ABBS 2005,38(04): Quality Control System of the Endoplasmic Reticulum and Related Diseases

Abstract        The quality control (QC) system of the
endoplasmic reticulum (ER) is an important monitoring mechanism in the protein maturation
process, which ensures export of properly folded proteins from the ER.
Incorrectly or incompletely folded proteins are retained in the ER for
refolding or degradation by the ER-residing proteasome. The
calnexin/calreticulin cycle and ER-associated degradation are the key elements
in QC. These two mechanisms work together to allow incorrectly folded proteins
have additional opportunities to achieve their native conformations. The QC
dysfunction is involved in many diseases caused by mutant proteins, many of
which are causes of neurodegenerative disorders. A better understanding of
molecular regulation in the QC system will uncover the molecular pathogenic
mechanisms of many diseases caused by protein misfolding and help discover
novel strategies for preventing or treating these diseases.

Key words        protein folding; quality control;
calnexin/calreticulin cycle; endoplasmic reticulum-associated degradation;
protein misfolding disease

Introduction

The endoplasmic
reticulum (ER) is a highly versatile protein processing factory that is
equipped with chaperones and folding enzymes essential for protein folding. ER
quality control (QC) guided by these chaperones is essential for life.
Correctly folded proteins are exported from the ER, but misfolded proteins are
retained and selectively degraded. Although the native conformation of a
protein lies in its primary amino acid sequence, the ER greatly enhances the
efficiency of protein folding. The compromised function of the ER QC system
often leads the organism to suffer the overexpression of misfolded proteins,
which may result in various diseases. The ER has unique oxidizing potential
that supports disulfide bond formation during protein folding [1]. Chaperones
and folding enzymes are abundant, greatly outnumbering the newly synthesized
substrates. These folding factors in general prevent aggregation and thereby
allow more efficient folding of a large variety of proteins. At least two main
chaperone classes, binding protein (BiP) and calnexin/calreticulin, are active
in the ER QC. Chaperones and folding enzymes are usually found in complexes.
Recent work emphasizes more than ever that chaperones act in concert with
co-factors and with each other [2]. Defective protein folding can lead to
clinically significant pathologies as seen in cystic fibrosis (CF), Alzheimer’s
disease (AD), diabetes, familial hypercholesterolemia, and amyotrophic lateral
sclerosis [3], as well as Huntington’s disease (HD) [4], Creutzfeld-Jacob
disease and alpha-1-antitrypsin (a1-AT) deficiency [5]. In
this review, we highlight the latest advances in understanding how these
chaperones and folding enzymes cooperate in assisting protein folding and
mediating QC.

