ChaperoninsValerio Consalvi, Sapienza Universita di Roma, Rome, Italy

Roberta Chiaraluce, Sapienza Universita di Roma, Rome, Italy

The challenge in protein folding is the production of the

native, active conformation from the primary sequence

information. In vivo, in the cytosolic environment, acqui-

sition of the native conformation is in competition with

proteolytic degradation and/or intracellular precipi-

tation. Chaperone proteins assist in finding the native

fold in all cellular phases, and these key components

of the cellular machinery occur ubiquitously through

archaea, bacteria and eukarya. Chaperonins are complex

polymeric proteins that form a closed environment, the

‘Anfinsen cage’, to allow correct protein folding triggered

by adenosine triphosphate hydrolysis, avoiding inter-

molecular aspecific aggregation. The chaperonins pre-

ventproteinunfoldingandmisfolding,events responsible

for several human diseases, such as cancer and amyloid

diseases. The chaperonins are considered potential drug

targets due to their role in protein misfolding, aggre-

gation and denaturation and in cellular signalling.

Introduction

The decoding of the linear primary sequence informationinto the unique three-dimensional tertiary structure thatcorresponds to the native and active conformation of aprotein has long been recognised as the protein foldingproblem. In vivo, despite all the necessary informationbeing contained in the primary sequence, the newly syn-thesised polypeptidemay require the contribution of helperproteins to fold properly. In the crowded cytosol environ-ment, where the macromolecular concentration may reachthe astonishing value of more than 300 gL21, the acqui-sition of the native conformation is in continuous com-petition with proteolytic degradation and/or intracellularprecipitation, and the formation of poorly soluble folding

intermediates may prevent the acquisition of the correctnative and biologically active conformation. Hence, pro-ductive folding in vivo is not simply a direct consequence ofprotein synthesis. The chaperones are proteins, which helpto achieve the native fold, and they are involved in thecorrect assembly of many proteins in various cellularcompartments. The nonnative proteins are the naturalligands of chaperones. These key components of the cellmachinery occur ubiquitously from archaea to bacteria toeukarya, and may exert their functions in all cellular pha-ses. See also: Protein Folding and Chaperones; ProteinFolding In Vivo; Protein Folding: Overview of Pathways

The Chaperonin Family ofChaperones

Chaperonins, ubiquitous molecular chaperones found inall kingdoms, are distinguished in two groups: those foundin eubacteria and eukaryotic organelles (group I) and inarchaea and in the eukaryotic cytosol (group II). Themolecular structures of chaperone families, and theirmechanisms of action, are notably divergent, despite thecommon cellular functions and some common structuralfeatures. They were originally discovered and named heatshock proteins, on the basis of their heat and/or stressinducibility. The simplest structural classification is basedon the molecular mass (Ranson et al., 1998; Hartl andHayer-Hartl, 2009), which is often reported in kilodaltonsafter the acronym ‘hsp’. The chaperones prevent the for-mation of misfolded structures and the deposition ofmassive intracellular aggregates, andmay contribute to therecovery of the nonnative proteins that may form understress conditions. After binding with the chaperones, thenonnative proteins find their destiny in the cell: nativefolding and/or quaternary association, intracellulartransportation and, sometimes, degradation (Hartl, 1996).The chaperonin family of chaperones is formed by complexpolymeric proteins that form a closed environment, the‘Anfinsen cage’, to encapsulate the nonnative protein andallow correct protein folding in the internal compartment,avoiding intermolecular aspecific aggregation. These largemultimeric folding machines are found in archaea

Advanced article

Article Contents

. Introduction

. The Chaperonin Family of Chaperones

. Substrate-binding Modes

. The GroEL Structure

. ATP-induced Structural Changes

. The GroEL–GroES Complex

. Chaperonins as Drug Targets

Online posting date: 15th November 2012

eLS subject area: Structural Biology

How to cite:Consalvi, Valerio; and Chiaraluce, Roberta (November 2012)Chaperonins. In: eLS. John Wiley & Sons, Ltd: Chichester.

