N-Glycosylation - The Sugar Code - page 98

Info Box 4 - ctd.

N -Glycans are required for successful glycoprotein synthesis, especially in
high-throughput situations, to prevent protein aggregation, cell death and
tissue damage. Their synthesis needs a huge energy- consuming machinery to
accomplish full functionality. Therefore, protein synthesis in the ER of verte-
brate cells is highly vulnerable to metabolic changes, such as amino acid starva-
tion (around 5-15 min) or glucose deprivation, which lowers the N - glycosylation
capacity due to LLO defi cit (around 5 min). Cells grown in such suboptimal
conditions induce a so- called unfolded protein response (UPR), not only to save
materials, but also to prevent an accumulation of incorrectly folded aggrega-
tion-prone underglycosylated proteins. During UPR, cells immediately, and
with high effi ciency, suppress eIF4-dependent protein synthesis, induce mRNA
transcription of all quality control components and guarantee their translation
into the ER by use of eIF4-independent ribosomal initiation processes. Molecu-
lar components of the ERAD pathway have been characterized biochemically
and genetically in yeast. Differences in N -glycan processing between yeast and
vertebrates, as well as cellular reactions under experimental conditions as men-
tioned above, complicate research on ERAD in higher eukaryotes.
ERAD is a pathogenetic factor in protein-folding diseases (caused by desta-
bilizing point mutations in glycoproteins) in two ways - either quality control
is not able to recognize and remove aggregation-prone mutated glycoproteins
(pathological gain of function) or it prevents nearly native, active enzymes from
reaching their fi nal destination (loss of function). An example for the former
is the apoptosis-inducing myocilin mutations causing open angle glaucoma,
and for the latter, Fabry disease-causing mutations in the lysosomal

α 1 - galactosidase A. In both cases, disease -causing manifestation in cultured

cells could be reversed by treatment with appropriate synthetic chaperones.

prohibited. Constitutive glucose cleavage in N - glycans released from CALX/
CALR-PDIA3, combined with an effi cient UGGG1, monitors the folding stage of
glycoproteins. When I pass UGGG1's quality control of protein folding, I will be
packed into COPII-coated tubulovesicular transport clusters that contrive my
transport towards the Golgi apparatus. Cholesterol-enriched transporters are used
as carriers for my glycosylphosphatidylinositol- anchored 120 - kDa cousins. Alter-
natively, the acceptor substrate for UGGG1 can be destroyed by the commonly
found 1,2 mannosidase MA1A1 (P33908) - the quality control terminator enzyme.

MA1A1 preferentially cleaves Man 9 - glycans at their A -branch (95%) and produces
the ER - export tag Man 8 - 6 GlcNAc 2 -A isomers without, however, determining the
fate of the glycoprotein.
Mammalian cells, in contrast to yeast, express several neutral

α

1,2 - mannosi-
dases in the secretory pathway (MA1; glycoside hydrolase family 47) in a tissue-
specifi c manner. They generate glycan structures free of

α

1,2 - linked mannoses - a
prerequisite for further trimming and elongation reactions. All processing

α

1,2
mannosidases are type II transmembrane proteins with long (35-85 amino acids)

α