Chemistry Reference
In-Depth Information
and TTA2, instead. The species differences in substrate specifi city of the peptide
sequence around the
site and the structure of the GPI anchors, as well as the
differences in the protein components of the complex, suggest that there may be
exploitable differences between mammalian and parasite GPI transamidase.
•
Step 12: modifi cations of the GPI anchor after attachment to proteins: removal of the
acyl chain from inositol
. In almost all cells except
Plasmodium
and erythrocytes, the
acyl group is usually removed from GPI-anchored proteins as result of inositol
deacylation that occurs directly after GPI -anchor attachment. Inositol deacylation
is mediated by mammalian PGAP1 (post-GPI attachment to proteins 1) and yeast
Bst1 (also called dihydrosphingosine phosphate lyase 1). Mutant cells defective
in deacylation show a clear delay in the maturation of GPI- anchored proteins in
the Golgi and accumulation of GPI-anchored proteins in the ER. Furthermore,
BST1 is important for ER-associated degradation of a misfolded, soluble protein,
where transport of such proteins to the Golgi may be a prerequisite for its
degradation (please see also Chapter 6, especially Info Box 4). The deacylation step
can also be inhibited
in vivo
and
in vitro
with diisopropylfl uorophosphate (DFP,
C
6
H
14
FO
3
P), a potent serine protease inhibitor such as PMSF mentioned in
steps 3 and 9, resulting in only inositol-acylated intermediates. PMSF and DFP
can also disrupt the dynamic equilibrium between mature acylated and non-
acylated GPI precursors. In
P. falciparum
, once acylated, the GPI intermediate
will never be deacylated. Having completed this step, we have reached the fi nal
destination, which is the end product of this biosynthetic route.
ω
The preceding explanations covered what is to know about general GPI biosynthe-
sis. In addition, the GPI glycan core undergoes structural variations in various
organisms, which arise from different substitutions (' decorations ' ) of the evolu-
tionary conserved core structure. In the following sections we will also see that
the lipid moiety of the newly synthesized GPIs may be substantially altered by a
process called remodeling. As with all types of natural glycoconjugates, each cel-
lular GPI anchor may be comprised of heterogeneous glycoforms. This heteroge-
neity can be further enhanced by variations of the lipid moieties. Thus, it is very
diffi cult, if not impossible, as with
N
-glycans from a glycoprotein, to obtain homo-
geneous GPIs and GPI-linked proteins or glycoproteins from living cells. As a
result, their chemical synthesis has attracted signifi cant attention (please see
below). Furthermore, the functions of GPIs or how mutant cells helped in gaining
insight into their biosynthesis will also be emphasized and discussed.
9.2
Remodeling of Lipid Moieties of GPI Proteins
The lipid moiety of the free GPI lipid or the GPI anchor attached to a protein is
modifi ed in a process called remodeling. In yeast cells, the mature GPI- anchored
proteins contain mainly ceramide or DAG with saturated long-fatty acids, whereas
conventional PI used for GPI biosynthesis contains unsaturated fatty acids.
Ceramides are also found in the GPI anchors in
Trypanosoma cruzi
(the causative
agent of Chagas disease), the free- living protozoan
Paramecium primaurelia
, the