DOI: 10.1002/9780470015902.a0003019.pub3

eLS & 2012, John Wiley & Sons, Ltd. www.els.net 1

(thermosome), in prokaryotic (GroEL/GroES) and eukar-yotic cytosol (TCP1-ring complex (TRiC)) and also inchloroplasts (cpn60/cpn10) and in mitochondria (hsp60/hsp10). The interaction of substrate proteins with chaper-onins is regarded as more than a passive interaction, whichdoes not impose any steric constraint and/or does notincrease the rate of the folding process (Radford, 2006;Jewett and Shea, 2010). The encapsulation in the chaper-onin increases the rate of folding of certain proteins, aneffect that was not anticipated in the original ‘Anfinsencage’ model.

Group I

Group I of the chaperonins, commonly termed the GroELsubfamily, always presents a 7-fold symmetry (Horwichet al., 2007; Yebenes et al., 2011). The hsp60 (GroEL inEscherichia coli) is arranged in a complex tetradecamer of

58 kDa subunits formed by a double-stacked seven subunitring (Radford, 2006). The two heptameric back-to-backrings enclose a cylindrical central cavity that can contain asingle substrate protein. The complete folding cyclerequires the additional presence of a heptameric ringformed by the 10 kDa subunits of the co-chaperonin hsp10(GroES in E. coli) (Figure 1). The co-chaperonin GroESbinds to GroEL in the presence of adenosine triphosphate(ATP) and caps the ‘cage’, thus sequestering the substratepolypeptide contained in the central cavity. The nonnativepolypeptide, after the first interaction with GroEL,becomes a solitary guest in the folding chamber whose roofis formed by GroES and walls by GroEL.

Group II

Group II of the eukaryal chaperonins, the TRiC subfamilyor chaperonin-containing t-complex polypeptide (CCT),

(a) (b)

(d)

GroES

GroEL

GroEL(c)

Figure 1 The group I chaperonin family. (a) The crystal structure of the asymmetric chaperonin complex GroEL and GroES with ADP (protein database code

1AON.pdb; at http://www.pdb.org/pdb/home/home.do). The 14 identical 58 kDa subunits in the two asymmetric rings of GroEL are represented with

different colours in space-filling format. In the top of the upper ring are visible the subunits of the heptameric cochaperonin GroES. (b) The bottom view of

the same structure as in (a) shows the internal cavity of the ‘folding cage’. (c) and (d) The crystal structure of GroEL in complex with ATP (protein database

code 1KP8.pdb). (c) The 14 identical subunits in GroEL are represented with different colours in space-filling format. (d) The equatorial, intermediate and

apical domains of each subunit in the two rings are shown in blue, green and orange, respectively. The figures were produced with DS Viewer Pro version 6.0

(Accelrys software, Inc.)

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has a cage-like toroidal structure similar to that of GroEL,formed by two rings of eight 55 kDa subunitswith a hetero-oligomeric architecture (Yebenes et al., 2011; Figure 2). Thiseukaryotic cytosolic chaperonin is especially active in thefolding of the cytoskeleton structural proteins tubulins andactins. The archaeal group II chaperonin, commonlynamed thermosome, is formed by two rings of eight or ninesubunits of one or three types, and is closely related to thegroup II chaperonin evolved in the eukaryal lineage (Ler-oux and Hartl, 2000; Figure 2). Both the archaeal andeukaryal chaperonins (as distinct from GroEL whichrequires the presence of a co-chaperonin to exert its func-tions) close the central ‘folding cage’, containing the sub-strate polypeptide, by means of protrusions from specificregions lining the mouth of the cavity. Both chaperoninsmay exist in an open and a closed conformation (Figure 2);the transition between the two conformational states isinduced by ATP binding and hydrolysis (Yebenes et al.,2011; Cong et al., 2012). The conformational change thatseals the ring may force the folding of substrate proteins,which are specifically bound in a quasifolded state (Val-puesta et al., 2002). See also: Chaperones, Chaperonin andHeat-Shock Proteins

