Compartmentation of protein folding in vivo: sequestration of non-native polypeptide by the chaperonin-GimC system

Comparison of hippocampal synaptosome proteins in young-adult and aged rats

The hippocampus is important in learning and memory functions but its ability to aid in these functions declines during aging. In this study, we examined hippocampal proteins whose expressions changed in the aging process. A comparison of synaptosome proteins of hippocampus prepared from young-adult (9-week-old) rats with those from aged (30-month-old) rats by two-dimensional fluorescence difference gel electrophoresis revealed 24 spots that were expressed differently among about 1000 spots detected in both young-adult and aged rat samples. Nineteen of these 24 spots were identified by peptide mass fingerprinting. These proteins included chaperone proteins and proteins related to the cytoskeleton, neurotransmission, signal transduction and energy supply. The cytoskeleton-related proteins included actin and T-complex 1, which is thought to play a role in actin folding. Actin was up-regulated but T-complex 1 was down-regulated in aged rat synapses. These results suggest that age-dependent changes of actin filament formation are related to neuronal dysfunction associated with aging.

Reduced bone mineral density in SOCS-2-deficient mice

Suppressor of cytokine signaling-2 (SOCS-2) is a member of the suppressor of cytokine signaling family, implicated in the negative regulation of cytokine action through inhibition of the Janus kinase (JAK) signal transducers and activators of transcription (STAT) signal transduction pathway. We have previously reported that SOCS-2-/- mice display an increased longitudinal skeletal growth associated with a deregulated GH/IGF-I signaling. The aim of the present study was to determine the role of SOCS-2 in the regulation of bone mineral density (BMD). Dual x-ray absorptiometry (DXA) analyses demonstrated that the areal BMD of the tibia was reduced in both 4-wk-old (-8.6%) and 15-wk-old (-6.0%) SOCS 2-/- mice compared with wild-type (WT) mice. The trabecular volumetric BMD, as measured by peripheral quantitative computerized tomography (pQCT) in the metaphyseal region of the distal femur, was reduced in both 4-wk-old (-10%) and 15-wk-old (-32%) SOCS 2-/- mice compared with WT mice. pQCT analyses in the diaphyseal region of tibia also revealed that the cortical volumetric BMD was reduced in both 4-wk-old (-7%) and 15-wk-old (-3%) SOCS 2-/- mice. The cortical cross-sectional area was reduced in 4-wk-old but not in 15-wk-old SOCS 2-/- mice. In conclusion, SOCS-2 inactivation results in reduced trabecular and cortical volumetric BMD. These effects are not consistent with an augmented GH/IGF-I signaling and, therefore, the mechanism behind the reduced BMD remains to be elucidated.

A robust toolkit for functional profiling of the yeast genome

Study of mutant phenotypes is a fundamental method for understanding gene function. The construction of a near-complete collection of yeast knockouts (YKO) and the unique molecular barcodes (or TAGs) that identify each strain has enabled quantitative functional profiling of Saccharomyces cerevisiae. By using these TAGs and the SGA reporter, MFA1pr-HIS3, which facilitates conversion of heterozygous diploid YKO strains into haploid mutants, we have developed a set of highly efficient microarray-based techniques, collectively referred as dSLAM (diploid-based synthetic lethality analysis on microarrays), to probe genome-wide gene-chemical and gene-gene interactions. Direct comparison revealed that these techniques are more robust than existing methods in functional profiling of the yeast genome. Widespread application of these tools will elucidate a comprehensive yeast genetic network.

Pathways of chaperone-mediated protein folding in the cytosol

Cells are faced with the task of folding thousands of different polypeptides into a wide range of conformations. For many proteins, the folding process requires the action of molecular chaperones. In the cytosol of prokaryotic and eukaryotic cells, molecular chaperones of different structural classes form a network of pathways that can handle substrate polypeptides from the point of initial synthesis on ribosomes to the final stages of folding.

