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Assembly chaperones: a perspective
R. John Ellis
School of Life Sciences, University of Warwick, Coventry CV4 7AL, UK
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Review
Cite this article: Ellis RJ. 2013 Assembly
chaperones: a perspective. Phil Trans R Soc B
368: 20110398.
http://dx.doi.org/10.1098/rstb.2011.0398
One contribution of 11 to a Theme Issue
‘Assembly chaperones in health and disease’.
Subject Areas:
biochemistry, cellular biology
Keywords:
molecular chaperones, self-assembly,
assembly chaperones, Rubisco,
histone chaperones
Author for correspondence:
R. John Ellis
e-mail: [email protected]
The historical origins and current interpretation of the molecular chaperone
concept are presented, with the emphasis on the distinction between folding
chaperones and assembly chaperones. Definitions of some basic terms in this
field are offered and misconceptions pointed out. Two examples of assembly
chaperone are discussed in more detail: the role of numerous histone chaperones in fundamental nuclear processes and the co-operation of assembly
chaperones with folding chaperones in the production of the world’s most
important enzyme.
1. Introduction
Perusal of the extensive literature about molecular chaperones might lead you
to conclude that their primary role is to reduce the tendency of newly synthesized and stress-denatured protein chains to misfold and aggregate
together by hydrophobic interactions in the cytosol and endoplasmic reticulum.
Such an emphasis on the role of chaperones in protein folding ignores the
fact that the term ‘molecular chaperone’ was coined originally to describe a
protein that is required for the assembly of folded histone proteins into oligomeric structures by electrostatic interactions with DNA inside the nucleus.
Thus the undoubted importance of chaperones in assisting protein folding [1]
has obscured their equally vital roles in protein assembly processes. Confusion
also persists about the precise relation between stress proteins and molecular
chaperones—are all chaperones also stress proteins, and are all stress proteins
also chaperones? How should we define the term ‘molecular chaperone’
today? In this review I offer an account of the historical origins of the chaperone
concept, suggest definitions of the terms used in this field and present a list of
some assembly chaperones. My aim is to offer a perspective for subsequent
articles that describe the roles of selected chaperones in protein assembly.
2. Origin
The term ‘molecular chaperone’ appeared first in 1978 in a paper from the laboratory of Ron Laskey to describe a nuclear protein required for the correct
assembly of nucleosomes from histones and DNA in extracts of amphibian
eggs [2]. Nucleosomes are oligomers of eight basic histone monomers bound
by electrostatic charge interactions to negatively charged eukaryotic DNA. Isolated nucleosomes can be dissociated into their histone and DNA components
by exposure to high salt concentrations. The principle of protein self-assembly,
established by the pioneer work of Anfinsen [3] and Caspar and Klug [4], states
that all the information required for protein chains to fold and assemble correctly with other proteins and/or nucleic acids is located in the primary
structure of those chains. Thus this principle predicts that a mixture of histones
and DNA placed in physiological salt concentrations should result in the
spontaneous assembly of nucleosomes. But when Laskey tried this experiment
with nucleosomes from Xenopus eggs, it was a spectacular failure; instead of
nucleosomes, an insoluble aggregate formed [2].
Experiments showed that addition of small amounts of Xenopus egg
homogenate prevents aggregate formation and allows nucleosome assembly.
The active factor was purified, found to be an abundant nuclear acidic protein
and named nucleoplasmin. This protein binds electrostatically to folded
histones, thereby reducing their basic charge, and permitting their inherent
& 2013 The Author(s) Published by the Royal Society. All rights reserved.
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The extension of the concept of a chaperone function to other
proteins had to wait until 1985. In that year I organized a
Royal Society Discussion meeting on the chloroplast protein
called Rubisco. This acronym stands for ribulose bisphosphate
carboxylase-oxygenase, the enzyme that brings carbon dioxide
into organic combination during photosynthesis. It is thus
arguably the world’s most important enzyme. It is also the
world’s most abundant enzyme, because it has a very low
turnover number; Rubisco is limiting to plant growth under
most environmental conditions so the plant has to make
large amounts to compete successfully [7]. It is also the
world’s most inefficient enzyme, because it is not saturated
at the current atmospheric level of carbon dioxide. Moreover,
Rubisco catalyses an oxygenase reaction that leads to a loss
of carbon dioxide during photosynthesis via the process of
photorespiration. None of these attributes mattered when
the enzyme first evolved, because at that time there was no
oxygen in the atmosphere, and the level of carbon dioxide
was much higher than it is today. Thus Rubisco has long
been the target of attempts to improve its properties for
today’s environment by genetic engineering [8]. All such
attempts have failed so far, one reason being the complex
involvement of chaperones in both its folding and assembly.