Substrate-binding Modes

The biosynthesis of a protein chain is a vectorial processinitiated from the N-terminus. The availability of all thenecessary folding information and constraints encoded inthe primary sequence is dependant on its complete bio-synthesis. This sequential event is not completed duringtranslation until all the residues emerge from the ribosome,and for large proteins the exit tunnel is too narrow to allowchain compaction inside the bacterial ribosome (Younget al., 2004). The formation of the native tertiary contactsmay be complicated by the competing reactions of aggre-gation among exposed hydrophobic residues not yet cor-rectly buried in the interior of the protein core, and situatedin close proximity because of molecular crowding.Cotranslational formation of folded domains would beexpected to partly solve this problem, but this may happenonly for fast folding domains capable of forming a native-like structure on the timescale of seconds, or for thoseprotein chains whose domains are formed by contiguoussegments of the sequence. The chaperonins are thereforedesigned to interact with many of those proteins that areeither not able to fold cotranslationally into their nativestructure, or that require assistance during a slow post-translational process of folding. See also: Protein Second-ary Structures: Prediction; Protein Tertiary Structures:Prediction from Amino Acid Sequences

Estimates of the number of E. coli proteins that aresubstrates of the chaperonin GroEL have been establishedat around approximately 5–15% of the total 2500 cyto-plasmic proteins, under normal cellular growth conditions,although only 84 proteins are predicted to depend totallyon GroEL to fold correctly (Ellis, 2005). Aggregation is

prevented by the direct interaction of the chaperonins withnonnative proteins prior to enclosing them in the interior ofthe ‘folding cage’. This entrapment should minimiseincorrect interactions within, and between, macro-molecules, and at the same time it may partially unfoldwrongly folded intermediates, providing them with a newstart in the search for their native conformation. Con-sidering the diversity of different protein chains that mayrequire folding assistance, the specificity of the macro-molecular binding interactions between the chaperoninand the substrate protein should be wide. The identity ofthe cytoplasmic substrate proteins in vivo, and the mode ofbinding, are intriguing aspects of the chaperonin’s functionthat have been difficult to elucidate, mainly because theprotein substrates of GroEL are unstructured. The lack ofstructure has hampered the crystallisation of chaperoninsbound to substrate(s), and the atomic resolution details ofsubstrate binding are unknown. Despite the lack of struc-tural studies on adducts of proteins with GroEL andGroES, information on the major factors determining theinteraction is available from biological studies, and it isknown that one of the protein substrates requires a min-imum of three adjacent binding sites for folding (Saibil,2000). The common denominator observed when usingmodel peptides to study themode of substrate binding in allthe chaperonins, independent of their family, is the plas-ticity of the flexible chaperonin region that is devoted tointeraction and binding with the substrate protein (Saibiland Ranson, 2002). See also: Crystallization of Proteinsand Protein–Ligand Complexes; Macromolecular Struc-ture Determination by X-ray CrystallographyIn the case of E. coliGroEL, the archetypal chaperonin,

the region involved in binding the substrate protein islocated in the apical domains of the 58 kDa subunits. Eachsubunit forms one part of the central body of the barrel-shaped heptamer by means of its intermediate and equa-torial domains (Figure 1). These latter domains form therigid part of the body, which is stacked back-to-backwith asecond seven subunit ring, whereas the intermediate andapical domains extend axially from both the equatorialtoroids with a more mobile conformation. The proteinsubstrates can bind to the hydrophobic sites of the apicaldomains lining the open mouth of the cavity via inter-actions similar to those that are used for the binding ofGroES, and for the closure of the central cavity. The regionof the apical domains involved in substrate binding can belarger than the hydrophobic groove that is also involved inGroES binding, and probably extends towards otherregions of the apical domain rich in hydrophobic residues.The apical domains are able to adapt their conformation todifferent types of model peptides, extended or a or b con-formation. Approximately, 50 proteins have been identi-fied in vivo bound to GroEL of the approximately 300proteins that are estimated to transit GroEL to reach thenative conformation. They belong mainly to the a–b anda–b–a sandwich folds, with ordinary isoelectric points anda preference for protein substrates of Mr=20–60 kDa.Several proteins have been observed to continue the

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(a) (b)

(c) (d)

TRiC

Open

Closed

(e) (f)

(h)(g)

Thermosome

Open

Closed

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interaction with GroEL throughout the course of theirlifetime, suggesting that, in addition to folding the nascentpolypeptides, the chaperonin may also be involved in thestructural maintenance of mature proteins.

