Chaperonin-mediated folding in the eukaryotic cytosol proceeds through rounds of release of native and nonnative forms

Pathway and mechanism of tubulin folding mediated by TRiC/CCT along its ATPase cycle revealed using cryo-EM

The eukaryotic chaperonin TRiC/CCT assists the folding of about 10% of cytosolic proteins through an ATP-driven conformational cycle, and the essential cytoskeleton protein tubulin is the obligate substrate of TRiC. Here, we present an ensemble of cryo-EM structures of endogenous human TRiC throughout its ATPase cycle, with three of them revealing endogenously engaged tubulin in different folding stages. The open-state TRiC-tubulin-S1 and -S2 maps show extra density corresponding to tubulin in the cis-ring chamber of TRiC. Our structural and XL-MS analyses suggest a gradual upward translocation and stabilization of tubulin within the TRiC chamber accompanying TRiC ring closure. In the closed TRiC-tubulin-S3 map, we capture a near-natively folded tubulin-with the tubulin engaging through its N and C domains mainly with the A and I domains of the CCT3/6/8 subunits through electrostatic and hydrophilic interactions. Moreover, we also show the potential role of TRiC C-terminal tails in substrate stabilization and folding. Our study delineates the pathway and molecular mechanism of TRiC-mediated folding of tubulin along the ATPase cycle of TRiC, and may also inform the design of therapeutic agents targeting TRiC-tubulin interactions.

Structural basis of plp2-mediated cytoskeletal protein folding by TRiC/CCT

The cytoskeletal proteins tubulin and actin are the obligate substrates of TCP-1 ring complex/Chaperonin containing TCP-1 (TRiC/CCT), and their folding involves co-chaperone. Through cryo-electron microscopy analysis, we present a more complete picture of TRiC-assisted tubulin/actin folding along TRiC adenosine triphosphatase cycle, under the coordination of co-chaperone plp2. In the open S1/S2 states, plp2 and tubulin/actin engaged within opposite TRiC chambers. Notably, we captured an unprecedented TRiC-plp2-tubulin complex in the closed S3 state, engaged with a folded full-length β-tubulin and loaded with a guanosine triphosphate, and a plp2 occupying opposite rings. Another closed S4 state revealed an actin in the intermediate folding state and a plp2. Accompanying TRiC ring closure, plp2 translocation could coordinate substrate translocation on the CCT6 hemisphere, facilitating substrate stabilization and folding. Our findings reveal the folding mechanism of the major cytoskeletal proteins tubulin/actin under the coordination of the biogenesis machinery TRiC and plp2 and extend our understanding of the links between cytoskeletal proteostasis and related human diseases.

Efficient Catalysis of Protein Folding by GroEL/ES of the Obligate Chaperonin Substrate MetF

The cylindrical chaperonin GroEL and its cofactor GroES mediate ATP-dependent protein folding in Escherichia coli by transiently encapsulating non-native substrate in a nano-cage formed by the GroEL ring cavity and the lid-shaped GroES. Mechanistic studies of GroEL/ES with heterologous protein substrates suggested that the chaperonin is inefficient, typically requiring multiple ATP-dependent encapsulation cycles with only a few percent of protein folded per cycle. Here we analyzed the spontaneous and chaperonin-assisted folding of the essential enzyme 5,10-methylenetetrahydrofolate reductase (MetF) of E. coli, an obligate GroEL/ES substrate. We found that MetF, a homotetramer of 33-kDa subunits with (β/α)₈ TIM-barrel fold, populates a kinetically trapped folding intermediate(s) (MetF-I) upon dilution from denaturant that fails to convert to the native state, even in the absence of aggregation. GroEL/ES recognizes MetF-I and catalyzes rapid folding, with ~50% of protein folded in a single round of encapsulation. Analysis by hydrogen/deuterium exchange at peptide resolution showed that the MetF subunit folds to completion in the GroEL/ES nano-cage and binds its cofactor flavin adenine dinucleotide. Rapid folding required the net negative charge character of the wall of the chaperonin cavity. These findings reveal a remarkable capacity of GroEL/ES to catalyze folding of an endogenous substrate protein that would have coevolved with the chaperonin system.

The TRiC/CCT Chaperonin and Its Role in Uncontrolled Proliferation

The cell cycle is a sophisticated space-time regulated mechanism where a wide variety of protein modules and complexes associate functioning in a concerted manner to regulate and transfer the genetic material to daughter cells. CCT (chaperonin containing TCP-1, also known as TRiC) is a molecular machine that forms a high molecular weight complex (1000 KDa). CCT is emerging as a key molecule during mitosis due to its essential role in the folding of many important proteins involved in cell division (Cdh1, Plk1, p27, Cdc20, PP2a regulatory subunits, tubulin or actin) suggesting its involvement in uncontrolled proliferation. The assembly is formed by eight different subunits called CCTα, β, γ, δ, ε, ζ, η and θ in mammals corresponding to CCT1-8 in yeast. CCT/TRiC is organized in a unique intra- and inter-ring arrangement. The chaperonin monomers share a common domain structure including an equatorial domain, which contains all the inter-ring contacts, most of the intra-ring contacts and the ATP binding site, whose binding and hydrolysis triggers the conformational changes that take place during the functional cycle. All chaperonins display an open substrate-receptive conformation, where the unfolded protein is recognized and trapped, and a closed conformation where the substrate is isolated from the bulk of the intracellular environment. In this chapter we discuss the complex set of intra- and inter-ring allosteric signals during chaperonin function.

TRiC/CCT Chaperonin: Structure and Function

The eukaryotic group II chaperonin TRiC/CCT assists the folding of 10% of cytosolic proteins including many key structural and regulatory proteins. TRiC plays an essential role in maintaining protein homeostasis, and dysfunction of TRiC is closely related to human diseases including cancer and neurodegenerative diseases. TRiC consists of eight paralogous subunits, each of which plays a specific role in the assembly, allosteric cooperativity, and substrate recognition and folding of this complex macromolecular machine. TRiC-mediated substrate folding is regulated through its ATP-driven conformational changes. In recent years, progresses have been made on the structure, subunit arrangement, conformational cycle, and substrate folding of TRiC. Additionally, accumulating evidences also demonstrate the linkage between TRiC oligomer or monomer and diseases. In this review, we focus on the TRiC structure itself, TRiC assisted substrate folding, TRiC and disease, and the potential therapeutic application of TRiC in various diseases.

