The protein N-glycosylation in plants

larger antennae b(1, 2)-linked to the a(1, 6)or the a(1, 3)-mannose constitutive of the core have been called In plants, most proteins of the extracellular compartcomplex-type N-glycans, and the modified N-glycans ment and the endomembrane system are glycosylated having only an a(1, 3)-fucose and/or a b(1, 2)-xylose by N-linked oligosaccharides. The N-glycosylation of residues linked to the core 3–22 have been proteins has a great impact both on their physicocalled paucimannosidic-type N-glycans as illustrated Fig. chemical properties and on their biological functions. 1. The later nomenclature has recently been proposed for Over the last ten years, a number of laboratories have insect N-linked glycans with similar structures deprived contributed considerably to the understanding of the of terminal residues attached to the a(1, 3)and a(1, 6)structure, the biosynthesis and the function of plant mannose residues linked to the b-mannose of the core N-linked glycans. In this review, data on this domain (Altmann, 1997). will be summarized and the recent results on the Nglycosylation of a vacuolar lectin, the bean phytohaemagglutinin (PHA) will also be included. This PHA, used Biosynthesis of N-linked glycans in plants as a model glycoprotein, was expressed in different plant systems and the N-glycosylation patterns of difThe N-glycosylation of plant proteins starts with the ferent recombinant PHA were compared. In addition transfer in the endoplasmic reticulum (ER) of the oligoto this study on plant-specific glycosylation, the same saccharide precursor 392 from a dolichol model glycoprotein was used to investigate whether lipid carrier to specific Asn residues on the nascent or not N-glycosylation and N-glycan maturation is polypeptide chain. The precursor is subsequently modified organ-specific in plants. by glycosidases and glycosyl transferases during the transport of glycoproteins to their final location as illus