section 16.1
Biosynthesis of Glycoproteins
9 l
FIGURE 16-13
Topography of glycosylation in the rough endoplasmic reticulum. Addition of O-linked N-acetylglucosamine to protein
and phosphatidylinositol anchoring. Ac, Acetyl; Ser, serine; L, lumenal; C, cytosolic. [Reproduced with permission
from C. Abeijon and C. B. Hirschberg, Topography of glycosylation reactions in the endoplasmic reticulum.
B iol. Sci.
17:34 (1992).]
peptide chain termination and occurs just after or concomi-
tant with cleavage of a C-terminal signal anchor sequence
from the protein. Studies using yeast and Thy-1-negative
T lymphocytes as well as intact ER vesicles suggest the fol-
lowing pathway: synthesis of the anchor oligosaccharide
portion with GlcNAc transfer to inositol, deacetylation of
GlcNAc, mannose transfer via dolichol-P-mannose after
GDP-mannose transfer to dolichol on the cytoplasmic side
of the ER, and translocation of the dolichol-P-mannose to
the lumen (Figure 16-13). Finally, the terminal mannose of
the core is phosphorylated with ethanolamine phosphate
which serves as the linker to the protein. The core GPI
may later be modified by attachment of fatty acids, man-
nose, N-acetylgalactosamine, and additional mannose in
the ER or galactose in the Golgi. GPI-anchored proteins
are absent on both red and white cells in the human dis-
paroxysmal nocturnal hemoglobinuria
(PNH). In
some cells the anchor is resistant to Pi-phospholipase C;
however, cells with a deacylase activity that removes an
acyl chain from inositol are sensitive to release of GPI-
anchored proteins (Figure 16-12). A mutation in the gene
coding for the first enzyme in the pathway for GPI anchor
synthesis results in PNH, which is an anemia caused
by complement-mediated hemolysis. Two complement
regulatory proteins that protect host cells from comple-
ment do not get properly GPI-anchored on the surface of
PNH erythrocytes causing them to be abnormally sensi-
tive to complement lysis. Patients with PNH, in addition to
having hemolytic anemia and the resulting hemoglobin-
uria, also have a reduced ability to combat infections and
a tendency for thrombosis. The excessive accumulation
of iron by overloaded kidneys due to the hemolysis has
caused some patients to set off airport metal detectors. The
gene, PIG-A, the cause of PNH, is located on the X chro-
mosome. Thus, a single PIG-A mutation can account for
the loss of GPI anchor synthesis even in females, depend-
ing upon which X chromosome is inactivated. While many
cells can survive without their GPI-anchored proteins, it
is the specific loss of specialized, complement-protective
proteins by erythrocytes that is the problem in PNH. While
only a small percentage of proteins in humans is anchored
to membranes using GPI anchors, in some species, such
as protozoa, many or all of the membrane-bound proteins
utilize GPI anchors. An understanding of the differences
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