The TRiC subfamily (group II) or ‘CCT’, despite thestructural similarity to group I bacterial chaperonins, isconsidered a specialised chaperone typically involved in thefolding of a few cytoskeletal proteins such as tubulins andactins that, exclusively for eukaryotes, are important forseveral cellular activities including muscle contraction,migration of organelles and chromosome segregation.Direct examination of the substrate spectrum of the TRiCsubfamily in vivo has demonstrated that approximately 5–10% of cytosolic cellular proteome require a direct inter-action with these eukaryal chaperonins as a preliminarystep to establish their correct fold (Yam et al., 2008; Conget al., 2012). Evidence obtained by the immunoprecipita-tion of TRiC–protein complexes (Leroux and Hartl, 2000)is contrary to preliminary observations of specific adap-tation of this chaperonin subfamily only for the specialisedfolding of cytoskeletal proteins. The size of the proteinsinteracting with TRiC ranges between 40 and 75 kDa,suggesting that TRiC substrates consist mostly of multi-domain proteins (Yam et al., 2008). The list of TRiC sub-strates includes firefly luciferase, a neurofilament, a viralcapsid protein, a myosin and the von Hippel–Lindautumour suppressor protein. Among the newly synthesisedcytosolic proteins, TRiC folds actin, tubulin and cell cycleregulators (Cong et al., 2012). In general, proteins of highb-sheet propensity and/or low a-helical content are thepreferred substrates of TRiC (Yam et al., 2008). In theprokaryotes, there are no homologues of these substrateproteins, and the GroEL/GroES complex is not able toassist the refolding of firefly luciferase. Interestingly, atumourigenic mutation in the von Hippel–Lindau tumoursuppressor protein prevents its release from the chaperoninand its incorporation into a functional complex. TRiCmayact in conjunction with cytosolic co-chaperones such asprefoldin, which protects the nascent chain from undesir-able reactions and transfers it to TRiC for subsequentfolding (Valpuesta et al., 2002; Lundin et al., 2010). Theinteraction of group II chaperonins with co-chaperones,such as hsp70, is necessary to deliver some protein sub-strates into the chaperonin cavity (Yebenes et al., 2011).Notably, the assisted protein folding in all the threedomains of life requires a complex machinery, whichinvolves several chaperone components to stabilise thenascent polypeptide chain, to initiate and complete thefolding process (Hartl and Hayer-Hartl, 2009). A majorproblem in understanding the binding of substrate proteinsto TRiC is caused by its hetero-oligomeric structure, whichsuggests the possibility of different binding specificities

among subunits. Those cytosolic proteins known to inter-act with TRiC have structures comparable with theGroELsubstrates, however a number of TRiC substrates cannotbe folded by other chaperonins, suggesting that TRiC/CCT may present unique structural and mechanisticproperties functionally distinct from other chaperonins(Cong et al., 2010; Leroux and Hartl, 2000; Spiess et al.,2004).Comparative primary sequence analysis between the

apical domains of GroEL and TRiC does not indicate anysignificant match between the conserved hydrophobicresidues of the GroEL, generally involved in bindinghydrophobic patches of nonnative proteins, and the cor-responding residues on the group II chaperonin sequence.The protrusions of the apical domain, which in this sub-family of chaperonins provide a closure for the centralchamber, are involved in the interactionwith the nonnativeproteins by means of the abundant hydrophobic residuespresent in this region. Hydrophobic interactions are con-sidered to be responsible of substrate recognition, as in thecase of von Hippel–Lindau tumour suppressor binding toCCT (Kubota et al., 2006; Yam et al., 2008). Actin seems tointeract with different binding sites located near the apicaldomains of CCT (Dekker et al., 2011). A sequence com-parison between GroEL and all the subunits of TRiCshows that some of the TRiC subunits present highsequence similarities with the hydrophobic residues of thecorresponding regions in GroEL. This observation mayimply the possibility of modulating specificities and tuningvariable affinities in anhetero-oligomer formedby subunitswith a different primary sequence. The TRiC chaperonin isan 1MDa hetero-oligomer of 16 subunits. ATP binding toTRiC and subsequent hydrolysis trigger a complex set ofstructural changes that induce the transition between theopen and closed states of the ring and the release of theunfolded protein inside the cavity for its folding (Yebeneset al., 2011; Munoz et al., 2011). The detailed subunitarrangement in TRiC has not been demonstrated conclu-sively because TRiC is refractary to crystallisation, how-ever low-resolution structural studies (Munoz et al., 2011)and cryo-electron microscopy (Cong et al., 2012) con-firmed that the TRiC particle is composed of two stackedoctameric rings enclosing a folding cavity (Figure 2). This16-subunit assemblywith eight distinct but similar subunitsarranged in two rings is likely to be conserved across alleukaryotes (Kalisman et al., 2012).

