Luminal lectins

Abstract

Asparagine-linked glycosylation (N-glycosylation) isa major post-translational modification of secretory and membrane proteins and influences important physical protein properties such as conformation, stability and solubility (Helenius and Aebi 2001). The majority of proteins that enter the secretory pathway receives multiple N-linked glycans. N-glycosylation is initiated cotranslationally in the lumen of the endoplasmic reticulum (ER) by oligosacchary ltransferase. This multisubunit protein complex scans nascent proteins for N-glycosylation consensus sequences (Asn-X-Ser/Thr) and catalyzes the transfer of a 14-saccharide core glycan to the asparagine residue (Fig. 1). About two-thirds of all consensus sites are glycosylated. After conjugation to the protein, the 14-saccharide core is trimmed in ERand Golgi by glycosidases and extended in the Golgi by glycosyltransferases (Kornfeld and Kornfeld 1985). ER glucosidases I and II remove the three glucose (Glc) residues, whereas ER a1,2 mannosidase I and Golgi α1,2 mannosidases 1A, 1B and 1C trim the α1,2-linked mannoses (Man). In the Golgi, two additional Man residues are cleaved and the N-glycans undergo complex glycosylation by the addition of N-acetylglucosamine (GlcNAc), fucose, galactose and sialic acid residues. After traversing the Golgi, glycoproteins carry various N-linked glycans differing in composition and structure. This heterogeneity allows mature glycoproteins to fulfill a plethora of functions including the presentation of interaction sites for other molecules (Varki 1993). In contrast, nascent glycoproteins in the early secretory pathway, termed high-mannose glycoproteins, display only few but distinct oligosaccharide structures which function as recognition tags for different sugar-binding proteins (lectins). With their carbohydrate recognition domain (CRD), the lectins bind newly synthesized glycoproteins and control their folding, degradation, transport and sorting. Here, we provide an overview of the different animal lectins localized to the lumen of the secretory pathway (Table 1) and describe them grouped according to their proposed function.