The archaeal Sec-dependent protein translocation pathway

Over the past three decades, transport of proteins across cellular membranes has been studied extensively in various model systems. One of the major transport routes, the so-called Sec pathway, is conserved in all domains of life. Very little is known about this pathway in the third domain of life, archaea. The core components of the archaeal, bacterial and eucaryal Sec machinery are similar, although the archaeal components appear more closely related to their eucaryal counterparts. Interestingly, the accessory factors of the translocation machinery are similar to bacterial components, which indicates a unique hybrid nature of the archaeal translocase complex. The mechanism of protein translocation in archaea is completely unknown. Based on genomic sequencing data, the most likely system for archaeal protein translocation is similar to the eucaryal co-translational translocation pathway for protein import into the endoplasmic reticulum, in which a protein is pushed across the translocation channel by the ribosome. However, other models can also be envisaged, such as a bacterial-like system in which a protein is translocated post-translationally with the aid of a motor protein analogous to the bacterial ATPase SecA. This review discusses the different models. Furthermore, an overview is given of some of the other components that may be involved in the protein translocation process, such as those required for protein targeting, folding and post-translational modification.

Genome-wide lethality screen identifies new PI4,5P2 effectors that regulate the actin cytoskeleton

To further understand the roles played by the essential phosphoinositide PI4,5P(2), we have used a synthetic lethal analysis, which systematically combined the mss4(ts) mutation, partially defective in PI4P 5-kinase activity, with each of approximately 4700 deletion mutations. This genomic screening technique uncovered numerous new candidate effectors and regulators of PI4,5P(2) in yeast. In particular, we identified Slm1 (Yil105c), a previously uncharacterized PI4,5P(2) binding protein. Like Mss4, Slm1 and its homolog Slm2 (Ynl047c) were required for actin cytoskeleton polarization and viability. Co-immunoprecipitation experiments revealed that Slm1 interacts with a component of TORC2, a Tor2 kinase-containing complex, which also regulates the actin cytoskeleton. Consistent with these findings, phosphorylation of Slm1 and Slm2 was dependent on TORC2 protein kinase activity, both in vivo and in vitro, and Slm1 localization required both PI4,5P(2) and functional TORC2. Together, these data suggest that Slm1 and Slm2 function downstream of PI4,5P(2) and the TORC2 kinase pathway to control actin cytoskeleton organization.

Kinetics and Binding Sites for Interaction of the Prefoldin with a Group II Chaperonin

Prefoldin is a jellyfish-shaped hexameric co-chaperone of the group II chaperonins. It captures a protein folding intermediate and transfers it to a group II chaperonin for completion of folding. The manner in which prefoldin interacts with its substrates and cooperates with the chaperonin is poorly understood. In this study, we have examined the interaction between a prefoldin and a chaperonin from hyperthermophilic archaea by immunoprecipitation, single molecule observation, and surface plasmon resonance. We demonstrate that Pyrococcus prefoldin interacts most tightly with its cognate chaperonin, and vice versa, suggesting species specificity in the interaction. Using truncation mutants, we uncovered by kinetic analyses that this interaction is multivalent in nature, consistent with multiple binding sites between the two chaperones. We present evidence that both N- and C-terminal regions of the prefoldin beta sub-unit are important for molecular chaperone activity and for the interaction with a chaperonin. Our data are consistent with substrate and chaperonin binding sites on prefoldin that are different but in close proximity, which suggests a possible handover mechanism of prefoldin substrates to the chaperonin.

Systematic quantification of gene interactions by phenotypic array analysis

A phenotypic array method, developed for quantifying cell growth, was applied to the haploid and homozygous diploid yeast deletion strain sets. A growth index was developed to screen for non-additive interacting effects between gene deletion and induced perturbations.

A phenotypic array method, developed for quantifying cell growth, was applied to the haploid and homozygous diploid yeast deletion strain sets. A growth index was developed to screen for non-additive interacting effects between gene deletion and induced perturbations. From a genome screen for hydroxyurea (HU) chemical-genetic interactions, 298 haploid deletion strains were selected for further analysis. The strength of interactions was quantified using a wide range of HU concentrations affecting reference strain growth. The selectivity of interaction was determined by comparison with drugs targeting other cellular processes. Bio-modules were defined as gene clusters with shared strength and selectivity of interaction profiles. The functions and connectivity of modules involved in processes such as DNA repair, protein secretion and metabolic control were inferred from their respective gene composition. The work provides an example of, and a general experimental framework for, quantitative analysis of gene interaction networks that buffer cell growth.

