offspring died after 9 days in embryo as a result of defects in vascularization and neural tube closure 1 Similarly deletion of the Mgat2 gene encoding N acetylglucosamine transferase II which is also necessary for processing complex type sugars caused frequent postnatal lethality and 99 of the mice died within the first week of birth 2 In the immune system oligosaccharides attached to both host and pathogenic proteins have many roles that are both structural and functional Box 2 In this review we discuss some of these roles focusing in particular on recognition events in which oligosaccharides generally initiate biological functions through mechanisms that involve multiple interactions of the monosaccharide residues with receptors This requirement for multivalency is a vital means of ensuring that physiological Corresponding author Pauline M Rudd pmr glycob ox ac uk Available online 14 August 2004 consequences generated by the activation of signalling pathways are not triggered inadvertently by single weak interactions but only in response to a strong stimulus Although individual monosaccharides have low affinity for protein receptors usually in the millimolar to micromolar range two mechanisms enable oligosaccharide ligands to interact with their receptors with high affinity or avidity The first mechanism requires the presence of subsites in the receptor each of which can accommodate a monosaccharide residue with very different affinities An example of such a receptor is the quality control lectin like chaperone calnexin which can interact with a tetrasaccharide epitope presented by an unfolded glycoprotein thereby retaining the glycoprotein in the ER see below The second mechanism depends on the presentation of multiple oligosaccharides in such a way that the sugars can interact with several carbohydrate recognition sites on the receptor An example of this mechanism is the functional binding of the carbohydrate recognition domains of MBL to specific aggregated IgG glycoforms that present arrays of terminal N acetylglucosamine residues see below Insights into some of the ligand receptor interactions that involve glycoproteins in the immune system have come from combining glycan analysis protein structural data and biochemical data with molecular modelling In general oligosaccharides confer stability on glycoproteins and protect them from proteases and nonspecific protein protein interactions but as we review here some oligosaccharides form specific recognition epitopes that have functional consequences on receptor binding Roughly 80 of secreted and cell surface proteins are glycosylated In the ER N linked sugars are usually attached co translationally to partially folded proteins through the side chains of asparagine residues contained in the consensus sequence Asn Xaa Ser Thr where Xaa is any amino acid except proline The fully folded proteins are transported to the Golgi where N linked glycans are further processed to complex type oligosaccharides and O glycans can be added to accessible side chains of serine or threonine residues 3 4 Notably molecular modelling of both proteins and their attached oligosaccharides provides graphic illustrations of the finding that the sugars frequently occupy a larger three dimensional space than do the protein domains to which they are attached Although glycoproteins usually consist of an ensemble of glycosylated www sciencedirect com 0167 7799 see front matter Q 2004 Elsevier Ltd All rights reserved doi 10 1016 j tibtech 2004 07 012 Review TRENDS in Biotechnology Vol 22 No 10 October 2004 525 Box 1 Glycoproteins comprise a mixture of glycosylated variants glycoforms The covalent attachment of sugars glycosylation is a common posttranslational modification of both secreted and cell surface proteins and one on which the cell expends a significant amount of energy About 30 different enzymes are required to attach and to process the oligosaccharides linked to asparagine N linked glycans and to serine or threonine O linked glycans residues Figure I N linked sugars are attached to proteins in the endoplasmic reticulum ER where a preformed oligosaccharide precursor attached to dolichol phosphate is transferred to some asparagine residues that are part of the motif Asn Xaa Ser Thr where Xaa is any amino acid except proline Further processing of the sugars on the nascent glycoprotein takes place first in the ER and then in the Golgi An important intermediate in the ER is the monoglucosylated glycan Glc1Man9 7GlcNAc2 which provides the folding protein with the means of entering the calnexin calreticulin quality control pathway O linked sugars are added