Thus it has not proved possible as yet to express chloroplast
Rubisco in active form in Escherichia coli [9].
Chloroplast Rubisco is a heteroligomer of eight catalytic
large subunits, synthesized inside the chloroplast, bound to
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3. Generalization
eight structural small subunits, made in the cytosol. Rubisco
is one of several proteins that fail to refold and assemble into
an active enzyme when diluted out from a denaturant in a
classic Anfinsen refolding experiment [10]. This failure
is because of the tendency of unfolded large subunits to
aggregate with one another and become insoluble. So, I
was surprised when work in my laboratory in 1980 revealed
that the unassembled large subunits synthesized inside isolated, intact chloroplasts are soluble. We established that
this is because they are bound non-covalently and transiently
to another protein before assembling into the Rubisco holoenzyme. We called this protein the Rubisco large subunit-binding
protein, and speculated that the complex of this protein with
newly synthesized large subunits is an obligatory intermediate
in the assembly of Rubisco [11]. This speculation was controversial because it implied that protein self-assembly is not
always a spontaneous process, but in some cases required the
transient involvement of pre-existing proteins.
I came across Laskey’s nucleoplasmin paper in 1985, and
realized that the role of the Rubisco large subunit-binding
protein could be thought of as similar to that of nucleoplasmin, that is, preventing aggregation by transiently masking
the interactive surfaces involved. For unfolded Rubisco
large subunits, these surfaces are hydrophobic in nature,
whereas for the folded histones they are charged, but the
principle is the same. At a Royal Society Rubisco meeting
in 1985, the proceedings of which appeared in the following
year [12], I made the suggestion that the Rubisco large
subunit-binding protein could be regarded as the second
example of a molecular chaperone.
I thought initially that nucleoplasmin and the Rubisco
large subunit-binding protein were special cases, evolved to
deal with proteins where aggregation is a particular problem.
But in 1987, a speculative paper by Hugh Pelham prompted
me to extend the chaperone idea to a much wider range of
proteins [13]. This paper makes no reference to nucleoplasmin and the Rubisco large subunit-binding protein, nor
uses the term ‘molecular chaperone’, but instead discusses
the possible roles of the heat-shock protein (HSP) 70 and
90 families in a range of assembly and disassembly processes.
Pelham proposed that heat shock proteins function in
unstressed cells by binding to exposed hydrophobic surfaces,
but are required in increased amounts in stressed cells to
unscramble aggregates, and to prevent further aggregation.
It occurred to me that all these ideas could be brought
together under the umbrella of a more fundamental chaperone
concept. I presented this idea at the NATO Advanced Study
Meeting in Copenhagen in 1987, where the Nature representative persuaded me to write a News and Views article. This
article appeared later that year with the opening sentence
‘At a recent meeting, I proposed the term “molecular chaperone” to describe a class of cellular proteins whose function is
to ensure that the folding of certain other polypeptides and
their assembly into oligomeric structures occur correctly’
(p. 378 [14]). It was this article that sparked the widespread
use of the term ‘molecular chaperone’ in the literature.
In 1988, my laboratory, together with that of the phage biochemist Roger Hendrix, published the finding that the Rubisco
large subunit-binding protein is 50 per cent identical with the
GroEL protein of E. coli [15]. GroEL was identified in several
laboratories in the 1970s as a bacterial protein required for
phages T4 and lambda to replicate. Its precise role was unclear,
but thought to involve the assembly of the phage head because
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self-assembly properties to predominate over their tendency to
aggregate non-specifically with negatively charged DNA [2].
