Alzheimer’s Disease
191
(residues 93–96) and DSGT (residues
289–292). Both aspartic acid residues have
been shown to be essential for enzymatic
activity. BACE1 contains intrinsic prote-
olytic activity; as a puriFed recombinant,
BACE1 can directly cleave APP
in vitro
.
Overexpression of BACE1 in cell lines dra-
matically increases
β
-secretase activity and
β
-cleavage products: APPs
β
and C99. In
addition, mice deFcient in BACE1 show a
normal phenotype, but A
β
generation is
abolished, further supporting that BACE1
is indeed a
β
-secretase.
BACE1 is initially synthesized in the
ER as an immature precursor protein,
proBACE1, which undergoes rapid mat-
uration involving N-glycosylation as well
as removal of the propeptide domain by
a furin-like convertase in the Golgi appa-
ratus. Unlike most zymogens, proBACE1
possesses
β
-secretase activity. The propep-
tide domain does not appear to signiF-
cantly inhibit protease activity, but rather
acts as a chaperone to assist proper folding
of the protease domain. Phosphorylation
of BACE1 at Ser498 appears to be impor-
tant for the intracellular trafFcking of the
protein between early endosomes and late
endosomes/TGN compartments.
A homologous aspartyl protease, BACE2
was shortly identiFed by searching the
EST database with the BACE1 sequence.
BACE1 and BACE2 share
64% amino
acid similarity. Together they deFne a
novel family of transmembrane aspartyl
proteases. Brain expression of BACE2 is
very low and most evidence suggests that
it does not play a major role in
β
-cleavage
of APP.
3.4
γ
-secretase – Presenilins and Cofactors
The enzyme that is clearly central to AD
pathogenesis is the
γ
-secretase, which
cleaves C99 and produces A
β
.E
v
id
en
c
e
suggests that it is an aspartyl protease with
the unusual property of cleaving substrates
within the transmembrane domain. Inde-
pendent from its role in processing APP,
γ
-secretase also mediates the intramem-
branous cleavage of several other type I
transmembrane proteins including Notch
and ErbB-4.
γ
-secretase cleavage of Notch
releases the Notch intracellular domain,
which translocates to the nucleus and acts
as a cofactor to modify transcription of
target genes.
The identity of the
γ
-secretase is rather
enigmatic. It appears to be a multiprotein,
high molecular weight complex that con-
sists of presenilins (PS) and three other
cofactors: nicastrin, APH-1 and PEN-2.
3.4.1
Presenilins
Presenilins are a family of polytopic trans-
membrane proteins that are implicated in
AD pathogenesis. PS homologs have been
found in species as diverse as
Drosophila
melanogaster
,
Caenorhabditis elegans
and
Arabidopsis
, but not in yeast. The function
of PS appears to be conserved. DeFciency
in sel-12, one of the nematode PS homolog,
causes egg-laying defects. This phenotype
can be rescued by expressing human PS1
in the mutant worms. In mammals, there
are two PS genes,
PS1
and
PS2
, that share
65% identity.
PS1
is believed to account
for the majority of PS functions, while
PS2
seems to play a complementary role.
Presenilins are found primarily within
intracellular membranes including the ER
and Golgi as well as the plasma membrane.
Structurally, they are predicted to have
eight transmembrane domains with both
the amino and carboxyl termini oriented
toward the cytoplasm (±ig. 3). A large cy-
toplasmic loop is postulated between TM6
and TM7. PS are synthesized as single
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