Molecular Chaperones

Mammalian secretory and membrane
proteins are synthesized and translocated into the ER by the ribosome and Sec61
translocation machinery. During translocation, newly synthesized proteins
immediately start to fold. Some soluble proteins fold relatively easily,
whereas others have more difficulty and require more assistance from chaperones
and folding enzymes. A chaperone is a kind of protein that will bind
transiently to newly synthesized proteins. Calnexin and calreticulin are
molecular chaperones involved in protein folding, assembly, and
retention/retrieval. Calreticulin binds Ca2+ in the lumen of
the ER with high capacity and also participates in the folding of newly
synthesized proteins and glycoproteins. It is a component of the
calreticulin/calnexin pathway [6]. Calreticulin consists of distinct structural
and functional domains. The N-domain of calreticulin, together with the central
P-domain, is responsible for the protein’s chaperone function. Recent studies
[7,8] with site-specific mutagenesis show that mutation of a single histidine
residue (His153) in calreticulin’s N-domain destroys the protein’s chaperone
function. The P-domain of calreticulin (residues 181–290) contains a proline-rich region, forms an
extended-arm structure, and interacts with other chaperones in the lumen of the
ER. The extended-arm structure is predicted to curve like that in calnexin, an
ER integral membrane chaperone that is similar to calreticulin, forming an
opening that is likely to accommodate substrate binding, including the
carbohydrate-binding site (Fig. 1). As a molecular chaperone,
calreticulin binds the monoglucosylated high mannose oligosaccharide
(Glc1Man9GlcNAc2) and recognizes the terminal glucose and four internal
mannoses in newly synthesized glycoproteins [9]. Both calreticulin and calnexin
act as lectins and molecular chaperones, and they bind monoglucosylated
proteins and associate with the thiol oxidoreductase ERp57, which is a protein
disulfide isomerase (PDI)-like protein resident in the ER and promotes
disulfide formation/isomerization in glycoproteins [10]. Together, calnexin,
ERp57 and calreticulin comprise the so-called “calreticulin/calnexin
cycle”, which is responsible for QC and folding in newly synthesized
(glyco) proteins. Folding substrates associate transiently with calnexin and
calreticulin and enter cycles of de-glucosylation/re-glucosylation, and the
process plays an important role in their association with the chaperones.
Changes within the ER, such as alterations in the concentration of Ca2+, Zn2+ or ATP, may affect the formation of these
chaperone complexes and thus the ability of calreticulin to assist in protein
folding [11]. The terminal glucose is trimmed by glucosidase (GLS) II and this
may be important in the regulation of substrate-calreticulin interactions. The
ER also contains a uridine diphosphate (UDP)-glucose, glycoprotein transferase
(UGGT), which can re-glucosylate chains that have been glucose-trimmed.
Together, UGGT and GLS II establish a cycle of de-glucosylation and
re-glucosylation. Importantly, UGGT discriminates between folded and unfolded
proteins, adding back a glucose residue to unfolded proteins only. This results
in “rebuilding” of the monoglucosylated oligosaccharide on unfolded substrates,
enabling them to interact with calreticulin and/or calnexin again. GLS then
removes the terminal glucose residue, releasing the bound glycoprotein from its
interaction with the chaperone again and again. This de-glucosylation/glucosylation
cycle may be repeated several times before a newly synthesized glycoprotein is
properly folded [12] (Fig. 2).

Calreticulin is
essential for normal calnexin chaperone function. In the absence of
calreticulin, calreticulin substrates are not “picked up” by calnexin
but accumulate in the ER lumen, resulting in the activation of the unfolded
protein response (UPR), which is activated to induce transcription of
ER-localized molecular chaperones [13]. Pancreatic ER kinase (PERK) and IreThere are many other
typical molecular chaperones in ER, such as BiP, glucose-regulated protein
(GRP) 78, GRP94, and PDI [2]. BiP, GRP78 and GRP94 are members of the heat
shock protein (HSP) family. They are the most abundant proteins in the ER
lumen. The importance of the BiP/GRP94 system in protein folding comes from
studies in yeast and mammalian cells. It is the second major ER chaperone
system, which is only dependent on the presence of unfolded regions on proteins
containing hydrophobic residues. In fact, some calnexin/calreticulin substrates
can bind to BiP instead, if N-linked glycosylation is blocked. It has been
proposed that during the translocation of a given glycoprotein into the ER a
choice is made between chaperone systems (BiP/GRP94 or calnexin/calreticulin)
depending on how soon an N-linked glycan occurs on the sequence [17]. PDI can
help the formation of disulfide bonds that are the critical structure of
protein secondary structure. PDI has two CXXC motifs as the active sites [18].
All of them depend on ATPase activity.

