Protein Glycosylation in Higher Plants: Recent Developments

Abstract

Within the last few years, very rapid progress has been made in the investigation of the biosynthesis of the most common type of glycoproteins, i.e., those having an N-glycosidic link between the oligosaccharide(s) and the amide nitrogen of asparagine residues on the protein. Pathways that involve the use of glycolipid intermediates have been delineated, and new, unexpected, phenomena such as oligosaccharide processing have been discovered. In this chapter, we shall review developments in this area with respect to higher plants, concentrating on the transfer of sugars to protein rather than on details of the synthesis of glycolipid precursors. An excellent review on the formation of lipid-linked sugars in plants and their functions in glycoprotein synthesis was recently published by Elbein (1980). The reader interested in the synthesis of O-glycosidically linked oligosaccharides of plant glycoproteins is referred to papers by Karr (1972) on the glycosylation of extensin and Soliday and Kolattukudy (1979) on the glycosylation of fungal cutinase as examples. We shall also exclude from this chapter the protein-linked carbohydrates proposed as primers or intermediates in polysaccharide synthesis. The existence, structure, and functioning of these are still controversial issues, and evidence is limited [for example, their consideration with respect to cellulose synthesis will be found in the paper by Hopp et al. (1978) and for starch synthesis in the paper by Lavintman et al., (1974)].