CIRCE element evolved for the coordinated transcriptional regulation of bacterial duplicate groELs

Chaperonin groEL genes are duplicated in approximately 20% of bacteria, and the duplicates are differentially transcribed due to their divergent functions. The coordinated regulation of this differential transcription is as yet undetermined. In this study, we reported that the controlling inverted repeat of chaperone expression (CIRCE) element (the HrcA-binding site located upstream of the promoter) evolved for the transcriptional regulation of duplicate groELs. CIRCE composition and locations were found to be phylogenetically conserved in bacterial taxa. Myxococcus xanthus DK1622 has two CIRCE elements (CIRCE1_groESL1 and CIRCE2_groESL1) in the promoter region of groESL1 and one CIRCE element (CIRCE_groEL2) before groEL2. We also found that negative HrcA and positive ?³² regulators coordinated the transcription of duplicate groELs, and that the double deletion in DK1622 eliminated transcriptional differences and reduced the heat-shock responses of groELs. In vitro binding assays showed that HrcA protein binding was biased towards CIRCE1_groESL1, followed by CIRCE_groEL2, but that HrcA proteins failed to bind with CIRCE2_groESL1. Mutation experiments revealed that single-nucleotide mutations in the inverted repeat regions changed the HrcA-binding abilities of CIRCEs. We constructed an in vivo transcription-regulation system in Escherichia coli to pair each of the regulators with a groEL promoter. The results indicated that the transcriptional regulation performed by HrcA and ?³² was biased towards the groEL2 and groEL1 promoters, respectively. Based on promoter-sequence characteristics, we proposed a model of the coordinated regulation of the transcription of duplicate groELs in M. xanthus DK1622.

Minichaperone (GroEL191-345) mediated folding of MalZ proceeds by binding and release of native and functional intermediates

The isolated apical domain of GroEL consisting of residues 191-345 (known as "minichaperone") binds and assists the folding of a wide variety of client proteins without GroES and ATP, but the mechanism of its action is still unknown. In order to probe into the matter, we have examined minichaperone-mediated folding of a large aggregation prone protein Maltodextrin-glucosidase (MalZ). The key objective was to identify whether MalZ exists free in solution, or remains bound to, or cycling on and off the minichaperone during the refolding process. When GroES was introduced during refolding process, production of the native MalZ was inhibited. We also observed the same findings with a trap mutant of GroEL, which stably captures a predominantly non-native MalZ released from minichaperone during refolding process, but does not release it. Tryptophan and ANS fluorescence measurements indicated that refolded MalZ has the same structure as the native MalZ, but that its structure when bound to minichaperone is different. Surface plasmon resonance measurements provide an estimate for the equilibrium dissociation constant KD for the MalZ-minichaperone complex of 0.21 ± 0.04 μM, which are significantly higher than for most GroEL clients. This showed that minichaperone interacts loosely with MalZ to allow the protein to change its conformation and fold while bound during the refolding process. These observations suggest that the minichaperone works by carrying out repeated cycles of binding aggregation-prone protein MalZ in a relatively compact conformation and in a partially folded but active state, and releasing them to attempt to fold in solution.

Characterization and Expression Pattern Analysis of the T-Complex Protein-1 Zeta Subunit in Musca domestica L (Diptera)

Chaperonins, belonging to the T-complex protein-1 (TCP-1) family, assist in the correct folding of nascent and misfolded proteins. It is well-known that in mammals, the zeta subunit of the TCP-1 complex (TCP-1ζ) plays a vital role in the folding and assembly of cytoskeleta proteins. This study reported for the first time the cloning, characterization and expression pattern analysis of the TCP-1ζ from Musca domestica, which was named as MdTCP-1ζ. The MdTCP-1ζ cDNA is 1,803 bp long with a 1,596 bp open reading frame that encodes a protein with 531 bp amino acids. The analysis of the transcriptional profile of MdTCP-1ζ using qRT-PCR revealed relatively high expression in the salivary glands and trachea at the tissues while among the developmental stages. The highest expression was observed only in the eggs suggesting that the MdTCP-1ζ may play a role in embryonic development. The expression of MdTCP-1ζ was also significantly induced after exposure to short-term heat shock and infection by Escherichia coli, Staphylococcus aureus, or Candida albicans. This suggested that MdTCP-1ζ may take part in the immune responses of housefly and perhaps contribute to the protection against cellular injury.

Silencing P2X7 receptor downregulates the expression of TCP-1 involved in lymphoma lymphatic metastasis

P2X₇R is an ATP-gated cation channel that participates in cell proliferation and apoptosis. TCP-1 assists with the protein folding. According to our previous research, the P2X₇R has a potential role in P388D1 lymphoid neoplasm cells dissemination to peripheral lymph nodes. In order to make a further exploration about the probable mechanism, the lymph nodes which metastasized by P2X₇R-silenced P388D1 cells or non-silenced cells were analyzed by 2DE and a MALDI-TOF-based proteomics approach. In the 64 proteins which were differentially expressed between two groups, TCP-1 was found to be significantly decreased in P2X₇R shRNA group compared to controls. This correlation was also found in subsequent experiments in vivo and in vitro. The positive correlation between P2X₇R and TCP-1 was also proved in both lymphoma and benign lymphadenopathy tissues from patients. It indicates that TCP-1 may be a crucial downstream molecular of P2X₇R and plays a novel role in lymphoid neoplasm metastasis.

CCT2 Mutations Evoke Leber Congenital Amaurosis due to Chaperone Complex Instability

Leber congenital amaurosis (LCA) is a hereditary early-onset retinal dystrophy that is accompanied by severe macular degeneration. In this study, novel compound heterozygous mutations were identified as LCA-causative in chaperonin-containing TCP-1, subunit 2 (CCT2), a gene that encodes the molecular chaperone protein, CCTβ. The zebrafish mutants of CCTβ are known to exhibit the eye phenotype while its mutation and association with human disease have been unknown. The CCT proteins (CCT α-θ) forms ring complex for its chaperon function. The LCA mutants of CCTβ, T400P and R516H, are biochemically instable and the affinity for the adjacent subunit, CCTγ, was affected distinctly in both mutants. The patient-derived induced pluripotent stem cells (iPSCs), carrying these CCTβ mutants, were less proliferative than the control iPSCs. Decreased proliferation under Cct2 knockdown in 661W cells was significantly rescued by wild-type CCTβ expression. However, the expression of T400P and R516H didn’t exhibit the significant effect. In mouse retina, both CCTβ and CCTγ are expressed in the retinal ganglion cells and connecting cilium of photoreceptor cells. The Cct2 knockdown decreased its major client protein, transducing β1 (Gβ1). Here we report the novel LCA mutations in CCTβ and the impact of chaperon disability by these mutations in cellular biology.

Overexpression of CCT8 and its significance for tumor cell proliferation, migration and invasion in glioma

Overexpression of chaperonin containing t-complex polypeptide 1 (TCP1), or CCT, has been reported in various classes of malignancies. However, little is known about the expression of t-complex protein subunits TCP1theta (CCT8) in gliomas. In this study, the expression of CCT8 protein was detected using blotting analysis and immunohistochemistry. CCT8 was found to be overexpressed in gliomas and to correlate with the WHO grade of gliomas. To further investigate the biological function of CCT8 in gliomas, CCT8-silenced U87 glioblastoma multiforme (GBM) and U251MG cells were constructed using a small interference RNA (siRNA) sequence. The knockdown effect of CCT8 on proliferation and invasion in these cells was analyzed using the CCK8, flow cytometry cycle, scratch, transwell invasion and fluorescence assays. Compared with the controls, the glioma cells expressing CCT8-siRNA exhibited a significantly decreased proliferation and invasion capacity, as well as a dysregulated cell cytoskeleton. This study showed that high CCT8 protein expression might be related to poor outcome of glioma, and that CCT8 regulates the proliferation and invasion of glioblastomas.