Pharmacological and Chemical
Chaperones

Conformational diseases
often result from mutations in proteins that are recognized as misfolded by the
QC system. Such recognition can lead to two different results: some misfolded
proteins can be efficiently ubiquitinated and degraded by the proteasome,
leading to a loss of function [19]; whereas others accumulate in cells, forming
aggregates that may have toxic consequences and are often referred to as “gain
of functions” [20]. In the past decade, efforts to overcome these defects
have led to the development of various interventions that successfully rescue
proteins from both aggregation and degradation pathways. In particular,
treatments with chemical compounds known as either chemical or pharmacological
chaperones have been found to stabilize some conformational mutants, promoting
their proper transport to their site of action where, in many cases, they can
be functional [21–23].
Identifying compounds that can bind to the mutant proteins has been easier for
proteins such as channels and receptors for which selective ligands have
already been characterized. Because of their involvement in many
pathophysiological conditions and the rich pharmacological diversity generated
through various drug-screening campaigns, G-protein-coupled receptors have
attracted considerable attention for the identification of pharmacological
chaperones [24].

Recent studies have
demonstrated that pharmacologically selective compounds, termed “pharmacoperones”, the short-hand expression for
pharmacological chaperones, can stabilize the misfolded receptors, facilitating
their export from the ER to the plasma membrane, where they can be active.
There are several examples, such as copper [25] and galactose [26]. Such
functional rescue suggests that pharmacoperones could represent novel
therapeutic agents for the treatment of protein conformational diseases.
Pharmacoperones were first discovered by the work carried out on V2 vasopressin
receptor mutants responsible for nephrogenic diabetes insipidus [27].
ER-retained mutant receptors appeared largely as immature core-glycosylated
receptor precursors, and they were processed to fully mature receptors
harbouring complex carbohydrate arborisation following treatment with the cell
permeable V2 vasopressin receptor antagonists. The antagonist treatment also
increased the turnover rate of the precursor form of the receptor without
affecting the half-life of the mature receptor, which is indicative of an
action on the biosynthetic processes. Taken together with the pharmacological
specificity of an action, these observations contributed to the emergence of
the concept that pharmacoperones assist the folding and ER export of mutant
G-protein-coupled receptors. The emerging hypothesis for the action of
pharmacoperones suggests that selective lipophilic ligands can penetrate the
plasma and ER membranes to bind to the partially folded receptor early during
biosynthesis. Ligand binding might alter the thermodynamic equilibrium in favor
of the correctly folded protein, increasing the likelihood of the protein
escaping the stringent Chemical chaperones are
chemical compounds of low molecular weight, such as dimethylsulphoxide (DMSO),
trimethylamine N-oxide (TMAO) and glycerol, which can bind with mutant proteins
unspecifically. Many data indicate that many of the mutant proteins adopt a
conformation compatible with cell-surface transport when the folding and/or the
degradation processes are slowed down by reducing the temperature (kinetic effect).
This temperature-dependent recovery of Cl– channels at the cell surface is mimicked by
treating cells with the chemical chaperones. They are believed to function by
stabilizing misfolded mutant proteins into conformations that are not targeted
for degradation and can escape the ER (conformational effect). The well-studied
examples are DF508 cystic
fibrosis transmembrane conductance regulator (CFTR) [28] and a1-AT [29]. For example, glycerol treatment of
cells that express DF508 CFTR
caused an eight-fold increase in cAMP-dependent Cl– currents. So some nascent DF508 molecules can fold correctly, thereby
escaping degradation. However, none of the chemical chaperones shown to be
active in cell systems could be used in clinical settings. This has led several
investigators to search for more specific treatments that could be tolerated in
humans.