Two characteristics of the mode of action of nucleoplasmin
were important in the subsequent development of the general concept of chaperone function that prevails today. First,
nucleoplasmin is required only transiently for nucleosome
assembly—it is not a component of the nucleosomes themselves. Secondly, nucleosomes can be assembled from histones
and DNA in the absence of nucleoplasmin if the high salt
concentration is reduced slowly by dialysis. Thus the role of
nucleoplasmin is not to provide steric information essential
for nucleosome assembly, but to reduce the strong basic
charge of the histones. The molecular details of how nucleoplasmin works are still being unravelled [5,6], but it is clear
that it does not involve the formation or breakage of covalent
bonds. The binding of nucleoplasmin can thus only be detected
by the use of non-denaturing techniques in the early stages of
nucleosome assembly. Later work revealed an additional role
of nucleoplasmin in decondensing sperm chromatin on fertilization of the egg, resulting in the replacement of the
protamine proteins of the sperm nucleosomes by the histone
proteins of the zygote. These properties of nucleoplasmin led
to the suggestion that ‘the role of the protein we have purified
is that of a “molecular chaperone” which prevents incorrect
interactions between histones and DNA’ ( p. 419 [2]).
Thus, the term ‘molecular chaperone’ was coined, not as a
metaphor or a whim, but because the properties of nucleoplasmin are a precise molecular analogy of the role of human
chaperones. The traditional role of human chaperones is to prevent incorrect interactions between pairs of human beings,
without either providing the steric information necessary for
their correct interaction or being present during married life.
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Before discussing how molecular chaperones are currently
viewed, I should like to offer some definitions of terms
used in this field.
(a) Definitions
Protein folding is the collapse of an extended primary translation product into a stable compact monomer. Monomers
possess primary, secondary and tertiary structure, but not
quaternary structure.
Protein assembly is the binding of monomers to one
another to produce a functional oligomer. Oligomers possess
quaternary structure.
The folding of a given polypeptide chain is characterized
by the formation of a stable fold specific to that sequence,
whereas assembly is characterized by the association of two
or more folded monomers into a biologically functional oligomer. The distinction between folding and assembly is not
absolute but quantitative, because in both processes there are
changes in the conformation of protein chains. These changes
in conformation are usually smaller during assembly than
during folding. Note also that, while folding is defined entirely
in structural terms, the definition of assembly contains a
biological criterion in addition to a chemical one. The word
‘functional’ is used to distinguish these oligomers from nonfunctional assemblies, and to make this explicit, non-functional
oligomers are commonly called aggregates or misassemblies.
This brings me to a fundamental point about the difference between chemistry and biology. What is important
in chemistry is structure, because structure determines the
properties of molecules. But what is important in biology
is not structure per se, but the function enabled by that
structure. This is because it is function that is selected for
in evolution—structure does not matter, provided it enables
functions that help the organism to survive in a competitive
world. For example, a variety of proteins called crystallins
are found in the lens of eyes, where they all provide required
properties of transparency and refractive index. But some
crystallins are metabolic enzymes and these vary between
species. Lactate dehydrogenase is a crystallin found in
avian and repilian eyes, but in the guinea pig it is replaced
by alcohol dehydrogenase [21]. So, the basic argument is
that, because proteins are the products of natural selection,
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Phil Trans R Soc B 368: 20110398
4. The molecular chaperone concept today
we should include biological criteria as well as chemical
criteria in our definitions.
Protein misfolding is the formation of a conformation that
cannot reach the native conformation on a biologically relevant
timescale. The term ‘native’ refers to the conformation of the
purified functional protein. The term ‘non-native’ is defined
as a conformation that is unable to reach the functional
conformation on any timescale.
This inclusion of a biological criterion distinguishes this
definition from others that are in use, such as the suggestion
that a misfolded conformation is one that has to unfold to
some extent before it can reach the native conformation
[22]. I propose this definition because the prefix ‘mis’ meaning ‘wrong’, when applied to proteins, can have a meaning
only in a biological context—a misfolded structure must
have a biological consequence. Note that, on my definition,
‘misfolded’ does not mean the same as ‘non-native’ because
a misfolded state could be a native state, i.e. a normal intermediate on its way to a functional state, but too slowly for
biological needs. Moreover, this definition aligns the meaning of the term ‘misfolding’ with that of ‘misassembly’ in a
pleasing symmetry.