Heterotrimeric G protein-mediated signaling and its non-canonical regulation in the heart

Heterotrimeric guanine nucleotide-binding proteins (G proteins) regulate a multitude of signaling pathways in mammalian cells by transducing signals from G protein-coupled receptors (GPCRs) to effectors, which in turn regulate cellular function. In the myocardium, G protein signaling occurs in all cardiac cell types and is centrally involved in the regulation of heart rate, pump function, and vascular tone and in the response to hemodynamic stress and injury. Perturbations in G protein-mediated signaling are well known to contribute to cardiac hypertrophy, failure, and arrhythmias. Most of the currently used drugs for cardiac and other diseases target GPCR signaling. In the canonical G protein signaling paradigm, G proteins that are located at the cytoplasmic surface of the plasma membrane become activated after an agonist-induced conformational change of GPCRs, which then allows GTP-bound Gα and free Gβγ subunits to activate or inhibit effector proteins. Research over the past two decades has markedly broadened the original paradigm with a GPCR-G protein-effector at the cell surface at its core by revealing novel binding partners and additional subcellular localizations for heterotrimeric G proteins that facilitate many previously unrecognized functional effects. In this review, we focus on non-canonical and epigenetic-related mechanisms that regulate heterotrimeric G protein expression, activation, and localization and discuss functional consequences using cardiac examples where possible. Mechanisms reviewed involve microRNAs, histone deacetylases, chaperones, alternative modes of G protein activation, and posttranslational modifications. Some of these newly characterized mechanisms may be further developed into novel strategies for the treatment of cardiac disease and beyond.

The G protein α chaperone Ric-8 as a potential therapeutic target

Resistance to inhibitors of cholinesterase (Ric-8)A and Ric-8B are essential genes that encode positive regulators of heterotrimeric G protein α subunits. Controversy persists surrounding the precise way(s) that Ric-8 proteins affect G protein biology and signaling. Ric-8 proteins chaperone nucleotide-free Gα-subunit states during biosynthetic protein folding prior to G protein heterotrimer assembly. In organisms spanning the evolutionary window of Ric-8 expression, experimental perturbation of Ric-8 genes results in reduced functional abundances of G proteins because G protein α subunits are misfolded and degraded rapidly. Ric-8 proteins also act as Gα-subunit guanine nucleotide exchange factors (GEFs) in vitro. However, Ric-8 GEF activity could strictly be an in vitro phenomenon stemming from the ability of Ric-8 to induce partial Gα unfolding, thereby enhancing GDP release. Ric-8 GEF activity clearly differs from the GEF activity of G protein-coupled receptors (GPCRs). G protein βγ is inhibitory to Ric-8 action but obligate for receptors. It remains an open question whether Ric-8 has dual functions in cells and regulates G proteins as both a molecular chaperone and GEF. Clearly, Ric-8 has a profound influence on heterotrimeric G protein function. For this reason, we propose that Ric-8 proteins are as yet untested therapeutic targets in which pharmacological inhibition of the Ric-8/Gα protein-protein interface could serve to attenuate the effects of disease-causing G proteins (constitutively active mutants) and/or GPCR signaling. This minireview will chronicle the understanding of Ric-8 function, provide a comparative discussion of the Ric-8 molecular chaperoning and GEF activities, and support the case for why Ric-8 proteins should be considered potential targets for development of new therapies.

GroEL/ES chaperonin modulates the mechanism and accelerates the rate of TIM-barrel domain folding

The GroEL/ES chaperonin system functions as a protein folding cage. Many obligate substrates of GroEL share the (βα)8 TIM-barrel fold, but how the chaperonin promotes folding of these proteins is not known. Here, we analyzed the folding of DapA at peptide resolution using hydrogen/deuterium exchange and mass spectrometry. During spontaneous folding, all elements of the DapA TIM barrel acquire structure simultaneously in a process associated with a long search time. In contrast, GroEL/ES accelerates folding more than 30-fold by catalyzing segmental structure formation in the TIM barrel. Segmental structure formation is also observed during the fast spontaneous folding of a structural homolog of DapA from a bacterium that lacks GroEL/ES. Thus, chaperonin independence correlates with folding properties otherwise enforced by protein confinement in the GroEL/ES cage. We suggest that folding catalysis by GroEL/ES is required by a set of proteins to reach native state at a biologically relevant timescale, avoiding aggregation or degradation.

Chaperonin containing TCP1, subunit 8 (CCT8) is upregulated in hepatocellular carcinoma and promotes HCC proliferation

The development of molecular pathogenesis of hepatocellular carcinoma (HCC) is complex and involves alterations in the expression and conformation of assorted oncoproteins and tumor suppressors. Chaperonin containing TCP1 (CCT) is a cytolic molecular chaperone complex that is required for the correct folding of numerous proteins. In this study, we investigated a possible involvement of CCT subunit 8 (CCT8) in HCC development. Immunohistochemical analysis was performed in 102 human HCC samples. High CCT8 expression was detected in clinical HCC samples compared with adjacent noncancerous tissues. The univariate and multivariate survival analyses were also performed to determine their prognostic significance. Western blot confirmed the high expression of CCT8 in HCC compared with adjacent normal tissue. Moreover, the biological significance of the aberrant expression of CCT8 was investigated in HCC cell lines. Expression of CCT8 was correlated directly with the histologic grades and tumor size of HCC and high expression of CCT8 was associated with a poor prognosis. CCT8 depletion by siRNA inhibited cell proliferation and blocked S-phase entry in HuH7 cells. These results suggested that CCT8 might be an oncogene and participate in HCC cell proliferation. These findings provide a potential therapeutic strategy for the treatment of HCC.

Modulation of STAT3 folding and function by TRiC/CCT chaperonin

Levels, folding, and function of the infamous cancer and inflammatory disease-related signaling molecule Stat3 are regulated by interaction with the chaperonin TRiC; manipulation of this interaction is a therapeutic avenue for exploration.

Signal transducer and activator of transcription 3 (Stat3) transduces signals of many peptide hormones from the cell surface to the nucleus and functions as an oncoprotein in many types of cancers, yet little is known about how it achieves its native folded state within the cell. Here we show that Stat3 is a novel substrate of the ring-shaped hetero-oligomeric eukaryotic chaperonin, TRiC/CCT, which contributes to its biosynthesis and activity in vitro and in vivo. TRiC binding to Stat3 was mediated, at least in part, by TRiC subunit CCT3. Stat3 binding to TRiC mapped predominantly to the β-strand rich, DNA-binding domain of Stat3. Notably, enhancing Stat3 binding to TRiC by engineering an additional TRiC-binding domain from the von Hippel-Lindau protein (vTBD), at the N-terminus of Stat3, further increased its affinity for TRiC as well as its function, as determined by Stat3's ability to bind to its phosphotyrosyl-peptide ligand, an interaction critical for Stat3 activation. Thus, Stat3 levels and function are regulated by TRiC and can be modulated by manipulating its interaction with TRiC.