ER-associated Protein Degradation

In cells, only proteins
that have folded or assembled correctly are able to leave the ER, and those
that fail to do so are disposed of by proteolytic degradation. This degradation
of non-functional proteins was thought to occur within the ER itself, but it
has since been recognized that these proteins are exported from the ER and
degraded in the cytosol by a process known as ER-associated degradation (ERAD)
[30]. This process is dependent on proteasomes and ubiquitin. Most
membrane and all soluble proteins are thought to be retrotranslocated to the
cytosol by a protein-conducting channel, the Sec61p complex, which also
mediates the import of proteins into the ER. Proteasomal degradation of
cytosolic proteins is preceded by polyubiquitination which functions as a
degradation signal for the proteasome. Polyubiquitination is mediated by the
coordinated action of ubiquitin-conjugating enzymes (E2s) and ubiquitin-protein
ligases (E3s), which determine the specificity of the ubiquitin proteasome
system. Proteins that fail to fold correctly, however, are retrotranslocated
from the ER to the cytosol, and are then recognized by ER-specific E3 ligases
that mediate polyubiquitination of the misfolded protein on the cytoplasmic
side of the ER, and are subsequently degraded by the proteasome. Several E2s
and E3s are located at the ER membrane with their domains active in
polyubiquitination facing the cytosolic side [31]. In addition to acting as a
degradation signal for the proteasome, polyubiquitination may help to ensure
the directionality of the retrotranslocation process by acting as a binding
signal for the AAA ATPase p97/Cdc48, which is required for retrotranslocation
of several ERAD substrates. Some examples demonstrate that retrotranslocation
and proteasomal degradation are not as tightly coupled. Several ERAD substrates
are retained in the ER when polyubiquitination is inhibited but are found in
the cytosol upon inhibition of the proteasome, for example, major
histocompatibility complex class I (MHC I) heavy chain or carboxypeptidase Y.
These findings suggest that polyubiquitination and retrotranslocation are
coupled processes that are functionally independent of proteasomal activity
[32].

QC System and Human Diseases

It is clear that a
protein’s conformation determines its function, and the folding of a protein
after synthesis plays a very important role in the process of the protein’s maturation.
QC is an inspector in the ER to ensure the correction of the process. If a
protein folds correctly, it will be secreted to the right location and exert
its function there. If it is unfolded or incorrectly folded, it will probably
lose its function and be degraded through ERAD or aggregates in the ER.
However, aggregation of misfolded or incorrectly folded proteins is lethal.
When the QC system breaks down, it will probably cause the dysfunction of
proteins, and it will lead to pathogenesis of diseases. Research has discovered
the relationship between QC and diseases, such as CF, a1-AT, polycystic liver disease (PCLD), Gaucher disease,
AD, PD, HD, and so on. Some diseases are caused by dysfunction of mutant
proteins, like PCLD. PRKCSH and SEC63 are the two genes that are
responsible for PCLD, which encode hepatocystin and sec63 respectively [33].
Both proteins are involved in translocation through the ER membrane and in the
oligosaccharide processing of newly synthesized glycoproteins. Mutant
hepatocystin results in reduction of the protein levels of normal hepatocystin
and glucosidase II a-subunit,
whereas mutant sec63 causes the cytosolic accumulation of full-length protein
precursors. Other diseases are caused by the QC excessive degradation of mutant
proteins. These proteins can function normally in certain conditions even
though they are mutated. But the QC system clears them because of misfolding
caused by their mutations, so that the functional proteins are lost. In the
case of CF, although the protein CFTR is mutated, it can be corrected when the
temperature is reduced to 26 º [28].

In this review, we
mainly focus on the role of QC in neurodegenerative diseases.

QC System and Neurodegenerative
Diseases

Protein misfolding and aggregation
occur in most neurodegenerative diseases. This fact has led to the widely
accepted view that protein QC plays a critical role in neuronal function and
survival. Its importance is underscored by studies showing that manipulating QC
pathways alters the pathogenesis of neurodegenerative diseases [34]. Early
research implied that aggregations of protein are the biochemical hallmark of
several neurodegenerative diseases.

In the last few years,
evidence has accumulated that supports the premise that the ubiquitin
proteasome system (UPS) [35], which some reports also call it the ubiquitin
proteasome pathway [36], plays a role in many neurodegenerative diseases. The
UPS is also a major player in cellular protein quality control, and is involved
in the degradation of misfolded and other aberrant proteins. Most
neurodegenerative diseases are characterized by intracellular deposits of
aggregated and misprocessed proteins, many of which are proteasomal components
and substrates. Furthermore, several mutations in UPS components have been
associated with neurodegenerative diseases, and it is therefore highly
conceivable that the UPS is involved in the neuropathogenesis of these
diseases.