The GroEL Structure

The structure ofGroEL oligomer has been elucidated fromthe analysis of the molecular structure obtained by several

Figure 2 The group II chaperonin family. (a–d) Models of Bos taurus TRiC structure obtained by cryo-electron microscopy in the open (a, bottom view; b,

side view) and closed (c, bottom view; d, side view) conformation. (e–g) Models of the archeal thermosome from Methanococcus maripaludis in the open (e,

bottom view; f, side view) and closed (g, bottom view; h, side view) conformation. The 16-subunit complex, formed by eight similar subunits arranged in

two stacked rings, is represented with different colours by solid ribbon. The models of TRiC (4A0O and 4A0W) and of thermosome structure (3LOS and 3YIF)

were obtained at http://www.pdb.org/pdb/home/home.do. The figures were produced with Discovery Studio 3.0 (Accelrys software, Inc.). Copyright by

public domain.

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physical techniques. The 14 subunits form two heptamericrings stacked back-to-back, which encapsulate a centralcavity of approximately 50 A with holes in the side of thestructure (Figure 1). Each of the 58 kDa GroEL subunits isfolded into three distinct domains. The equatorial domainis very rich in helical structure and contains the binding sitefor ATP (Figure 3). This is the most rigid part of the mol-ecule, placed at the interface between the 7-fold rings, andundergoes only minor changes while functioning. Theintermediate domain is formed by both a and b secondarystructure elements and forms the wall of the GroELchamber. The intermediate domain functions as anarticulated joint, by means of its connection to the equa-torial and apical domains (Figure 3). The apical domain,a+b protein chain, is the outermost portion of the ring. Itprovides the interactive binding elements for GroES andfor the substrate proteins. The apical and intermediatedomains can move in concert as a single unit of the mol-ecule during the alternating cycle of ATP binding andhydrolysis to each equatorial domain in the two rings. Theability ofGroEL to adopt awide range of conformations toadjust to different proteins is the result of its structure, andthe intermediate domain plays a key role in controlling theallosteric mechanism of substrate binding and release(Saibil and Ranson, 2002). The concomitant and fullycoordinated movements of all the apical and intermediate

domains within the same ring result in a widening andelongation of the inner folding chamber to almost twice itsinitial size. At the same time, the concerted twist of all theapical domains of the same ring rotates the hydrophobicfaces of these domains away from each other and leads tothe exposure of hydrophilic residues in the central cavity.The rotation progressively occludes the ‘folding cage’,which is now ready to bind one molecule of GroES, aheptamer of 10 kDa subunits (Figure 1).

ATP-induced Structural Changes

ATP binds to GroEL with positive cooperativity and atight-binding equilibriumwithin the seven subunits of eachring, but with negative cooperativity between rings.Negative cooperativity between the two rings means thatthe high-affinity binding of ATP to one ring reduces thebinding affinity to the other, and the result is an asymmetricstructural conformation of the two heptameric rings, des-pite the identical subunit composition (Figure 1), becausethe nucleotide can bind to only one of the two availablerings. At low ATP concentration, both rings present highnucleotide affinity but, because of negative cooperativity,ATP binds to only one of the two rings, causing a decreasein the affinity of the other. The ring in the ATP-free state

Pi

ATP ATP ADP ADP

ADPADP ATPATP

ATP

GroES

ADP

+

Native

Nonnative

Native

+ADP

ATP

Pi

Nonnative

180°

ADP ADP

Figure 3 The alternating cycle of ATP binding and hydrolysis to GroEL. The GroEL tetradecamer and GroES are represented in a schematic view from a cut

along the longitudinal plane. The GroEL is formed by two seven-subunit rings stacked back-to-back. The equatorial, intermediate and apical domains of each

subunit in the two rings are shown in blue, green and orange, respectively. Two nonnative proteins binding to the opposite rings are represented in blue and