Stat3 is a multidomain transcription factor that contributes to many cellular functions by transmitting signals for over 40 peptide hormones from the cell surface to the nucleus. Understanding how multidomain proteins achieve their fully folded and functional state is of substantial biological interest. As Stat3 signaling is up-regulated in many pathological conditions, including cancer and inflammatory diseases, insight into what controls its folding may be useful for the identification of vulnerabilities that can be therapeutically exploited. We demonstrate that the major protein-folding machine or chaperonin within eukaryotic cells, TRiC/CCT, is required for Stat3 to fold during its synthesis and for Stat3 to be fully functional within the cell. We also find that TRiC can refold chemically denatured Stat3 and provide evidence that the CCT3 subunit of TRiC binds to the DNA-binding domain of Stat3. We also show that Stat3 activity is decreased by down-modulating levels of TRiC and can be increased by increasing Stat3's interaction with TRiC. TRiC therefore regulates both Stat3 protein levels and its function, making Stat3 modulation by manipulation of its interaction with TRiC a potential approach for the treatment of cancer and inflammatory diseases.

PHLP2 is essential and plays a role in ciliogenesis and microtubule assembly in Tetrahymena thermophila

Recent studies have implicated the phosducin-like protein-2 (PHLP2) in regulation of CCT, a chaperonin whose activity is essential for folding of tubulin and actin. However, the exact molecular function of PHLP2 is unclear. Here we investigate the significance of PHLP2 in a ciliated unicellular model, Tetrahymena thermophila, by deleting its single homolog, Phlp2p. Cells lacking Phlp2p became larger and died within 96 h. Overexpressed Phlp2p-HA localized to cilia, basal bodies, and cytosol without an obvious change in the phenotype. Despite similar localization, overexpressed GFP-Phlp2p caused a dominant-negative effect. Cells overproducing GFP-Phlp2p had decreased rates of proliferation, motility and phagocytosis, as compared to wild type cells or cells overproducing a non-tagged Phlp2p. Growing GFP-Phlp2p-overexpressing cells had fewer cilia and, when deciliated, failed to regenerate cilia, indicating defects in cilia assembly. Paclitaxel-treated GFP-Phlp2p cells failed to elongate cilia, indicating a change in the microtubules dynamics. The pattern of ciliary and cytosolic tubulin isoforms on 2D gels differed between wild type and GFP-Phlp2p-overexpressing cells. Thus, in Tetrahymena, PhLP2 is essential and under specific experimental conditions its activity affects tubulin and microtubule-dependent functions including cilia assembly.

Identification of PDCL3 as a novel chaperone protein involved in the generation of functional VEGF receptor 2

Angiogenesis, a hallmark step in tumor metastasis and ocular neovascularization, is driven primarily by the function of VEGF ligand on one of its receptors, VEGF receptor 2 (VEGFR-2). Central to the proliferation and ensuing angiogenesis of endothelial cells, the abundance of VEGFR-2 on the surface of endothelial cells is essential for VEGF to recognize and activate VEGFR-2. We have identified phosducin-like 3 (PDCL3, also known as PhLP2A), through a yeast two-hybrid system, as a novel protein involved in the stabilization of VEGFR-2 by serving as a chaperone. PDCL3 binds to the juxtamembrane domain of VEGFR-2 and controls the abundance of VEGFR-2 by inhibiting its ubiquitination and degradation. PDCL3 increases VEGF-induced tyrosine phosphorylation and is required for VEGFR-2-dependent endothelial capillary tube formation and proliferation. Taken together, our data provide strong evidence for the role of PDCL3 in angiogenesis and establishes the molecular mechanism by which it regulates VEGFR-2 expression and function.

Ric-8 regulation of heterotrimeric G proteins

Resistance to inhibitors of cholinesterase 8 proteins (Ric-8A and Ric-8B) collectively bind the four classes of heterotrimeric G protein α subunits. Ric-8A and Ric-8B act as non-receptor guanine nucleotide exchange factors (GEFs) toward the Gα subunits that each binds in vitro and seemingly regulate diverse G protein signaling systems in cells. Combined evidence from worm, fly and mammalian systems has shown that Ric-8 proteins are required to maintain proper cellular abundances of G proteins. Ric-8 proteins support G protein levels by serving as molecular chaperones that promote Gα subunit biosynthesis. In this review, the evidence that Ric-8 proteins act as non-receptor GEF activators of G proteins in signal transduction contexts will be weighed against the evidence supporting the molecular chaperoning function of Ric-8 in promoting G protein abundance. I will conclude by suggesting that Ric-8 proteins may act in either capacity in specific contexts. The field awaits additional experimentation to delineate the putative multi-functionality of Ric-8 towards G proteins in cells.

Molecular chaperoning function of Ric-8 is to fold nascent heterotrimeric G protein α subunits

We have shown that resistance to inhibitors of cholinesterase 8 (Ric-8) proteins regulate an early step of heterotrimeric G protein α (Gα) subunit biosynthesis. Here, mammalian and plant cell-free translation systems were used to study Ric-8A action during Gα subunit translation and protein folding. Gα translation rates and overall produced protein amounts were equivalent in mock and Ric-8A-immunodepleted rabbit reticulocyte lysate (RRL). GDP-AlF4(-)-bound Gαi, Gαq, Gα13, and Gαs produced in mock-depleted RRL had characteristic resistance to limited trypsinolysis, showing that these G proteins were folded properly. Gαi, Gαq, and Gα13, but not Gαs produced from Ric-8A-depleted RRL were not protected from trypsinization and therefore not folded correctly. Addition of recombinant Ric-8A to the Ric-8A-depleted RRL enhanced GDP-AlF4(-)-bound Gα subunit trypsin protection. Dramatic results were obtained in wheat germ extract (WGE) that has no endogenous Ric-8 component. WGE-translated Gαq was gel filtered and found to be an aggregate. Ric-8A supplementation of WGE allowed production of Gαq that gel filtered as a ∼100 kDa Ric-8A:Gαq heterodimer. Addition of GTPγS to Ric-8A-supplemented WGE Gαq translation resulted in dissociation of the Ric-8A:Gαq heterodimer and production of functional Gαq-GTPγS monomer. Excess Gβγ supplementation of WGE did not support functional Gαq production. The molecular chaperoning function of Ric-8 is to participate in the folding of nascent G protein α subunits.

The alpha- and beta-tubulin folding pathways

The alpha-beta tubulin heterodimer is the subunit from which microtubules are assembled. The pathway leading to correctly folded alpha- and beta-tubulins is unusually complex: it involves cycles of ATP-dependent interaction of newly synthesized tubulin subunits with cytosolic chaperonin, resulting in the production of quasi-native folding intermediates, which must then be acted upon by additional protein cofactors. These cofactors form a supercomplex containing both alpha- and beta-tubulin polypeptides, from which native heterodimer is released in a GTP-dependent reaction. Here, we discuss the current state of our understanding of the function of cytosolic chaperonin and cofactors in tubulin folding.