QC System and AD

AD is a well-researched
disease. The aberrant and misprocessed proteins that accumulate in the brains
of AD patients constitute the neuropathological hallmarks of the disease. The
two most pronounced hallmarks are neurofibrillary tangles, formed by
intracellular accumulations of the hyperphosphorylated protein tau, and
plaques, which are extracellular deposits of the 40–42 amino acid amyloid peptide, processed from the amyloid
precursor protein. The presence of ubiquitin and ubiquitinated proteins in the
brain is the initial clue suggesting that the UPS is involved in the
pathogenesis of AD. An aberrant form of ubiquitin (UBB+1) also accumulates in the neuropathological hallmarks of
AD. This UBB+1 is translated from ubiquitin-B mRNA, which
contains a dinucleotide deletion near a GAGAG repeat. The two nucleotides are
likely to be deleted during or after transcription, as the mutation cannot be
detected in the UbB gene of AD patients. UBB+1 accumulates in the
earliest affected brain areas of patients with AD, such as neurons in the
transentorhinal hippocampal cortex area. All three proteins mentioned earlier,
tau, amyloid peptide and UBB+1, accumulate
in the AD brain, and all were reported to affect the proteasomal pathway [37].

QC System and HD

HD is also induced by
the aggregation of aberrant proteins. HD is caused by an expansion of the
polyglutamine tract in the protein named huntingtin. The bio-hallmark of mutant
huntingtin is the formation of intranuclear inclusions and cytoplasmic
aggregates in neurons in vulnerable brain areas. Expression of mutant huntingtin
in cultured cells also causes the formation of intranuclear inclusions and
aggregates in the cytoplasm. The inclusions and aggregates are usually formed
by small N-terminal huntingtin fragments and are co-localized with other
cellular proteins involved in proteolysis, vesicle trafficking and protein
degradation. The formation of huntingtin aggregates and intranuclear inclusions
has been proposed to play a role in HD pathogenesis [38].

Polyubiquitinated
proteins are targeted for degradation by the proteasome, which is a large
enzymatic complex found in all eukaryotic cells. The proteolytic core of the
proteasome or “20S proteasome” is a 28 subunit multi-catalytic
particle consisting of four heptagonal rings. The 20S proteasome can also be
associated with one or two 11S (PA28) particles, which consist of a– and b-subunits
that can be induced by interferon g (IFN-g) and result in so-called immunoproteasomes. The
association of 11S and 20S particles is thought to also lead to a rearrangement
of a-subunit chains,
resulting in the widening of the openings to the 20S barrel, thereby
facilitating the access of substrates and the exit of peptide fragments at the
other end of the proteolytic chamber. In response to IFN-g, hybrid proteasomes can also assemble into particles
consisting of 20S proteasomes with a 19S particle at one end and an 11S
particle at the other [39]. As a result of this switch in regulatory particles,
there is an increase in ATP-independent degradation of small peptides, but not
proteins [40]. In mammals, IFN-g also induces
changes in the 20S proteasome; and the three catalytic subunits in 20S
particles are replaced by IFN-g inducible
subunits, namely LMP2 (bi1), LMP10 (bi2) and LMP7 (bi5) [41]. Proteasomes are referred to as
immunoproteasomes when they contain the inducible subunits. Oxidative
modification might make proteins susceptible to degradation by the
immunoproteasome, facilitating peptide generation and antigen presentation
[42]. A significant increase in LMP2 and LMP7 subunits was found in HD,
indicating an induction of the immunoproteasome. This increase correlates with
a rise in 20S proteasome activity assayed with fluorogenic substrates that are
processed in a Ub-independent manner. The immunoproteasome induction seems to
take place specifically in degenerating neurons in both huntingtin transgenic
mice (HD94) and HD patient brain extracts [43]. Cell lines expressing
polyglutamine expansion constructs also show an increase in LMP2 subunit
expression [44]. These results imply that the immunoproteasome can be involved
in neurodegeneration.

