Glycosylation has been studied mainly in mammals, yeasts and plants, but has barely been studied in algae. This study focuses on the structural determination of N-linked glycans within the 66-kDa glycoprotein, part of the unique sulfated complex cell-wall polysaccharide of the red microalga Porphyridium sp.[1]. This glycoprotein is tightly bound, to the cell-wall polysaccharide and consists of a novel polypeptide, and a sugar moiety of approximately 8-kDa. N-glycan structures were elucidated by a combination of NP/RP-HPLC, positive ion MALDI-TOF MS, negative ion ESI-MS and MS/MS spectrometry[1].
These methodologies revealed that the unique sugar moieties of the glycoprotein consisted of four prominent N-linked glycans, each composed of the same four monosaccharides, N-acetylglucosamine, mannose, 6-O-methylmannose and xylose, in different ratios[1]. The present study comprises the first report of N-glycans with the terminal xylose attached to the 6-mannose branch of the 6-antenna and to the 3-oxygen of the penultimate (core) GlcNAc. Another novel finding was that all four glycans contain three O-methylmannose residues in positions that have never been reported before[1].
Bsed on the 66-kDa N-glycan structures, for the first time two suggested pathways of the 66-kDa cell-wall N-glycosylation can be discussed [2]. Each one of the suggested pathways has a similar basic form based on the Man9 topology structure constructed along the pathways typical of the ER. In both the suggested glycosylation pathways, xylose and methyl transferases play prominent roles in the glycosylation. The findings of this study will thus facilitate the identification of glycan biosynthetic components, thereby making an invaluable contribution to a comprehensive understanding of N-glycosylation in red microalgae.
[1] Levy-Ontman O, Arad SM, Harvey DJ, Parsons TB, Fairbanks A, Tekoah Y (2011) J Biol Chem 286: 21340–21352.
[2] Levy-Ontman O. (2012) "N-glycosylation of the 66-kDa cell-wall glycoprotein within a red microalga" In: Stefana Petrescu (Ed.) Intech open access publishing .