Protein misassembly is the association of two or more polypeptide chains to form non-functional oligomers. A widely
used term to describe protein misassemblies is ‘aggregates’,
especially to describe so-called amyloid structures, such as
those associated with neurodegenerative diseases. A minor
problem with this term is that functional amyloids have
been discovered in both prokaryoyes and eukaryotes [23],
so it is preferable to use the term ‘misassembly’. Note that
misassembled states are always misfolded, by definition.
Molecular chaperones are a large and diverse group of
proteins that share the property of assisting non-covalent
folding and unfolding, and the assembly and disassembly,
of other macromolecular structures, but are not permanent
components of these structures when they are performing
their normal biological functions [24]. Chaperone substrates
include RNA as well as proteins, because nucleic acids have
less information content than proteins and so suffer more
often from misfolding and misassembly. The reason for this
difference in information content is that there are only
4 bases but 20 amino acids [25].
It is important to note that this definition is functional, and
not structural, and contains no constraints on how different molecular chaperones may act. The qualification ‘non-covalent’ is
used to exclude those proteins that catalyse co- and posttranslational modifications. Protein disulphide isomerase
might appear to be an exception, but it is both a covalent modification enzyme and a molecular chaperone [26]. There is no
reason in principle why molecular chaperones should not
possess additional functions, such as cell–cell signalling [27].
It is for this reason that I suggest that it is more useful to think
of a molecular chaperone as a function, rather than a molecule;
on this basis, a chaperone function can be a property of a molecule that may have additional and quite different functions.
Chaperone function is to prevent and/or reverse incorrect
interactions that may result when potentially interactive surfaces are exposed to the crowded intracellular or extracellular
environments. These surfaces occur on nascent and newly synthesized protein and RNA chains, on mature proteins unfolded
by environmental stress or covalent modification, and on
folded proteins in native and near-native conformations. Incorrect interactions are defined as those that result in biologically
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a mutation in GroEL results in the head proteins forming an
insoluble aggregate [16], but what its role in uninfected cells
might be was not addressed. In 1988 it was also reported that
antisera to GroEL identified a protein inside mitochondria
from various plant and animal species [17].
My postdoc, Sean Hemmingsen and I realized that there
was now evidence from both bacteria and chloroplasts that
linked the involvement of highly similar pre-existing proteins
in the assembly of newly synthesized proteins in a manner
that fitted the general concept of molecular chaperones that
I had proposed in 1987. Sean named these chloroplast and
bacterial proteins the ‘chaperonins’, and he and I wrote the
first comprehensive proposal for the existence of a widespread class of cellular proteins that acted as molecular
chaperones [18]. Detailed histories of the discovery of the
molecular chaperone function have been published [19,20].
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(b) Misconceptions
(1) All chaperones assist protein folding. The falsity of the belief
that ‘all chaperones assist protein folding’ is demonstrated,
not only by the origins of the chaperone concept, but also
by the subject matter of this theme issue.
(2) Chaperones fold proteins. The related claim that ‘chaperones
fold proteins’ could be taken to imply that some chaperones provide steric information necessary for some
proteins to fold. If this were the case, the principle of
protein self-assembly would be falsified. But what the chaperone concept proposed from its inception is that the
principle of spontaneous self-assembly should be replaced
by the principle of assisted self-assembly [18]. Thus the
principle of self-assembly is retained, but modified to
include the need for chaperones that reduce unproductive
side reactions, particularly aggregation.
(3) All chaperones are promiscuous for their substrates. The
initial work on molecular chaperones was triggered by
the identification of the chaperonins, and so concentrated
on the mechanism of action of the chaperonin called
GroEL, together with the chaperone HSP70, both found
in E. coli [1]. These chaperones assist the folding of
many different, newly synthesized protein chains, so
the belief arose that chaperones are necessarily promiscuous. Now that over 100 families of chaperones have been
identified, it is clear that this is not the case for many of
them, such as PapD, HSP47, Lin Syc, ExbB, PrtM/PrsA
and proseqences. For example, PapD is a chaperone
found in the periplasmic space of E. coli, where it is essential for the correct assembly of pili. The gene for PapD
can be deleted without altering the viability of the
cell—it just cannot make pili any more [29]. But GroEL
is essential for the survival of E. coli under all conditions
because it assists the folding of over 80 different proteins,
including some essential ones.