Molecular clamp mechanism of substrate binding by hydrophobic coiled-coil residues of the archaeal chaperone prefoldin

Prefoldin (PFD) is a jellyfish-shaped molecular chaperone that has been proposed to play a general role in de novo protein folding in archaea and is known to assist the biogenesis of actins, tubulins, and potentially other proteins in eukaryotes. Using point mutants, chimeras, and intradomain swap variants, we show that the six coiled-coil tentacles of archaeal PFD act in concert to bind and stabilize nonnative proteins near the opening of the cavity they form. Importantly, the interaction between chaperone and substrate depends on the mostly buried interhelical hydrophobic residues of the coiled coils. We also show by electron microscopy that the tentacles can undergo an en bloc movement to accommodate an unfolded substrate. Our data reveal how archael PFD uses its unique architecture and intrinsic coiled-coil properties to interact with nonnative polypeptides.

Selective Contribution of Eukaryotic Prefoldin Subunits to Actin and Tubulin Binding

Eukaryotic prefoldin (PFD) is a heterohexameric chaperone with a jellyfish-like structure whose function is to deliver nonnative target proteins, principally actins and tubulins, to the eukaryotic cytosolic chaperonin for facilitated folding. Here we demonstrate that functional PFD can spontaneously assemble from its six constituent individual subunits (PFD1-PFD6), each expressed as a recombinant protein. Using engineered forms of PFD assembled in vitro, we show that the tips of the PFD tentacles are required to form binary complexes with authentic target proteins. We show that PFD uses the distal ends of different but overlapping sets of subunits to form stable binary complexes with different target proteins, namely actin and alpha- and beta-tubulin. We also present data that suggest a model for the order of these six subunits within the hexamer. Our data are consistent with the hypothesis that PFD, like the eukaryotic cytosolic chaperonin, has co-evolved specifically to facilitate the folding of its target proteins.

Hsp70 and Hsp90--a relay team for protein folding

Molecular chaperones are a functionally defined set of proteins which assist the structure formation of proteins in vivo. Without certain protective mechanisms, such as binding nascent polypeptide chains by molecular chaperones, cellular protein concentrations would lead to misfolding and aggregation. In the mammalian system, the molecular chaperones Hsp70 and Hsp90 are involved in the folding and maturation of key regulatory proteins, like steroid hormone receptors, transcription factors, and kinases, some of which are involved in cancer progression. Hsp70 and Hsp90 form a multichaperone complex, in which both are connected by a third protein called Hop. The connection of and the interplay between the two chaperone machineries is of crucial importance for cell viability. This review provides a detailed view of the Hsp70 and Hsp90 machineries, their cofactors and their mode of regulation. It summarizes the current knowledge in the field, including the ATP-dependent regulation of the Hsp70/Hsp90 multichaperone cycle and elucidates the complex interplay and their synergistic interaction.

Control of nutrient-sensitive transcription programs by the unconventional prefoldin URI

Prefoldins (PFDs) are members of a recently identified, small-molecular weight protein family able to assemble into molecular chaperone complexes. Here we describe an unusually large member of this family, termed URI, that forms complexes with other small-molecular weight PFDs and with RPB5, a shared subunit of all three RNA polymerases. Functional analysis of the yeast and human orthologs of URI revealed that both are targets of nutrient signaling and participate in gene expression controlled by the TOR kinase. Thus, URI is a component of a signaling pathway that coordinates nutrient availability with gene expression.

TRiC/CCT cooperates with different upstream chaperones in the folding of distinct protein classes

The role in protein folding of the eukaryotic chaperonin TRiC/CCT is only partially understood. Here, we show that a group of WD40 beta-propeller proteins in the yeast cytosol interact transiently with TRiC upon synthesis and require the chaperonin to reach their native state. TRiC cooperates in the folding of these proteins with the ribosome-associated heat shock protein (Hsp)70 chaperones Ssb1/2p. In contrast, newly synthesized actin and tubulins, the major known client proteins of TRiC, are independent of Ssb1/2p and instead use the co-chaperone GimC/prefoldin for efficient transfer to the chaperonin. GimC can replace Ssb1/2p in the folding of WD40 substrates such as Cdc55p, but combined deletion of SSB and GIM genes results in loss of viability. These findings expand the substrate range of the eukaryotic chaperonin by a structurally defined class of proteins and demonstrate an essential role for upstream chaperones in TRiC-assisted folding.