Chaperone-mediated assembly of G protein complexes

G protein signaling depends on the ability of the individual subunits of the G protein heterotrimer to assemble into functional complexes. Formation of the G protein βγ (Gβγ) dimer is particularly challenging because it is an obligate dimer in which the individual subunits are unstable on their own. Recent studies have revealed an intricate chaperone system that brings the Gβ and Gγ subunits together. This system includes the cytosolic chaperonin containing TCP-1 (CCT) and its co-chaperone phosducin-like protein 1 (PhLP1). CCT assists Gβ in achieving its β-propeller structure, while PhLP1 releases Gβ from CCT and facilitates its interaction with Gγ. Once Gβγ is formed, PhLP1 remains bound until it is displaced by the Gα subunit and the G protein heterotrimer is brought together. Another obligate dimer is the complex between the G protein β(5) subunit and a regulator of G protein signaling protein (Gβ(5)-RGS). Gβ(5)-RGS also requires CCT for Gβ(5) folding, but PhLP1 plays a different role. It stabilizes the interaction between Gβ(5) and CCT, perhaps to increase folding efficiency. After Gβ(5) folding PhLP1 must subsequently release, allowing the RGS protein to bind and form the Gβ(5)-RGS dimer directly on CCT. Gβ(5)-RGS is then freed from CCT to interact with its membrane anchoring protein and form a stable complex that turns off the G protein signal by catalyzing GTP hydrolysis on Gα.

Synthesis and assembly of G protein βγ dimers: comparison of in vitro and in vivo studies

The heterotrimeric GTP-binding proteins (G proteins) are the canonical cellular machinery used with the approximately 700 G protein-coupled receptors (GPCRs) in the human genome to transduce extracellular signals across the plasma membrane. The synthesis of the constituent G protein subunits, and their assembly into Gβγ dimers and G protein heterotrimers, determines the signaling repertoire for G-protein/GPCR signaling in cells. These synthesis/assembly -processes are intimately related to two other overlapping events in the intricate pathway leading to formation of G protein signaling complexes, posttranslational modification and intracellular trafficking of G proteins. The assembly of the Gβγ dimer is a complex process involving multiple accessory proteins and organelles. The mechanisms involved are becoming increasingly appreciated, but are still incompletely understood. In vitro and in vivo (cellular) studies provide different perspectives of these processes, and a comparison of them can provide insight into both our current level of understanding and directions to be taken in future investigations.

Ric-8 proteins are molecular chaperones that direct nascent G protein α subunit membrane association

Ric-8A (resistance to inhibitors of cholinesterase 8A) and Ric-8B are guanine nucleotide exchange factors that enhance different heterotrimeric guanine nucleotide-binding protein (G protein) signaling pathways by unknown mechanisms. Because transgenic disruption of Ric-8A or Ric-8B in mice caused early embryonic lethality, we derived viable Ric-8A- or Ric-8B-deleted embryonic stem (ES) cell lines from blastocysts of these mice. We observed pleiotropic G protein signaling defects in Ric-8A(-/-) ES cells, which resulted from reduced steady-state amounts of Gα(i), Gα(q), and Gα(13) proteins to <5% of those of wild-type cells. The amounts of Gα(s) and total Gβ protein were partially reduced in Ric-8A(-/-) cells compared to those in wild-type cells, and only the amount of Gα(s) was reduced substantially in Ric-8B(-/-) cells. The abundances of mRNAs encoding the G protein α subunits were largely unchanged by loss of Ric-8A or Ric-8B. The plasma membrane residence of G proteins persisted in the absence of Ric-8 but was markedly reduced compared to that in wild-type cells. Endogenous Gα(i) and Gα(q) were efficiently translated in Ric-8A(-/-) cells but integrated into endomembranes poorly; however, the reduced amounts of G protein α subunits that reached the membrane still bound to nascent Gβγ. Finally, Gα(i), Gα(q), and Gβ(1) proteins exhibited accelerated rates of degradation in Ric-8A(-/-) cells compared to those in wild-type cells. Together, these data suggest that Ric-8 proteins are molecular chaperones required for the initial association of nascent Gα subunits with cellular membranes.

Chaperonin TRiC/CCT participates in replication of hepatitis C virus genome via interaction with the viral NS5B protein

To identify the host factors implicated in the regulation of hepatitis C virus (HCV) genome replication, we performed comparative proteome analyses of HCV replication complex (RC)-rich membrane fractions prepared from cells harboring genome-length bicistronic HCV RNA at the exponential and stationary growth phases. We found that the eukaryotic chaperonin T-complex polypeptide 1 (TCP1)-ring complex/chaperonin-containing TCP1 (TRiC/CCT) plays a role in the replication possibly through an interaction between subunit CCT5 and the viral RNA polymerase NS5B. siRNA-mediated knockdown of CCT5 suppressed RNA replication and production of the infectious virus. Gain-of-function activity was shown following co-transfection with whole eight TRiC/CCT subunits. HCV RNA synthesis was inhibited by an anti-CCT5 antibody in a cell-free assay. These suggest that recruitment of the chaperonin by the viral nonstructural proteins to the RC, which potentially facilitate folding of the RC component(s) into the mature active form, may be important for efficient replication of the HCV genome.

Dealing with misfolded proteins: examining the neuroprotective role of molecular chaperones in neurodegeneration

Human neurodegenerative diseases arise from a wide array of genetic and environmental factors. Despite the diversity in etiology, many of these diseases are considered "conformational" in nature, characterized by the accumulation of pathological, misfolded proteins. These misfolded proteins can induce cellular stress by overloading the proteolytic machinery, ultimately resulting in the accumulation and deposition of aggregated protein species that are cytotoxic. Misfolded proteins may also form aberrant, non-physiological protein-protein interactions leading to the sequestration of other normal proteins essential for cellular functions. The progression of such disease may therefore be viewed as a failure of normal protein homeostasis, a process that involves a network of molecules regulating the synthesis, folding, translocation and clearance of proteins. Molecular chaperones are highly conserved proteins involved in the folding of nascent proteins, and the repair of proteins that have lost their typical conformations. These functions have therefore made molecular chaperones an active area of investigation within the field of conformational diseases. This review will discuss the role of molecular chaperones in neurodegenerative diseases, highlighting their functional classification, regulation, and therapeutic potential for such diseases.

Disruption of the chaperonin containing TCP-1 function affects protein networks essential for rod outer segment morphogenesis and survival

Type II Chaperonin Containing TCP-1 (CCT, also known as TCP-1 Ring Complex, TRiC) is a multi-subunit molecular machine thought to assist in the folding of ∼ 10% of newly translated cytosolic proteins in eukaryotes. A number of proteins folded by CCT have been identified in yeast and cultured mammalian cells, however, the function of this chaperonin in vivo has never been addressed. Here we demonstrate that suppressing the CCT activity in mouse photoreceptors by transgenic expression of a dominant-negative mutant of the CCT cofactor, phosducin-like protein (PhLP), results in the malformation of the outer segment, a cellular compartment responsible for light detection, and triggers rapid retinal degeneration. Investigation of the underlying causes by quantitative proteomics identified distinct protein networks, encompassing ∼ 200 proteins, which were significantly affected by the chaperonin deficiency. Notably among those were several essential proteins crucially engaged in structural support and visual signaling of the outer segment such as peripherin 2, Rom1, rhodopsin, transducin, and PDE6. These data for the first time demonstrate that normal CCT function is ultimately required for the morphogenesis and survival of sensory neurons of the retina, and suggest the chaperonin CCT deficiency as a potential, yet unexplored, cause of neurodegenerative diseases.