QC System and Prion Diseases

Prion (proteinaceous
infectious particles) diseases are fatal neurodegenerative disorders that have attracted
enormous scientific attention because they exemplify a novel mechanism of
biological information transfer based on the transmission of protein
conformation rather than on the inheritance of nucleic acid sequence. PrPSc is the key
protein involved in the origin of prion disease. It is a conformationally
altered isoform of the cellular protein PrPC, a protein of unknown function that is encoded by a
cellular gene [45]. PrPSc and probably other neurotoxic forms of PrP are
structurally distinct from PrPC, and these
molecules would be recognized as “abnormal” by the ER QC machinery.
They are retained in the ER and degraded by proteasomes. It is possible that
PrPSc damages neurons by activating stress-induced
signaling pathways that are engaged by the accumulation of misfolded proteins
in the ER, such as UPR. Furthermore, some other PrP molecules carrying
disease-related mutations are only partially trapped, and a fraction of them
are able to reach the cell surface. Retained PrP might trigger ER stress response
pathways, those that reach the surface are known to become aggregated and
weakly proteinase K-resistant and might damage cells by altering membrane
properties, interacting abnormally with other proteins, or other mechanisms.
Interestingly, in Tg (PG14) mice, an animal model of prion, mutant PrP
accumulates continuously as the animals age, until a threshold level is reached
for the development of neuropathology and clinical symptoms. This phenomenon
could reflect a gradual decrement in the efficiency of the proteasomal
degradation system during aging, with consequent build-up of mutant PrP in the
ER. If ER retention of mutant PrPs does occur in familial prion diseases, then
this finding would link these diseases to other human disorders, both inherited
and sporadic, that are thought to involve the action of protein QC mechanisms
[46]. These include other neurodegenerative diseases such as AD, HD and PD, as
well as disorders that affect peripheral organ systems such as CF, congenital
hypothyroidism, and a1-AT
deficiency. The common theme of all of these diseases is thought to be the
misfolding of a polypeptide that is recognized by the cell as abnormal and is
then subject to retention in the ER (for secreted or membrane proteins) or sequestration
in cytoplasmic inclusions (for soluble proteins), followed by proteasomal
attack.

Summary

It remains unclear how
the ER decides at the molecular level between the protein folding and ERAD
pathways. The kinetics of the folding and degradation pathways may simply
differ, as glucosidase activity is suggested to be higher than mannosidase
activity. This may favor the folding cycle over ERAD. After multiple rounds of
chaperone binding, proteins that inefficiently fold are eventually targeted for
degradation. However, as the molecular mechanisms of a growing number of
genetically inherited diseases are uncovered, it is increasingly appreciated
that errors in folding and cellular trafficking are more frequent than
anticipated. Thus, the development of strategies aimed at promoting proper
folding and maturation of mutant proteins could provide new therapies for a
wide spectrum of diseases.

References

 1   Tu BP, Weissman JS. Oxidative protein folding
in eukaryotes: Mechanisms and consequences. J Cell Biol 2004, 164: 341–346

 2   Kleizen B, Braakman I. Protein folding and quality
control in the endoplasmic reticulum. Curr Opin Cell Biol 2004, 16: 343–349

 3   Thomas PJ, Qu BH, Pedersen PL. Defective protein
folding as a basis of human disease. Trends Biochem Sci 1995, 20: 456–459

 4   Qin ZH, Wang Y, Sapp E, Cuiffo B, Wanker E,
Hayden MR, Kegel KB et al. Huntingtin bodies sequester vesicle-associated
proteins by a polyproline-dependent interaction. J Neurosci 2004, 24: 269–281

 5   Perlmutter DH. Liver injury in alpha1-antitrypsin
deficiency: An aggregated protein induces mitochondrial injury. J Clin Invest
2002, 110: 1579–1583

