Glycosylation of Model and ‘Lower’ Organisms - The Sugar Code - page 145

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Many plant glycoproteins, such as horseradish peroxidase (HRP), contain core

α 1,3-fucose and xylose (see Figure 8.3), but these moieties are recognised as

foreign to mammalian immune systems; indeed, core

1,3 - fucose is a major
epitope for the antiserum raised against HRP (anti-HRP). The presence of this
residue also on invertebrate glycoproteins (see Figures 8.4 and 8.5 ) accounts
for the cross-reactivity observed between many plant and invertebrate glycans
(i.e., anti-bee venom recognises plant glycoproteins and anti-HRP recognises
insect glycans). Core

α

1,3-fucose is present on many allergens and is an epitope
for IgE antibodies from many patients allergic to bee venom, plant pollens and
plant foods. Other residues absent from mammals, such as O - linked arabinose,
are also recognised by the IgE from some patients. The clinical signifi cance of
anti-carbohydrate IgE is controversial, even if some glycoproteins can elicit
histamine release in vitro.

α

Table 8.1 Comparison of the phenotypes of organisms lacking complex N - glycans due to the
absence of G n T - I .

Organism Anti - HRP staining Phenotype

Core

α

1 - 6 - fucose

Fly Abolished Abolished Neuroanatomical defect

Worm Present Abolished Bacterial infection sensitive

Plant Abolished Not relevant Heat stress sensitive

Mouse Never present Probably absent Embryonic lethal

The absence of GnT -I has a distinct impact on the glycomic potential of all multicellular species
studied; however, in worms the anti-HRP epitope (based on core α 1,3 - fucose) is retained, in

contrast to the situation in fl ies and plants.

the other hand, a defect in glucosidase I (the enzyme which catalyses the fi rst
processing step after transfer of the glycan to protein, resulting in Glc 2 Man 9 GlcNAc 2 )
is severe and the observed phenotype may be due to a lack of proper processing
of the glycans on proteins associated with cellulose biosynthesis [9]. Thus, the fi rst
N -glycan processing steps are biologically imperative for plants (perhaps due to
their 'cofactor' role in protein folding), but the 'decorations' are not essential, even
if they are highly conserved.
Other forms of glycosylation include modifi cation of proteins with galactose and
arabinose [10] or GPI anchors (see Chapter 9), galactolipids in plastids [11] , and a
wide range of cell wall polysaccharides, such as cellulose, hemicelluloses and
pectins. In the case of these polysaccharides, which are unique to plants, the use
of mutant Arabidopsis strains is proving important in the elucidation of genes
involved in their biosynthesis.