Interplay between protein homeostasis networks in protein aggregation and proteotoxicity

The misfolding and aggregation of disease proteins is characteristic of numerous neurodegenerative diseases. Particular neuronal populations are more vulnerable to proteotoxicity while others are more apt to tolerate the misfolding and aggregation of disease proteins. Thus, the cellular environment must play a significant role in determining whether disease proteins are converted into toxic or benign forms. The endomembrane network of eukaryotes divides the cell into different subcellular compartments that possess distinct sets of molecular chaperones and protein interaction networks. Chaperones act as agonists and antagonists of disease protein aggregation to prevent the accumulation of toxic intermediates in the aggregation pathway. Interacting partners can also modulate the conformation and localization of disease proteins and thereby influence proteotoxicity. Thus, interplay between these protein homeostasis network components can modulate the self-association of disease proteins and determine whether they elicit a toxic or benign outcome.

BBS6, BBS10, and BBS12 form a complex with CCT/TRiC family chaperonins and mediate BBSome assembly

Bardet-Biedl syndrome (BBS) is a human genetic disorder resulting in obesity, retinal degeneration, polydactyly, and nephropathy. Recent studies indicate that trafficking defects to the ciliary membrane are involved in this syndrome. Here, we show that a novel complex composed of three chaperonin-like BBS proteins (BBS6, BBS10, and BBS12) and CCT/TRiC family chaperonins mediates BBSome assembly, which transports vesicles to the cilia. Chaperonin-like BBS proteins interact with a subset of BBSome subunits and promote their association with CCT chaperonins. CCT activity is essential for BBSome assembly, and knockdown of CCT chaperonins in zebrafish results in BBS phenotypes. Many disease-causing mutations found in BBS6, BBS10, and BBS12 disrupt interactions among these BBS proteins. Our data demonstrate that BBS6, BBS10, and BBS12 are necessary for BBSome assembly, and that impaired BBSome assembly contributes to the etiology of BBS phenotypes associated with the loss of function of these three BBS genes.

Reconciling theories of chaperonin accelerated folding with experimental evidence

For the last 20 years, a large volume of experimental and theoretical work has been undertaken to understand how chaperones like GroEL can assist protein folding in the cell. The most accepted explanation appears to be the simplest: GroEL, like most other chaperones, helps proteins fold by preventing aggregation. However, evidence suggests that, under some conditions, GroEL can play a more active role by accelerating protein folding. A large number of models have been proposed to explain how this could occur. Focused experiments have been designed and carried out using different protein substrates with conclusions that support many different mechanisms. In the current article, we attempt to see the forest through the trees. We review all suggested mechanisms for chaperonin-mediated folding and weigh the plausibility of each in light of what we now know about the most stringent, essential, GroEL-dependent protein substrates.

Differential substrate specificity of group I and group II chaperonins in the archaeon Methanosarcina mazei

Chaperonins are macromolecular machines that assist in protein folding. The archaeon Methanosarcina mazei has acquired numerous bacterial genes by horizontal gene transfer. As a result, both the bacterial group I chaperonin, GroEL, and the archaeal group II chaperonin, thermosome, coexist. A proteome-wide analysis of chaperonin interactors was performed to determine the differential substrate specificity of GroEL and thermosome. At least 13% of soluble M. mazei proteins interact with chaperonins, with the two systems having partially overlapping substrate sets. Remarkably, chaperonin selectivity is independent of phylogenetic origin and is determined by distinct structural and biochemical features of proteins. GroEL prefers well-conserved proteins with complex alpha/beta domains. In contrast, thermosome substrates comprise a group of faster-evolving proteins and contain a much wider range of different domain folds, including small all-alpha and all-beta modules, and a greater number of large multidomain proteins. Thus, the group II chaperonins may have facilitated the evolution of the highly complex proteomes characteristic of eukaryotic cells.

Yeast phosducin-like protein 2 acts as a stimulatory co-factor for the folding of actin by the chaperonin CCT via a ternary complex

The eukaryotic chaperonin-containing TCP-1 (CCT) folds the cytoskeletal protein actin. The folding mechanism of this 16-subunit, 1-MDa machine is poorly characterised due to the absence of quantitative in vitro assays. We identified phosducin-like protein 2, Plp2p (=PLP2), as an ATP-elutable binding partner of yeast CCT while establishing the CCT interactome. In a novel in vitro CCT-ACT1 folding assay that is functional under physiological conditions, PLP2 is a stimulatory co-factor. In a single ATP-driven cycle, PLP2-CCT-ACT1 complexes yield 30-fold more native actin than CCT-ACT1 complexes. PLP2 interacts directly with ACT1 through the C-terminus of its thioredoxin fold and the CCT-binding subdomain 4 of actin. The in vitro CCT-ACT1-PLP2 folding cycle of the preassembled complex takes 90 s at 30 degrees C, several times slower than the canonical chaperonin GroEL. The specific interactions between PLP2, CCT and ACT1 in the yeast-component in vitro system and the pronounced stimulatory effect of PLP2 on actin folding are consistent with in vivo genetic approaches demonstrating an essential and positive role for PLP2 in cellular processes involving actin in Saccharomyces cerevisiae. In mammalian systems, however, several members of the PLP family, including human PDCL3, the orthologue of PLP2, have been shown to be inhibitory toward CCT-mediated folding of actin in vivo and in vitro. Here, using a rabbit-reticulocyte-derived in vitro translation system, we found that inhibition of beta-actin folding by PDCL3 can be relieved by exchanging its acidic C-terminal extension for that of PLP2. It seems that additional levels of regulatory control of CCT activity by this PLP have emerged in higher eukaryotes.

The role of Gbetagamma subunits in the organization, assembly, and function of GPCR signaling complexes

The role of Gbetagamma subunits in cellular signaling has become well established in the past 20 years. Not only do they regulate effectors once thought to be the sole targets of Galpha subunits, but it has become clear that they also have a unique set of binding partners and regulate signaling pathways that are not always localized to the plasma membrane. However, this may be only the beginning of the story. Gbetagamma subunits interact with G protein-coupled receptors, Galpha subunits, and several different effector molecules during assembly and trafficking of receptor-based signaling complexes and not simply in response to ligand stimulation at sites of receptor cellular activity. Gbetagamma assembly itself seems to be tightly regulated via the action of molecular chaperones and in turn may serve a similar role in the assembly of specific signaling complexes. We propose that specific Gbetagamma subunits have a broader role in controlling the architecture, assembly, and activity of cellular signaling pathways.

Molecular chaperones antagonize proteotoxicity by differentially modulating protein aggregation pathways

The self-association of misfolded or damaged proteins into ordered amyloid-like aggregates characterizes numerous neurodegenerative disorders. Insoluble amyloid plaques are diagnostic of many disease states. Yet soluble, oligomeric intermediates in the aggregation pathway appear to represent the toxic culprit. Molecular chaperones regulate the fate of misfolded proteins and thereby influence their aggregation state. Chaperones conventionally antagonize aggregation of misfolded, disease proteins and assist in refolding or degradation pathways. Recent work suggests that chaperones may also suppress neurotoxicity by converting toxic, soluble oligomers into benign aggregates. Chaperones can therefore suppress or promote aggregation of disease proteins to ameliorate the proteotoxic accumulation of soluble, assembly intermediates.