 6   Ellgaard L, Molinari M, Helenius A. Setting
the standards: Quality control in the secretory pathway. Science 1999, 286:
1882–1888

 7   Guo L, Groenendyk J, Papp S, Dabrowska M,
Knoblach B, Kay C, Parker JM et al. Identification of an N-domain
histidine essential for chaperone function in calreticulin. J Biol Chem 2003,
278: 50645–50653

 8   Michalak M, Robert Parker JM, Opas M. Ca2+ signaling and calcium binding chaperones of
the endoplasmic reticulum. Cell Calcium 2002, 32: 269–278

 9   Michalak M, Lynch J, Groenendyk J, Guo L,
Robert Parker JM, Opas M. Calreticulin in cardiac development and pathology.
Biochim Biophys Acta 2002, 1600: 32–37

10  High S, Lecomte FJ, Russell SJ, Abell BM, Oliver JD. Glycoprotein
folding in the endoplasmic reticulum: A tale of three chaperones? FEBS Lett
2000, 476: 38–41

11  Trombetta, ES, Parodi AJ. Quality control and protein folding in
the secretory pathway. Annu Rev Cell Dev Biol 2003, 19: 649–676

12  Roth J, Ziak M, Zuber C. The role of glucosidase II and
endomannosidase in glucose trimming of asparagine-linked oligosaccharides.
Biochimie 2003, 85: 287–294

13  Shen X, Zhang K, Kaufman RJ. The unfolded protein response¾a stress
signaling pathway of the endoplasmic reticulum. J Chem Neuroanat 2004, 28: 79–92

14  Knee R, Ahsan I, Mesaeli N, Kaufman RJ, Michalak M. Compromised
calnexin function in calreticulin-deficient cells. Biochem Biophys Res Commun
2003, 304: 661–666

15  Ellgaard L, Riek R, Herrmann T, Guntert P, Braun D, Helenius A,
Wuthrich K. NMR structure of the calreticulin P-domain. Proc Natl Acad Sci USA
2001, 98: 3133–3138

16  Schrag JD, Bergeron JJ, Li Y, Borisova S, Hahn M, Thomas DY, Cygler
M. The structure of calnexin, an ER chaperone involved in quality control of
protein folding. Mol Cell 2001, 8: 633–644

17  Molinari M, Helenius A. Chaperone selection during glycoprotein
translocation into the endoplasmic reticulum. Science 2000, 288: 331–333

18  Bottomley MJ, Batten MR, Lumb RA, Bulleid NJ. Quality control in
the endoplasmic reticulum: PDI mediates the ER retention of unassembled procollagen
C-propeptides. Curr Biol 2001, 11: 1114–1118

19  Sitia R, Braakman I. Quality control in the endoplasmic reticulum
protein factory. Nature 2003, 426: 891–894

20  Selkoe DJ. Folding proteins in fatal ways. Nature 2003, 426: 900–904

21  Bernier V, Lagace M, Bichet DG, Bouvier M. Pharmacological chaperones:
Potential treatment for conformational diseases. Trends Endocrinol Metab 2004,
15: 222–228

22  23  Perlmutter DH. Chemical chaperones: A pharmacological strategy for
disorders of protein folding and trafficking. Pediatr Res 2002, 52: 832–836

24  Bernier V, Bichet DG, Bouvier M. Pharmacological chaperone action
on G-protein-coupled receptors. Curr Opin Pharmacol 2004, 4: 528–533

25  Kim BE, Smith K, Meagher CK, Petris MJ. A conditional mutation
affecting localization of the Menkes disease copper ATPase. Suppression by
copper supplementation. J Biol Chem 2002, 277: 44079–44084

26  Frustaci A, Chimenti C, Ricci R, Natale L, Russo MA, Pieroni M, Eng
CM et al. Improvement in cardiac function in the cardiac variant of
Fabry’s disease with galactose-infusion therapy. N Engl J Med 2001, 345: 25–32