red. The two protein substrates progressively reach their native conformations inside the internal cavity. The apical and intermediate domains move in a

concerted action on binding of ATP. ATP hydrolysis is the timer of the folding process in the ‘cage’. The folding by GroEL involves each of the two opposite

rings alternately in the sequential steps represented in the scheme. The chaperonin accepts a nonnative protein in the empty ring when the opposite ring is

in the GroEL–GroES–ADP complex. The ring becomes competent for folding the nonnative protein on the binding of ATP and GroES. GroES binds alternately

to the two rings; its dissociation from GroEL is controlled by ATP binding to the opposite ring and is accompanied by the release of the freshly folded protein.

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binds the nonnative protein with a high affinity, whereas inthe ATP-bound state the affinity for the protein substratesis strongly reduced. The binding and hydrolysis of ATP isthus responsible for the start of a two-phase system, inwhich each ring is switched between the two states withstrong and poor affinity for the nonnative protein sub-strates. The binding of ATP to the equatorial domainsslightly decreases their flexibility, whereas the largestchanges on binding of ATP are produced in the inter-mediate and apical domains. These two domains are rad-ically reoriented relative to each other, with a significanttwist of the apical domains. The predominant effect on thestructure is an elongation of the heptamer, and the majorfunctional consequence is the progressive occlusion of thehydrophobic sites, with a significant decrease in the bindingaffinity for protein substrates and the concomitant expos-ure of the proper binding surfaces for GroES.

The GroEL–GroES Complex

The GroES subunit is a single-domain protein formed bynine b strands and a long mobile loop that becomes morestructuredon interactionwith the substrate-binding sites ofthe twisted apical domains on GroEL. GroES binding israpid in the presence of ATP, but slower with adenosinediphosphate (ADP), which stabilises the formation of thebullet-shaped 1:1 asymmetric complex between the co-chaperonin and GroEL (Figure 1). The massive conforma-tional change in the GroEL structure is the most remark-able aspect of the binding of the GroES heptamer over thetop of the folding chamber containing the protein sub-strate. The subunits of GroEL double the size of theinternal folding cavity, and the substrate protein bound tothe apical domains can thus be forcibly stretched andpartially unfolded during the movement of the substrate-binding sites. The protein encapsulated in the ‘folding cage’during the approximately 15 s of the ATPase cycle has theopportunity to fold into its native state at infinite dilutionand in a polar environment, due to a radical change in thesurface properties of the internal cavity, which is convertedinto an enlarged chamber lined with hydrophilic residues.ATP hydrolysis is not only the timer of the folding process,but preparesGroEL to releaseGroES, and is also the signalfor binding of a new protein ligand to the opposite ring(Figure 3). The importance of the interactions between thetwo rings through the back-to-back contact sites is evident.ATP hydrolysis also induces a conformational change tothe open opposite ring, and a new nonnative protein isbound to the more accessible substrate-binding sites,before the release of the previously folded one (Figure 3). Anew round of ATP binding to the opposite ring is thennecessary to trigger the release of GroES, and the escape ofthe freshly folded protein into the bulk solution. The cyclecan now start on the opposite ring, where the ATPaseactivity promotes the folding of the second substrate(Figure 3). Substrate protein that has failed to find the right

fold is then either recaptured by another chaperonin, or istargeted for proteolysis and degradation.