Development of free-energy-based models for chaperonin containing TCP-1 mediated folding of actin

A free-energy-based approach is used to describe the mechanism through which chaperonin-containing TCP-1 (CCT) folds the filament-forming cytoskeletal protein actin, which is one of its primary substrates. The experimental observations on the actin folding and unfolding pathways are collated and then re-examined from this perspective, allowing us to determine the position of the CCT intervention on the actin free-energy folding landscape. The essential role for CCT in actin folding is to provide a free-energy contribution from its ATP cycle, which drives actin to fold from a stable, trapped intermediate I3, to a less stable but now productive folding intermediate I2. We develop two hypothetical mechanisms for actin folding founded upon concepts established for the bacterial type I chaperonin GroEL and extend them to the much more complex CCT system of eukaryotes. A new model is presented in which CCT facilitates free-energy transfer through direct coupling of the nucleotide hydrolysis cycle to the phases of actin substrate maturation.

Defining the TRiC/CCT interactome links chaperonin function to stabilization of newly made proteins with complex topologies

Folding within the crowded cellular milieu often requires assistance from molecular chaperones that prevent inappropriate interactions leading to aggregation and toxicity. The contribution of individual chaperones to folding the proteome remains elusive. We here demonstrate that the eukaryotic chaperonin TRiC/CCT (TCP1-Ring Complex or Chaperonin Containing TCP1) has broad binding specificity in vitro similar to the prokaryotic chaperonin GroEL. However, in vivo TRiC substrate selection is not based solely on intrinsic determinants; instead, specificity is dictated by factors present during protein biogenesis. The identification of cellular substrates revealed that TRiC interacts with folding intermediates of a subset of structurally and functionally diverse polypeptides. Bioinformatics analysis revealed an enrichment in multidomain proteins and regions of beta strand propensity that are predicted to be slow-folding and aggregation-prone. Thus, TRiC may have evolved to protect complex protein topologies within its central cavity during biosynthesis and folding.

Defining the TRiC/CCT interactome links chaperonin function to stabilization of newly made proteins with complex topologies

Folding within the crowded cellular milieu often requires assistance from molecular chaperones that prevent inappropriate interactions leading to aggregation and toxicity. The contribution of individual chaperones to folding the proteome remains elusive. Here we demonstrate that the eukaryotic chaperonin TRiC/CCT (TCP1-ring complex or chaperonin containing TCP1) has broad binding specificity in vitro, similar to the prokaryotic chaperonin GroEL. However, in vivo, TRiC substrate selection is not based solely on intrinsic determinants; instead, specificity is dictated by factors present during protein biogenesis. The identification of cellular substrates revealed that TRiC interacts with folding intermediates of a subset of structurally and functionally diverse polypeptides. Bioinformatics analysis revealed an enrichment in multidomain proteins and regions of beta-strand propensity that are predicted to be slow folding and aggregation prone. Thus, TRiC may have evolved to protect complex protein topologies within its central cavity during biosynthesis and folding.

The CCT/TRiC chaperonin is required for maturation of sphingosine kinase 1

Sphingosine kinase 1 (SK1) catalyses the generation of sphingosine 1-phosphate (S1P), a bioactive phospholipid that influences a diverse range of cellular processes, including proliferation, survival, adhesion, migration, morphogenesis and differentiation. SK1 is controlled by various mechanisms, including transcriptional regulation, and post-translational activation by phosphorylation and protein-protein interactions which can regulate both the activity and localisation of this enzyme. To gain a better understanding of the regulatory mechanisms controlling SK1 activity and function we performed a yeast two-hybrid screen to identify SK1-interacting proteins. Using this approach we identified that SK1 interacts with subunit 7 (eta) of cytosolic chaperonin CCT (chaperonin containing t-complex polypeptide, also called TRiC for TCP-1 ring complex), a hexadecameric chaperonin that binds unfolded polypeptides and mediates their folding and release in an ATP-dependent manner. Further analysis of the SK1-CCTeta interaction demonstrated that other CCT/TRiC subunits also associated with SK1 in HEK293T cell lysates in an ATP-sensitive manner, suggesting that the intact, functional, multimeric CCT/TRiC complex associated with SK1. Furthermore, pulse-chase studies indicated that CCT/TRiC binds specifically to newly translated SK1. Finally, depletion of functional CCT/TRiC through the use of RNA interference in HeLa cells or temperature sensitive CCT yeast mutants reduced cellular SK1 activity. Thus, combined this data suggests that SK1 is a CCT/TRiC substrate, and that this chaperonin facilitates folding of newly translated SK1 into its mature active form.

Overexpressed ribosomal proteins suppress defective chaperonins in Saccharomyces cerevisiae

The chaperonin Cct complex of the yeast Saccharomyces cerevisiae is composed of eight different subunits encoded by eight essential genes, CCT1-CCT8. This Cct complex is responsible for the folding of a number of proteins including actin and tubulin. We have isolated and characterized 22 multicopy suppressors of the temperature-sensitive allele, cct4-1, which encodes an altered protein with a G345D replacement that diminishes ATP hydrolysis. Fourteen of the suppressors encode ribosomal proteins, four have roles in ribosome biogenesis, two have phosphatase activities, one is involved in protein synthesis and one of the suppressors corresponded to Cct4p. Some of the suppressors also acted on certain cct1, cct2, cct3 and cct6 mutations. We suggest that certain overexpressed ribosomal and other proteins can act as weak chaperones, phenotypically alleviating the partial defects of mutationally altered Cct subunits.

Transducin gamma-subunit sets expression levels of alpha- and beta-subunits and is crucial for rod viability

Transducin is a prototypic heterotrimeric G-protein mediating visual signaling in vertebrate photoreceptor cells. Despite its central role in phototransduction, little is known about the mechanisms that regulate its expression and maintain approximately stoichiometric levels of the alpha- and betagamma-subunits. Here we demonstrate that the knock-out of transducin gamma-subunit leads to a major downregulation of both alpha- and beta-subunit proteins, despite nearly normal levels of the corresponding transcripts, and fairly rapid photoreceptor degeneration. Significant fractions of the remaining alpha- and beta-subunits were mislocalized from the light-sensitive outer segment compartment of the rod. Yet, the tiny amount of the alpha-subunit present in the outer segments of knock-out rods was sufficient to support light signaling, although with a markedly reduced sensitivity. These data indicate that the gamma-subunit controls the expression level of the entire transducin heterotrimer and that heterotrimer formation is essential for normal transducin localization. They further suggest that the production of transducin beta-subunit without its constitutive gamma-subunit partner sufficiently stresses the cellular biosynthetic and/or chaperone machinery to induce cell death.

