N-LINKED OLIGOSACCHARIDE PROCESSING IS NOT NECESSARY FOR GLYCOPROTEIN-SECRETION IN PLANTS

The role of N-glycans in the secretion of glycoproteins by suspension- cultured sycamore cells was studied. The transport of glycoproteins to the extracellular compartment was investigated in the presence of a g lycan-processing inhibitor, castanospermine. Castanospermine has been selected because it inhibits homogeneously glycan maturation in sycamo re cells and leads to the accumulation of a single immature N-glycan. The structure of this glycan has been identified as Glc(3)Man(7)GlcNAc (2) by labeling experiments, affinity chromatography on concanavalin A -Sepharose and proton NMR. In contrast with previous results showing t hat N-glycosylation is a prerequisite for secretion of N-linked glycop roteins, this secretion is not affected by the presence of castanosper mine. As a consequence, the presence of this unprocessed glycan is suf ficient for an efficient secretion of glycoproteins in the extracellul ar compartment of suspension-cultured sycamore cells.