Plant nuclear pore complex proteins are modified by novel oligosaccharides with terminal N-acetylglucosamine

Only a few nuclear pore complex (NPC) proteins, mainly in vertebrates and yeast but none in plants, have been well characterized. As an initial step to identify plant NPC proteins, we examined whether NPC proteins from tobacco are modified by N-acetylglucosamine (GlcNAc). Using wheat germ agglutinin, a lectin that binds specifically to GlcNAc in plants, specific labeling was often found associated with or adjacent to NPCs. Nuclear proteins containing GlcNAc can be partially extracted by 0.5 M salt, as shown by a wheat germ agglutinin blot assay, and at least eight extracted proteins were modified by terminal GlcNAc, as determined by in vitro galactosyltransferase assays. Sugar analysis indicated that the plant glycans with terminal GlcNAc differ from the single O-linked GlcNAc of vertebrate NPC proteins in that they consist of oligosaccharides that are larger in size than five GlcNAc residues. Most of these appear to be bound to proteins via a hydroxyl group. This novel oligosaccharide modification may convey properties to the plant NPC that are different from those of vertebrate NPCs.