N-Glycosylation - The Sugar Code - page 92

depends on fully assembled LLOs - a specifi city that guarantees accurate N - glyco-
sylation under high LLO throughput conditions that are predominant under ele-
vated secretory circumstances. Ribophorin I was necessary for my glycosylation,
although it is dispensable for the glycosylation of soluble secretory and polytope
membrane proteins [4]. OST activity is crucial in vertebrate development, but its
individual subunits may be not. Mutations in OST components such as TUSC3
(Q13454) or MAGT1 (Q9H0U3) cause congenital disorders of glycosylation (CDG)
(CDG-Io or CDG-Ip; please see Chapter 22.1 for overview on CDGs).

6.3
Trimming Reactions by

- Glucosidases and Interactions with ER Lectins α

Although I get decorated with several Glc 3 Man 9 GlcNAc 2 oligosaccharides inside
the ER, I no longer have this glycan structure when I reach the plasma membrane
(Figure 6.1). Extensive trimming and elongation reactions occur along the secre-
tory pathway that remodel my N - glycans (Figures 6.2 and 6.3 ; Info Box 2 ).
Oligosaccharide trimming starts immediately ( t 1/2

2 min) after transfer from
glycolipid to my nascent polypeptide chain. Mannosyl-oligosaccharide

<

α - glucosi-

dase - I [GCS1 (Q13724), 1,2 - linked

glucose to drive and stabilize the oligosaccharide transfer reaction. GCS1 is
retained in the ER as a newly synthesized homotetramer, due to its cytoplasmic

α - glucosidase - I] hydrolyzes the outermost α

Info Box 2

Removal of most sugars, occurring just after transfer of the oligosaccharide
from lipid to protein, was the most confusing part of the N - glycosylation
pathway in vertebrates. These trimming reactions resulted in the removal of
not only all three glucoses, as in Saccharomyces cerevisiae , but also most of the
mannoses before condensing reactions rebuild glycan structures with high
species - specifi c diversity. Why did premetazoan cells conserve such a compli-
cated and energy - consuming synthesis - degradation pathway for N - glycans
along their evolution? High-mannose glycans evolved when extensive manno-
sylation at the cell surface was essential. Inside the cells they provided the basis
for new pathways introduced during or after the prokaryotic- to - eukaryotic tran-
sition. In particular, they participated in the disulfi de bond formation pro-
cesses, and became active compounds for quality control of protein folding,
assembly, and degradation. These diverse interrelated processes in the early
secretory pathway made conservation of LLO composition vital. Why, however,
are almost all mannoses removed in vertebrates after glycoproteins have
achieved native conformations? In mammals, oligomannosyl structures play
important roles during both cell invasion and induction of innate immunity,
suggesting that epigenetic pressure forced early metazoans to remove man-
noses to prevent oligomannosyl-dependent cell invasion.