(4) All chaperones hydrolyse ATP. The initial emphasis on GroEL
and HSP70 also led to the erroneous view that chaperones
necessarily hydrolyse ATP. Many chaperones, such as
nucleoplasmin, PapD, calnexin and protein disulphide
isomerase, do not require ATP to function.
(5) Molecular chaperonins. The occasional use of the term ‘molecular chaperonin’ in some journals suggests that some
people use these terms casually without reference to
either their meaning or their origin. The term ‘chaperonin’
is also sometimes wrongly used as synonomous with ‘chaperone’, but the chaperonins are just one particular family
of chaperone—the family that contains GroEL. Families of
Despite this, the idea persists. For example, a recent
survey of 300 articles listed in PubMed in 2009 found that
one-third of them stated that molecular chaperones are
stress proteins and that stress proteins are molecular chaperones. A meta-analysis of the relations between the two sets
revealed that the majority of chaperone genes (66% for
humans and 72% for Arabidopsis) are not induced by heat
[32]. These figures are probably too small, because this analysis considered only well-studied chaperone families, and not
many other families of chaperone, including the growing
number of assembly chaperones, the subject of this theme
issue. So, most chaperones are not stress proteins, and most
stress proteins are not chaperones.
5. Assembly chaperones
Because changes in protein conformation occur both when new
protein chains are emerging from the ribosome and collapsing
into monomers, and during the subsequent stages of oligomer
assembly, the distinction between folding chaperones and
assembly chaperones is not absolute. For example, the trigger
factor is a folding chaperone that binds to many newly synthesized chains as they emerge from the exit tunnel of the
prokaryotic ribosome, while nucleoplasmin is an assembly
chaperone that binds to folded histone proteins before they
bind to DNA. Between these two extreme examples, many of
the intermediate stages in the formation of functional oligomers also require chaperoning to minimize unproductive
side reactions. The same observation applies to mature proteins
destined either for degradation or for refolding after stressinduced denaturation. The situation is complicated further by
the fact that some members of the same protein family, as
defined by sequence similarity, act as folding chaperones,
while other members act as assembly chaperones.
I am unaware of any comprehensive list of assembly chaperones, but table 1 lists some of the best-studied examples.
I shall discuss two of these in more detail.
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Like many fields, the chaperone field is noted for the persistence of various myths, perhaps because people come across
this term in casual use at conferences without researching its origins. Examples of misconceptions found in the
literature include the following.
chaperone are defined by sequence similarity, i.e. each
family contains proteins with similar sequences.
(6) Chemical chaperones. The term ‘chemical chaperone’ is
used to describe small molecules such as glycerol,
dimethylsulphoxide and trimethylamine N-oxide that
act as protein-stabilizing agents. This terminology is
unfortunate, and can confuse students who sometimes
ask me whether that means that proteins are not chemicals! The term should be replaced by ‘pharmacological
chaperone’ or ‘kosmotrope’, the term used by physical
chemists to describe small molecules that stabilize proteins. Kosmotrope is the opposite of ‘chaotrope’, used
to describe molecules that unfold proteins.
(7) Chaperones are another name for heat-shock proteins. It is a mistake to think that chaperones such as HSP70 and HSP90
are present only under stress conditions. The first papers
to present the chaperone concept made no suggestion
that they were necessarily stress (or heat-shock) proteins,
although some clearly were [14,15,18]. It was argued
instead that some chaperones are stress proteins, because
the need for the chaperone function increases as proteins
unfold under stress conditions. Subsequent reviews
emphasized that only a subset of chaperones are stress proteins, and thus that chaperones and stress proteins are
overlapping sets, not identical sets [30,31].
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non-functional products; such products are often cytotoxic,
e.g. those implicated in neurodegenerative disease.
Cochaperones are proteins that modulate chaperone activity.
