Letter to the Glyco-Forum: Asparagine Linked Glycosylation in Giardia
Phillips W. Robbins and
John Samuelson
Department of Molecular and Cell Biology, Boston University Goldman School of Dental Medicine, 715 Albany Street, Boston, MA 02118-2932
This letter is written in response to the paper by Morelle et al. (2005)
, who provide structural evidence for the presence of typical mammalian type asparagine N-linked glycans containing mannose, fucose, and sialic acid on secreted glycoproteins of Giardia. Their experiments involve growing Giardia in a complex medium containing 10% bovine serum, rinsing them in protein-free medium, and then collecting glycoproteins shed by the protists into protein-free medium during a six-hour incubation. We suggest that the N-glycans characterized by Morelle et al. most likely result from bovine serum rather than Giardia for the following reasons:
- The N-glycans closely resemble those of mammalian species (Varki, 1999)in particular, the complex N-glycans build on a mannose core with acetylglucosamine, galactose, and sialic acid. Because these N-glycans do not result from metabolic labeling of Giardia, the authors cannot rule out the possibility that the N-glycans derive from bovine serum.
- We recently performed bioinformatic and biochemical surveys of the asparagine linked glycosylation pathways of numerous protists (Samuelson et al., 2005
). In the case of Giardia, we find that the genome, which has been sequenced in its entirety, encodes the first enzyme in the lipid-linked pathway (Alg 7), as well as the peptides required for addition of the second GlcNAc to generate dolichol pyrophosphate diacetyl chitobiose (Chantret et al., 2005
). The Giardia genome lacks the rest of the genes encoding enzymes needed to synthesize the typical fourteen sugar oligosaccharide present on the dolichol-linked precursor in yeast, mammals, and most other eukaryotes (Varki, 1999). Typical genes for enzymes needed for endoplasmic reticulum and Golgi processing of the N-linked glycans are also missing from the Giardia genome.
- We directly demonstrated that N-linked glycosylation in Giardia is limited to the addition of N-acetylglucosamine residues to secreted glycoproteins by metabolic labeling (Samuelson et al., 2005
). These experiments demonstrate the presence of only N-acetylglucosamine and diacetyl chitobiose attached to dolichol pyrophosphate and to oligosaccharides released from bulk glycoproteins by N-glycanase. In the same manuscript, we demonstrate that Entamoeba and Trichomonas also make N-glycans based upon severely truncated dolichol-linked precursors.
References
Chantret, I, Dancourt, J, Barbat, A, and Moore, SE (2005) Two Proteins Homologous the N- and C-terminal domains of the Bacterial Glycosyltransferase Murg Are Required for the Second Step of Dolichyl-Linked Oligosaccharide Synthesis in Saccharomyces cerevisiae. J. Biol. Chem., 280, 92369242.[Abstract/Free Full Text]
Morelle, W, Jimenez, J-C, Cieniewski-Bernard, C, Dei-Cas, E, and Michalski, J-C (2005) Characterization of the N-Linked Glycans of Giardia intestinalis. Glycobiology 15, 549559.[Abstract/Free Full Text]
Samuelson, J, Banerjee, S, Magnelli, P, Cui, J, Kelleher, DJ, Gilmore, R, and Robbins, PW (2005) The Diversity of Dolichol-Linked Precursors to Asn-Linked Glycans Likely Results from Secondary Loss of Sets of Glycosyltransferases. Proc. Natl. Acad. Sci. USA, 102, 15481553.[Abstract/Free Full Text]