Chaperonins as Drug Targets

The cell responds to changes in environmental conditions,such as thermal and chemical stress, with increased pro-duction of heat shock proteins. The chaperonins are thenatural antagonists of the dramatic consequences of pro-tein unfolding and misfolding, events responsible for sev-eral human diseases (Dobson, 2004), and may also beinvolved in protein degradation by the ubiquitin–protea-some pathway. Abnormal conformations or assembly ofproteins is the cause of several degenerative diseases,including cystic fibrosis, Alzheimer, Parkinson and Hun-tington diseases. These diseases may be related to theimbalance between normal chaperone activity and pro-duction of toxic protein species, and the increase of chap-erone expression and/or activity as a possible therapeuticstrategy has been suggested (Barral et al., 2004).Moreover,mutations in genes encoding chaperone proteins areresponsible for at least five humandiseases, and thenumberof pathological conditions associated with chaperoninsdysfunctions will probably increase (Slavotinek and Bie-secker, 2001; Barral et al., 2004). Chaperonins have alsobeen implicated in bacterial infections, autoimmune dis-eases and complex pathologies such as arthritis and ath-erosclerosis (Ranford andHenderson, 2002). In particular,chaperonins of somepathogenic bacteria andhumanhsp60have been demonstrated to be antigens recognised by thecells of the innate immune system, and can specificallyactivate certain cell types to release inflammation medi-ators (Ranford and Henderson, 2002). The role of cha-peronins in initiating and/or sustaining the immunologicalcellular response is by no means completely unravelled,despite their presence in tissue and biological fluids duringtissue alterations and chronic inflammation. These studiesmay provide new avenues for the development of immu-notherapeutics, as shown by the therapy of tuberculosis byhsp65 deoxyribonucleic acid vaccination (Santos-Junioret al., 2005). Study of the potential of molecular chap-erones, such as heat shock proteins and chaperonins, to beused in the development of therapeutics based on theimmune system, or on their resistance to pathologicalstimuli, is a challenge for the future. In particular, themolecular chaperone hsp90 has emerged as a promisingtarget for the treatment of cancer because the folding,stability and maturation of many proteins in cancer cellsdepend on hsp90 (Donnelly and Blagg, 2008). See also:History of Drug Discovery

References

Barral JM,Broadley SA, SchaffarGandHartl FU (2004)Roles of

molecular chaperones in protein misfolding diseases. Seminars

in Cell and Developmental Biology 15: 17–29.

Chaperonins

eLS & 2012, John Wiley & Sons, Ltd. www.els.net 7

Cong Y, Baker ML, Jakana J et al. (2010) 4.0-A resolution cryo-

EMstructure of themammalian chaperoninTRiC/CCT reveals

its unique subunit arrangement. Proceedings of the National

Academy of Sciences of the USA 107: 4967–4972.

Cong Y, Schroder GF, Meyer AS et al. (2012) Symmetry-free

cryo-EM structures of the chaperonin TRiC along its ATPase-

driven conformational cycle. EMBO Journal 31: 720–730.

Dekker C, Roe SM, McCormack EA et al. (2011) The crystal

structure of yeast CCT reveals intrinsic asymmetry of eukary-

otic cytosolic chaperonins. EMBO Journal 30: 3078–3090.

Dobson CM (2004) Principles of protein folding, misfolding

and aggregation. Seminars in Cell and Developmental Biology

15: 3–16.

Donnelly A and Blagg BS (2008) Novobiocin and additional

inhibitors of the Hsp90 C-terminal nucleotide-binding pocket.

Current Medicinal Chemistry 15: 2702–2717.

Ellis RJ (2005) Chaperomics: in vivo GroEL function defined.

Current Biology 15: R661–R663.

Hartl FU (1996)Molecular chaperones in cellular protein folding.

Nature 381: 571–580.

Hartl FU and Hayer-Hartl M (2009) Converging concepts of

protein folding in vitro and in vivo. Nature Structural and

Molecular Biology 16: 574–581.

Horwich AL, FentonWA, Chapman E and Farr GW (2007) Two

families of chaperonin: physiology and mechanism. Annual

Review of Cell and Developmental Biology 23: 115–145.

Jewett AI and Shea JE (2010) Reconciling theories of chaperonin

accelerated folding with experimental evidence. Cellular and

Molecular Life Sciences 67: 255–276.

Kalisman N, Adams CM and Levitt M (2012) Subunit order of

eukaryotic TRiC/CCT chaperonin by cross-linking, mass spec-

trometry, and combinatorial homologymodeling.Proceedings of

the National Academy of Sciences of the USA 109: 2884–2889.

Kubota S, Kubota H and Nagata K (2006) Cytosolic chaperonin

protects folding intermediates of Gbeta from aggregation by

recognizing hydrophobic beta-strands. Proceedings of the

National Academy of Sciences of the USA 103: 8360–8365.

LerouxMRandHartl FU (2000) Protein folding: versatility of the

cytosolic chaperonin TRiC/CCT.Current Biology 10: 260–264.

Lundin VF, LerouxMRand Stirling PC (2010) Quality control of

cytoskeletal proteins and human disease. Trends in Biochemical

Sciences 35: 288–297.

