Occurrence of complex type free N-glycans with a single GlcNAc residue at the reducing termini in the fresh-water plant, Egeria densa.
In our previous study, we found unique free N-glycans (FNGs), which carry a single GlcNAc residue (GN1) at the reducing-end side and the Lewis-a epitope at the non-reducing-end side, in the culture broth of rice cells. Based on the FNG structural features and the substrate specificity of plant ENGase, we hypothesized that there might be a novel biosynthetic mechanism responsible for the production of these unique GN1-FNGs, in which high-mannose type (HMT)-GN1-FNGs produced in the cytosol from misfolded glycoproteins by ENGase are transported back into the endoplasmic reticulum and processed to plant complex type (PCT)-GN1-FNGs in the Golgi apparatus. Until now, however, PCT-GN1-FNGs had only been found in the culture broth of rice cultured cells and never in plants, suggesting that the formation of PCT-GN1-FNGs might be generated under special or artificial conditions. In this study, we confirm the presence of PCT-GN1-FNGs, HMT-GN1-FNGs and PCT-GN2-FNGs in the fresh-water plant Egeria densa. These results suggest that a mechanism responsible for the production of PCT-GN1-FNG is present in native plant tissues.