N-GLYCOPROTEIN BIOSYNTHESIS IN PLANTS - RECENT DEVELOPMENTS AND FUTURE-TRENDS

N-glycosylation is a major modification of proteins in plant cells. Th is process starts in the endoplasmic reticulum by the co-translational transfer of a precursor oligosaccharide to specific asparagine residu es of the nascent polypeptide chain. Processing of this oligosaccharid e into high-mannose-type, paucimannosidic-type, hybrid-type or complex -type N-glycans occurs in the secretory pathway as the glycoprotein mo ves from the endoplasmic reticulum to its final destination. At the en d of their maturation, some plant N-glycans have typical structures th at differ from those found in their mammalian counterpart by the absen ce of sialic acid and the presence of beta(1,2)-xylose and alpha(1,3)- fucose residues. Glycosidases and glycosyltransferases that respective ly catalyse the stepwise trimming and addition of sugar residues are g enerally considered as working in a co-ordinated and highly ordered fa shion to form mature N-glycans. On the basis of this assembly line con cept, fast progress is currently made by using N-linked glycan structu res as milestones of the intracellular transport of proteins along the plant secretory pathway. Further developments of this approach will n eed to more precisely define the topological distribution of glycosylt ransferases within a plant Golgi stack. In contrast with their acknowl edged role in the targeting of lysosomal hydrolases in mammalian cells , N-glycans have no specific function in the transport of glycoprotein s into the plant vacuole. However, the presence of N-glycans, regardle ss of their structures, is necessary for an efficient secretion of pla nt glycoproteins. In the biotechnology field, transgenic plants are ra pidly emerging as an important system for the production of recombinan t glycoproteins intended for therapeutic purposes, which is a strong m otivation to speed up research in plant glycobiology. In this regard, the potential and limits of plant cells as a factory for the productio n of mammalian glycoproteins will be illustrated.