Some cochaperones are also chaperones, e.g. HSP40. There
is an expanding literature on cochaperones, especially those
that modulate the functions of HSP70 and HSP90 proteins. Cochaperones modulate the activity of their cognate
chaperones in a variety of different ways [28].
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Table 1. Examples of assembly chaperone.
chaperone(s)
reference
nucleosome
nucleoplasmin,
[33,34]
nucleophosmin, Asf1,
CAF-1 etc.
RbcX
PAC1/2/3, hUmp1
[35]
[36]
pilus
hemoglobin
PapD
AHSP
[29]
[37]
nuclear hormone
HSP90
[38]
receptors and
protein kinases
ribosome
myosin
RAC, NAC, Jjj1
UNC-45
[39]
[40]
spliceosomal snRNPs
plCln
[41]
(a) Nuclear chaperones
The first molecular chaperone to be discovered, nucleoplasmin, was characterized by its ability to bind histones
[2], but subsequent research identified many other nuclear
chaperones involved in all phases of the synthesis, transport,
assembly and disassembly of histones [33]. The nuclei of
eukaryotic cells are crowded with molecules presenting high
densities of electrostatic charge to the intranuclear environment, i.e. basic proteins and nucleic acids. This presentation
creates the potential for incorrect interactions between molecules bearing many charges of opposite sign. This potential
is reduced by the activities of various nuclear assembly chaperones that function during the deposition of histones onto newly
replicated DNA, the accumulation of histones in oogenesis,
the replacement of sperm protamines by histones after fertilization, the repair of damage to DNA outside S phase and
selective gene expression in non-dividing cells. Thus histone
chaperones bind to histones during their synthesis in the cytosol, escort them into the nucleus and assist the removal and
deposition of histones during transcription, replication and
DNA repair. A more recent discovery is that histone chaperones are also involved in the post-translational modification
of histones that act as epigenetic markers [34].
The number of known histone chaperones is over 25 and
continues to grow. De Koning et al. [33] suggest that they can
be divided into three main classes:
(1) those that bind, transport or transfer histones without
involving additional partners, such as Asf1;
(2) multichaperone complexes that combine several histone
chaperone subunits, such as the CAF1 complex; and
(3) chaperones that provide histone binding capacity within
large enzymatic complexes, such as Arp4.
Histone chaperones can also be distinguished by their specificity for different histones. Table 2 provides a snapshot of
histone chaperones and the processes that they influence.
During DNA replication in S-phase, chaperones assist the
disassembly of the existing nucleosomes, and recycle them to
form new nucleosomes on the newly synthesized DNA. The
extra histones required during DNA replication are
(b) Rubisco chaperones
Experiments show that chloroplast Rubisco is a major factor
limiting plant growth, so many attempts have been made
to try to improve its properties by genetic engineering [7,8].
All such attempts have failed. One problem is the strong tendency of unassembled large subunits to aggregate together.
The discovery of the chaperonins [15] raised hopes that coexpression of chaperonin genes with chloroplast Rubisco
genes in E. coli would allow correct assembly. However, the
only success in this type of experiment has been some cyanobacterial Rubiscos (called form I), and with a simpler form of
Rubisco (called form II) found in anaerobic bacteria, consisting of just two large subunits [42]. Rubiscos from these
prokaryotes have no agricultural relevance.
The same limitation applies to attempts to renature
Rubisco from the denatured state in vitro. The form II Rubisco
from anaerobic bacteria can be successfully renatured in vitro,
provided that the chaperonins from E. coli (GroEL and
GroES) are present, but chloroplast Rubisco cannot, nor can
the similar Rubisco found in some cyanobacteria (called
form I). The simplest assumption is that additional chaperones may be required for the assembly of form I Rubisco in
some species, including species of green plants, and recently,
support has been gained for this idea from studies on the
in vitro assembly of a cyanobacterial Rubisco after release of
large subunits from GroEL. These studies suggest that an
additional chaperone called RbcX is required for the assembly
of cyanobacterial Rubisco in some species [35].