Munoz IG, Yebenes H, Zhou M et al. (2011) Crystal structure of

the open conformation of the mammalian chaperonin CCT in

complex with tubulin.Nature Structural andMolecular Biology

18: 14–19.

Ranford JC and Henderson (2002) Chaperonins in disease:

mechanisms, models, and treatments. Molecular Pathology 55:

209–213.

Radford SE (2006) GroEL: more than just a folding cage. Cell

125: 831–833.

Ranson NA, White HE and Saibil HR (1998) Chaperonins. Bio-

chemical Journal 333: 233–242.

SaibilH (2000)Molecular chaperones: containers and surfaces for

folding, stabilising or unfolding proteins. Current Opinion in

Structural Biology 10: 251–258.

Saibil HR and Ranson NA (2002) The chaperonin folding

machine. Trends in Biochemical Sciences 27: 627–632.

Santos-Junior RR, Sartori A, De Franco M et al. (2005) Immu-

nomodulation and protection induced by DNA-hsp65 vaccin-

ation in an animal model of arthritis.Human Gene Therapy 16:

1338–1345.

Slavotinek AM and Biesecker LG (2001) Unfolding the role of

chaperones and chaperonins in human disease. Trends in Gen-

etics 17: 528–535.

Spiess C, Meyer AS, Reissmann S and Frydman J (2004) Mech-

anism of the eukaryotic chaperonin: protein folding in the

chamber of secrets. Trends in Cell Biology 14: 598–604.

Valpuesta JM,Martin-Benito J,Gomez-Puertas P, Carrascosa JL

and Willison KR (2002) Structure and function of a protein

folding machine: the eukaryotic cytosolic chaperonin CCT.

FEBS Letters 529: 11–16.

Yam AY, Xia Y, Lin HT et al. (2008) Defining the TRiC/CCT

interactome links chaperonin function to stabilization of newly

made proteins with complex topologies. Nature Structural and

Molecular Biology 15: 1255–1262.

Yebenes H, Mesa P, Munoz IG, Montoya G and Valpuesta JM

(2011) Chaperonins: two rings for folding. Trends Biochemical

Sciences 36: 424–432.

Young JC, Agashe VR, Siegers K and Hartl FU (2004) Pathways

of chaperone-mediated protein folding in the cytosol. Nature

Reviews Molecular Cell Biology 5: 781–791.

Further Reading

AgasheVRandHartl FU (2000)Roles ofmolecular chaperones in

cytoplasmic protein folding. Seminars in Cell and Develop-

mental Biology 11: 15–25.

Booth CR, Meyer AS, Cong Y et al. (2008) Mechanism of lid

closure in the eukaryotic chaperonin TRiC/CCT. Nature

Structural & Molecular Biology 15: 746–753.

Clare DK, Vasishtan D, Stagg S et al. (2012) ATP-triggered

conformational changes delineate substrate-binding and

-foldingmechanics of theGroEL chaperonin.Cell 149: 113–123.

Molecular Chaperone Group http://people.cryst.bbk.ac.uk/

� ubcg16z/chaperone.html

Dunn AY, Melville MW and Frydman J (2001) Review: cellular

substrates of the eukaryotic chaperonin TRiC/CCT. Journal of

Structural Biology 135: 176–184.

Ellis RJ (2000) Chaperone substrates inside the cell. Trends in

Biochemical Sciences 25: 210–212.

Ellis RJ (2003) Protein folding: importance of the Anfinsen cage.

Current Biology 13: R881–R883.

GottesmanME and HendricksonWA (2000) Protein folding and

unfolding by E. coli chaperones and chaperonins. Current

Opinion in Microbiology 3: 197–202.

Horovitz A and Willison KR (2005) Allosteric regulation of

chaperonins.Current Opinion in Structural Biology 15: 646–651.

Horwich AL and Fenton WA (2009) Chaperonin-mediated pro-

tein folding: using a central cavity to kinetically assist

polypeptide chain folding. Quarterly Reviews of Biophysics 42:

83–116.

Zoubeidi A, Chi K and Gleave M (2010) Targeting the cytopro-

tective chaperone, clusterin, for treatment of advanced cancer.

Clinical Cancer Research 16: 1088–1093.

Chaperonins

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