Aggregation, Protein
25
1
Introduction
Protein misfolding and aggregation have
been recognized for many years as com-
mon processes. Aggregation can occur
under various conditions. The aggregation
of which we are speaking is very different
from the precipitation of a native protein
at the isoelectric point or upon salting
out, which can be reversed under appro-
priate conditions. In the precipitate, the
protein remains in a native conformation.
The aggregates, however, are formed from
partially folded intermediates and result
from intermolecular interactions, which
compete with intramolecular interactions.
Thermal denaturation of proteins is fre-
q
u
e
n
t
l
ya
c
c
om
p
a
n
i
e
db
yt
h
ef
o
rm
a
t
i
o
n
of aggregates leading to the irreversibil-
ity of the process. As early as 1931,
Wu, in a review on protein denaturation,
distinguished between aggregation and
precipitation. The aggregated species are
not in equilibrium with the soluble species,
complicating experimental approaches.
Aggregation has been reported to occur
during the
in vitro
refolding of monomeric
as well as oligomeric proteins, lowering
the refolding yield. As mentioned above,
the use of very low protein concentra-
tions could prevent protein aggregation.
However, during the folding of nascent
polypeptide chains biosynthesized within
prokaryotic and eukaryotic cells, aggre-
gates can accumulate. The overexpression
of genes in foreign hosts often result
in aggregated nonnative proteins called
inclusion bodies
,wh
ichared
isorderedag
-
gregates, leading to serious limitation in
the production of recombinant proteins.
It is a real problem needing a lot of
effort to fully exploit the sequence infor-
mation contained in the genome projects.
Ordered aggregates resulting in amyloid
Fbrils lead to a number of serious hu-
man diseases such as Alzheimer’s disease
and
the
transmissible
spongiform
en-
cephalopathies. The formation of amyloid
aggregates has also been reported in
in
vitro
experiments.
Experimental
and
theoretical
studies
together have provided signiFcant insights
into the mechanisms of protein folding,
also allowing a better understanding of the
aggregation processes.
The following different aspects of pro-
tein aggregation must be considered:
1. Theoretical and methodological aspects
of
protein
folding,
misfolding,
and
aggregation including the detection of
aggregates
and
the
mechanisms
of
aggregation processes.
2. Protein aggregation in the cellular en-
vironment including the folding into
th
ec
e
l
l
,th
er
o
l
eo
fm
o
l
e
cu
l
a
rch
ap
-
erones, and the formation of different
aggregate morphologies, as well as the
pathological consequences.
2
Protein Folding, Misfolding, and
Aggregation
2.1
The New View of Protein Folding
The question of the mechanisms of pro-
tein folding has intrigued scientists for
many decades. As early as the 1930s, at-
tempts to refold denatured proteins were
published, but signiFcant progress began
to be made when AnFnsen successfully
refolded, denatured, and reduced ribonu-
clease into the fully active enzyme. In 1973,
he stated the fundamental principle of pro-
tein folding referred to as the AnFnsen
postulate: ‘‘all the information necessary
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