27  Morello JP, Petäjä-Repo UE, Bichet DG, Bouvier M. Pharmacological chaperones:
A new twist on receptor folding. Trends Pharmacol Sci 2000, 21: 466–469

28  Sharma M, Pampinella F, Nemes C. Benharouga M, So J, Du K, Bache KG
et al. Misfolding diverts CFTR from recycling to degradation: Quality
control at early endosomes. J Cell Biol 2004, 164: 923–933

29  Le A, Steiner JL, Ferrell GA, Shaker JC, Sifers RN. Association
between calnexin and a secretion-incompetent variant of human a 1-antitrypsin. J Biol Chem 1994, 269: 7514–7519

30  Lord JM, Davey J, Frigerio L, Roberts LM. Endoplasmic
reticulum-associated protein degradation. Semin Cell Dev Biol 2000, 11: 159–164

31  Weissman AM. Themes and variations on ubiquitylation. Nat Rev Mol
Cell Biol 2001, 2: 169–178

32  Schmitz A, Herzog V. Endoplasmic reticulum-associated degradation:
Exceptions to the rule. Eur J Cell Biol 2004, 83: 501–509

33  Drenth JP, Martina JA, van de Kerkhof R, Bonifacino JS, Jansen JB.
Polycystic liver disease is a disorder of cotranslational protein processing.
Trends Mol Med 2005, 11: 37–42

34  Bonini NM. Chaperoning brain degeneration. Proc Natl Acad Sci USA
2002, 99: 16407–16411

35  Hershko A, Ciechanover A. The ubiquitin system. Annu Rev Biochem
1998, 67: 425–479

36  Glickman MH, Ciechanover A. The ubiquitin-proteasome proteolytic
pathway: Destruction for the sake of construction. Physiol Rev 2002, 82: 373–428

37  de Vrij FM, Fischer DF, van Leeuwen FW, Hol EM. Protein quality
control in Alzheimer’s disease by the ubiquitin proteasome system. Prog
Neurobiol 2004, 74: 249–270

38  QIN ZH, GU ZL. Huntingtin processing in pathogenesis of 39  Cascio P, Call M, Petre BM, Walz T, Goldberg AL. Properties of the
hybrid form of the 26S proteasome containing both 19S and PA28 complexes. EMBO
J 2002, 21: 2636–2645

40  Whitby FG, Masters EI, Kramer L, Knowlton JR, Yao Y, Wang CC, Hill
CP. Structural basis for the activation of 20S proteasomes by 11S regulators.
Nature 2000, 408: 115–120

41  Rivett AJ, Bose S, Brooks P, Broadfoot KI. Regulation of proteasome
complexes by g-interferon
and phosphorylation. Biochimie 2001, 83: 363–366

42  Teoh CY, Davies KJ. Potential roles of protein oxidation and the
immunoproteasome in MHC class I antigen presentation: The ‘PrOxI’ hypothesis.
Arch Biochem Biophys 2004, 423: 88–96

43  Diaz-Hernandez M, Hernandez F, Martin-Aparicio E, Gomez-Ramos P,
Moran MA, Castano JG, Ferrer I et al. Neuronal induction of the
immunoproteasome in Huntington’s disease. J Neurosci 2003, 23: 11653–11661

44  Ding Q, Lewis JJ, Strum KM, Dimayuga E, Bruce-Keller AJ, Dunn JC,
Keller JN. Polyglutamine expansion, protein aggregation, proteasome activity,
and neural survival. J Biol Chem 2002, 277: 13935–13942

45  Collinge J. Prion diseases of humans and animals: Their causes and
molecular basis. Annu Rev Neurosci 2001, 24: 519–550

46  Chiesa R, Harris DA. Prion diseases: What is the neurotoxic
molecule? Neurobiol Dis 2001, 8: 743–763