Chaperonins: The hunt for the Group II mechanism

Chaperonins are multi-subunit complexes that enhance the efficiency of protein-folding reactions by capturing protein substrates in their central cavities. They occur in all prokaryotic and eukaryotic cell types and, alone amongst molecular chaperones, chaperonin knockouts are always lethal. Chaperonins come in two forms; the Group I are found in bacteria, mitochondria and plastids [W.A. Fenton, A.L. Horwich, Q. Rev. Biophys. 36 (2003) 229-256, [1]] and the Group II in the eukaryotic cytoplasm and in archaea [N.J. Cowan, S.A. Lewis, Adv. Protein Chem. 59 (2001) 73-104, [2]]. Both use energy derived from ATP binding and hydrolysis to drive a series of structural rearrangements that enable them to capture, engulf and then release polypeptide chains that have either not yet acquired the native, biologically active state or have been denatured in the cell.

Molecular chaperones and selection against mutations

Background

Molecular chaperones help to restore the native states of proteins after their destabilization by external stress. It has been proposed that another function of chaperones is to maintain the activity of proteins destabilized by mutation, weakening the selection against suboptimal protein variants. This would allow for the accumulation of genetic variation which could then be exposed during environmental perturbation and facilitate rapid adaptation.

Results

We focus on studies describing interactions of chaperones with mutated polypeptides. There are some examples that chaperones can alleviate the deleterious effects of mutations through increased assistance of destabilized proteins. These experiments are restricted to bacteria and typically involve overexpression of chaperones. In eukaryotes, it was found that the malfunctioning of chaperones aggravated phenotypic aberrations associated with mutations. This effect could not be linked to chaperone-mediated stabilization of mutated proteins. More likely, the insufficient activity of chaperones inflicted a deregulation of multiple cellular systems, including those responsible for signaling and therefore important in development. As to why the assistance of mutated proteins by chaperones seems difficult to demonstrate, we note that chaperone-assisted folding can often co-exist with chaperone-assisted degradation. There is growing evidence that some chaperones, including those dependent on Hsp90, can detect potentially functional but excessively unstable proteins and direct them towards degradation instead of folding. This implies that at least some mutations are exposed rather than masked by the activity of molecular chaperones.

Conclusion

It is at present impossible to determine whether molecular chaperones are mostly helpers or examiners of mutated proteins because experiments showing either of these roles are very few. Depending on whether assistance or disposal prevails, molecular chaperones could speed up or slow down evolution of protein sequences. Similar uncertainties arise when the concept of chaperones (mostly Hsp90) as general regulators of evolvability is considered. If the two roles of chaperones are antagonistic, then any (even small) modification of the chaperone activities to save mutated polypeptides could lead to increased misfolding and aggregation of other proteins. This would be a permanent burden, different from the stochastic cost arising from indiscriminate buffering of random mutations of which many are maladaptive.

Reviewers

This article was reviewed by A. S. Kondrashov, J. Höhfeld (nominated by A. Eyre-Walker) and D. A. Drummond (nominated by C. Adami). For the full reviews, please go to the Reviewers' comments section.

ATP-induced allostery in the eukaryotic chaperonin CCT is abolished by the mutation G345D in CCT4 that renders yeast temperature-sensitive for growth

Saccharomyces cerevisiae yeast cells containing the chaperonin CCT (chaperonin-containing t-complex polypeptide 1 (TCP-1)) with the G345D mutation in subunit CCT4 (anc2-1) are temperature-sensitive for growth and display defects in organization of actin structure, budding and cell shape. In this first structure-function analysis of CCT, we show that this mutation abolishes both intra- and inter-ring cooperativity in ATP binding by CCT. The finding that a single mutation in only one subunit in each CCT ring has such drastic effects highlights the importance of allostery for its in vivo function. These results, together with other kinetic data for wild-type CCT reported in this study, provide support for the sequential model for ATP-dependent allosteric transitions in CCT.

MKKS is a centrosome-shuttling protein degraded by disease-causing mutations via CHIP-mediated ubiquitination

McKusick-Kaufman syndrome (MKKS) is a recessively inherited human genetic disease characterized by several developmental anomalies. Mutations in the MKKS gene also cause Bardet-Biedl syndrome (BBS), a genetically heterogeneous disorder with pleiotropic symptoms. However, little is known about how MKKS mutations lead to disease. Here, we show that disease-causing mutants of MKKS are rapidly degraded via the ubiquitin-proteasome pathway in a manner dependent on HSC70 interacting protein (CHIP), a chaperone-dependent ubiquitin ligase. Although wild-type MKKS quickly shuttles between the centrosome and cytosol in living cells, the rapidly degraded mutants often fail to localize to the centrosome. Inhibition of proteasome functions causes MKKS mutants to form insoluble structures at the centrosome. CHIP and partner chaperones, including heat-shock protein (HSP)70/heat-shock cognate 70 and HSP90, strongly recognize MKKS mutants. Modest knockdown of CHIP by RNA interference moderately inhibited the degradation of MKKS mutants. These results indicate that the MKKS mutants have an abnormal conformation and that chaperone-dependent degradation mediated by CHIP is a key feature of MKKS/BBS diseases.

Function of phosducin-like proteins in G protein signaling and chaperone-assisted protein folding

Members of the phosducin gene family were initially proposed to act as down-regulators of G protein signaling by binding G protein betagamma dimers (Gbetagamma) and inhibiting their ability to interact with G protein alpha subunits (Galpha) and effectors. However, recent findings have over-turned this hypothesis by showing that most members of the phosducin family act as co-chaperones with the cytosolic chaperonin complex (CCT) to assist in the folding of a variety of proteins from their nascent polypeptides. In fact rather than inhibiting G protein pathways, phosducin-like protein 1 (PhLP1) has been shown to be essential for G protein signaling by catalyzing the folding and assembly of the Gbetagamma dimer. PhLP2 and PhLP3 have no role in G protein signaling, but they appear to assist in the folding of proteins essential in regulating cell cycle progression as well as actin and tubulin. Phosducin itself is the only family member that does not participate with CCT in protein folding, but it is believed to have a specific role in visual signal transduction to chaperone Gbetagamma subunits as they translocate to and from the outer and inner segments of photoreceptor cells during light-adaptation.

Circadian proteomics of the mouse retina

The circadian clock in the retina regulates a variety of physiological phenomena such as disc shedding and melatonin release. Although these events are critical for retinal functions, it is almost unknown how the circadian clock controls the physiological rhythmicity. To gain insight into the processes, we performed a proteomic analysis using 2-DE to find proteins whose levels show circadian changes. Among 415 retinal protein spots, 11 protein spots showed circadian rhythmicity in their intensities. We performed MALDI-TOF MS and NanoLC-MS/MS analyses and identified proteins contained in the 11 spots. The proteins were related to vesicular transport, calcium-binding, protein degradation, metabolism, RNA-binding, and protein foldings, suggesting the clock-regulation of neurotransmitter release, transportation of the membrane proteins, calcium-binding capability, and so on. We also found a rhythmic phosphorylation of N-ethylmaleimide-sensitive fusion protein and identified one of the amino acid residues modified by phosphorylation. These findings provide a new perspective on the relationship between the physiological functions of the retina and the circadian clock system.