Functional characterization of an archaeal GroEL/GroES chaperonin system: significance of substrate encapsulation

In all three kingdoms of life chaperonins assist the folding of a range of newly synthesized proteins. As shown recently, Archaea of the genus Methanosarcina contain both group I (GroEL/GroES) and group II (thermosome) chaperonins in the cytosol. Here we report on a detailed functional analysis of the archaeal GroEL/GroES system of Methanosarcina mazei (Mm) in comparison to its bacterial counterpart from Escherichia coli (Ec). We find that the groESgroEL operon of M. mazei is unable to functionally replace groESgroEL in E. coli. However, the MmGroES protein can largely complement a mutant EcGroES protein in vivo. The ATPase rate of MmGroEL is very low and the dissociation of MmGroES from MmGroEL is 15 times slower than for the EcGroEL/GroES system. This slow ATPase cycle results in a prolonged enclosure time for model substrate proteins, such as rhodanese, in the MmGroEL:GroES folding cage before their release into the medium. Interestingly, optimal functionality of MmGroEL/GroES and its ability to encapsulate larger proteins, such as malate dehydrogenase, requires the presence of ammonium sulfate in vitro. In the absence of ammonium sulfate, malate dehydrogenase fails to be encapsulated by GroES and rather cycles on and off the GroEL trans ring in a non-productive reaction. These results indicate that the archaeal GroEL/GroES system has preserved the basic encapsulation mechanism of bacterial GroEL and suggest that it has adjusted the length of its reaction cycle to the slower growth rates of Archaea. Additionally, the release of only the folded protein from the GroEL/GroES cage may prevent adverse interactions of the GroEL substrates with the thermosome, which is not normally located within the same compartment.

A Novel Step in β-Tubulin Folding Is Important for Heterodimer Formation inSaccharomyces cerevisiae

Undimerized β-tubulin is toxic in the yeast S. cerevisiae. It can arise if levels of β-tubulin and α-tubulin are unbalanced or if the tubulin heterodimer dissociates. We are using the toxicity of β-tubulin to understand early steps in microtubule morphogenesis. We find that deletion of PLP1 suppresses toxic β-tubulin formed by disparate levels of α- and β-tubulin. That suppression occurs either when α-tubulin is modestly underexpressed relative to β-tubulin or when β-tubulin is inducibly and strongly overexpressed. Plp1p does not affect tubulin expression. Instead, a significant proportion of the undimerized β-tubulin in plp1Δ cells is less toxic than that in wild-type cells. It is also less able to combine with α-tubulin to form a heterodimer. As a result, plp1Δ cells have lower levels of heterodimer. Importantly, plp1Δ cells that also lack Pac10, a component of the GimC/PFD complex, are even less affected by free β-tubulin. Our results suggest that Plp1p defines a novel early step in β-tubulin folding.

Coexistence of group I and group II chaperonins in the archaeon Methanosarcina mazei

Two distantly related classes of cylindrical chaperonin complexes assist in the folding of newly synthesized and stress-denatured proteins in an ATP-dependent manner. Group I chaperonins are thought to be restricted to the cytosol of bacteria and to mitochondria and chloroplasts, whereas the group II chaperonins are found in the archaeal and eukaryotic cytosol. Here we show that members of the archaeal genus Methanosarcina co-express both the complete group I (GroEL/GroES) and group II (thermosome/prefoldin) chaperonin systems in their cytosol. These mesophilic archaea have acquired between 20 and 35% of their genes by lateral gene transfer from bacteria. In Methanosarcina mazei Gö1, both chaperonins are similarly abundant and are moderately induced under heat stress. The M. mazei GroEL/GroES proteins have the structural features of their bacterial counterparts. The thermosome contains three paralogous subunits, alpha, beta, and gamma, which assemble preferentially at a molar ratio of 2:1:1. As shown in vitro, the assembly reaction is dependent on ATP/Mg2+ or ADP/Mg2+ and the regulatory role of the beta subunit. The co-existence of both chaperonin systems in the same cellular compartment suggests the Methanosarcina species as useful model systems in studying the differential substrate specificity of the group I and II chaperonins and in elucidating how newly synthesized proteins are sorted from the ribosome to the proper chaperonin for folding.