RbcX functions by inhibiting the tendency of partially
folded large subunits to aggregate together, and allows
them to interact correctly with small subunits. Rubisco
large subunits released from ribosomes are enclosed one at
a time inside the cavity in GroEL capped by GroES. Inside
this cavity, the single protein chain folds without the
danger of it aggregating with similar folding chains. The
partly folded chain is then released from the cage, a process
that requires ATP hydrolysis, but is still not entirely free
from the aggregation hazard, because it exposes a disordered
C-terminal peptide of about 60 residues. A conserved binding
motif of seven residues in this flexible segment is recognized
by a hydrophobic cleft in RbcX. In this way, dimers of large
subunit are bound to dimers of RbcX, and so are prevented
from aggregating. This binding motif is absent from form II
Rubiscos, which comprise only two large subunits.
The RbcX-large subunit dimers assemble into octamers.
Rubisco small subunits then displace RbcX from these octamers, forming the Rubisco holoenzyme. Like most assembly
chaperones, but unlike many folding chaperones such as
GroEL/GroES, RbcX is highly specific for its protein substrates
and does not require ATP hydrolysis to function. Figure 1
Phil Trans R Soc B 368: 20110398
Rubisco
proteasome
5
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oligomer
synthesized during S phase, and histone chaperones mediate
their transport into the nucleus and their deposition in new
nucleosomes. The passage of RNA polymerase II during transcription requires local chromatin rearrangements, and this
requires histone replacement and exchange. This exchange
can involve acetylation of some histones, which weakens
their binding to DNA and thus increases the accessibility of
DNA locally.
It is becoming increasingly obvious that assembly chaperones are involved in many, if not all, of the fundamental
processes that occur in the nucleus.
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6
RbcL
GroES
GroEL
1
2
3
6
5
4
Figure 1. Model of GroEL/ES and RbcX2-assisted folding and assembly of cyanobacterial Rubisco. 1. Folded Rubisco large subunit (RbcL) with a disordered C-terminal region
is transiently released from GroEL/ES. 2. RbcX2 binds to the exposed RbcL C-terminus. 3. RbcL dimers are formed, ‘stapled’ together by the interaction of RbcX2 with the
C-terminus of one RbcL and the N-terminal domain of the adjacent subunit. 4. Stable dimers assemble to RbcL8-(RbcX2)8 complexes. 5. Binding of Rubisco small subunit
(RbcS) weakens the RbcL–RbcX2 interaction. Dissociation of RbcX2 and binding of RbcS may occur in a stepwise manner, populating intermediates. 6. RbcX2 dissociates,
enabling the C-terminal region of RbcL to adopt its final position and allowing maturation of Rubisco. Reproduced from [35] with permission.
Table 2. Some histone chaperones (modified from reference [33]).
class
class 1: single chaperones
histone specificity
name
function
H3 – H4
Asf1
donor for CAF-1 and HIRA
H3 – H4
H3 – H4
Fkbp39p
HIRA
rDNA silencing
histone deposition without replication
H3 – H4
H3 – H4
N1/N2
Spt6
H3/H4 storage in Xenopus oocytes
transcription initiation and elongation
H3 – H4
H2A – H2B
Rtt106
nucleophosmin, nucleoplasmin
heterochromatin silencing
transport, replication and storage in Xenopus oocytes
class 2: multi-chaperones
H3 – H4
CAF-1 complex
deposition coupled to DNA replication & repair
class 3: chaperones within
H2A – H2B
H3 – H4
FACT complex
Hif1
elongation during transcription
assisting histone acetyl acetylase (HAT)
H3 – H4
not determined
Rsf-1
Arp4/7/8/9
remodelling of chromatin
remodelling of chromatin
not determined
Acf1
remodelling of chromatin
enzymatic complexes
illustrates the complementary roles of the chaperonins and
RbcX in the folding and assembly of the form I Rubisco from
cyanobacteria, as deduced from in vitro renaturation studies
with purified components [35].
Homologues of RbcX are found in plants [43], so an
obvious strategy for future research on the chloroplast
Rubiscos is to see whether they can also be renatured in
vitro by supplying cognate chaperonins and RbcX. Or could
it be that even more chaperones are required to assemble
the world’s most important enzyme?
I thank Ulrich Hartl for kindly commenting on the draft manuscript.
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