Getting a grip on non-native proteins

It is an underappreciated fact that non-native polypeptides are prevalent in the cellular environment. Native proteins have the folded structure, assembled state and cellular localization required for activity. By contrast, non-native proteins lack function and are particularly prone to aggregation because hydrophobic residues that are normally buried are exposed on their surfaces. These unstable entities include polypeptides that are undergoing synthesis, transport to and translocation across membranes, and those that are unfolded before degradation. Non-native proteins are normal, biologically relevant components of a healthy cell, except in cases in which their misfolding results from disease-causing mutations or adverse extrinsic factors. Here, we explore the nature and occurrence of non-native proteins, and describe the diverse families of molecular chaperones and coordinated cellular responses that have evolved to prevent their misfolding and aggregation, thereby maintaining quality control over these potentially damaging protein species.

The Hsp70 and TRiC/CCT chaperone systems cooperate in vivo to assemble the von Hippel-Lindau tumor suppressor complex

The degree of cooperation and redundancy between different chaperones is an important problem in understanding how proteins fold in the cell. Here we use the yeast Saccharomyces cerevisiae as a model system to examine in vivo the chaperone requirements for assembly of the von Hippel-Lindau protein (VHL)-elongin BC (VBC) tumor suppressor complex. VHL and elongin BC expressed in yeast assembled into a correctly folded VBC complex that resembles the complex from mammalian cells. Unassembled VHL did not fold and remained associated with the cytosolic chaperones Hsp70 and TRiC/CCT, in agreement with results from mammalian cells. Analysis of the folding reaction in yeast strains carrying conditional chaperone mutants indicates that incorporation of VHL into VBC requires both functional TRiC and Hsp70. VBC assembly was defective in cells carrying either a temperature-sensitive ssa1 gene as their sole source of cytosolic Hsp70/SSA function or a temperature-sensitive mutation in CCT4, a subunit of the TRiC/CCT complex. Analysis of the VHL-chaperone interactions in these strains revealed that the cct4ts mutation decreased binding to TRiC but did not affect the interaction with Hsp70. In contrast, loss of Hsp70 function disrupted the interaction of VHL with both Hsp70 and TRiC. We conclude that, in vivo, folding of some polypeptides requires the cooperation of Hsp70 and TRiC and that Hsp70 acts to promote substrate binding to TRiC.

Ribosome-tethered molecular chaperones: the first line of defense against protein misfolding?

Folding of many cellular proteins is facilitated by molecular chaperones. Analysis of both prokaryotic and lower eukaryotic model systems has revealed the presence of ribosome-associated molecular chaperones, thought to be the first line of defense against protein aggregation as translating polypeptides emerge from the ribosome. However, structurally unrelated chaperones have evolved to carry out these functions in different microbes. In the yeast Saccharomyces cerevisiae, an unusual complex of Hsp70 and J-type chaperones associates with ribosome-bound nascent chains, whereas in Escherichia coli the ribosome-associated peptidyl-prolyl-cis-trans isomerase, trigger factor, plays a predominant role.

Function and regulation of cytosolic molecular chaperone CCT

Molecular chaperones are a group of proteins that assists in the folding of newly synthesized proteins or in the refolding of denatured proteins. The cytosolic chaperonin-containing t-complex polypeptide 1 (CCT) is a molecular chaperone that plays an important role in the folding of proteins in the eukaryotic cytosol. Actin, tubulin, and several other proteins are known to be folded by CCT, and an estimated 15% of newly translated proteins in mammalian cells are folded with the assistance of CCT. CCT differs from other chaperonin family proteins in its subunit composition, which consists of eight subunit species comprising the CCT 16-mer double-ring-like complex. CCT preferentially recognizes quasinative (or partially folded) intermediates, whereas its Escherichia coli homologue GroEL recognizes more unfolded intermediates, especially those displaying hydrophobic surfaces. Molecular evolutionary analyses have suggested that each subunit species has a specific function in addition to contributing to a common ATPase activity. Consistent with this view, it has been suggested that each subunit recognizes specific substrate proteins (or their parts) and that they collectively modulate the ATPase activity of the complex. The overall expression of CCT in mammalian cells is primarily dependent on cell growth, but each subunit exhibits an individual patterns of expression. Recent progress in CCT research is reviewed, focusing particularly on CCT function and expression. From these observations, the possible roles of the distinct subunits in CCT-assisted folding in the eukaryotic cytosol are discussed.

Structure of eukaryotic prefoldin and of its complexes with unfolded actin and the cytosolic chaperonin CCT

The biogenesis of the cytoskeletal proteins actin and tubulin involves interaction of nascent chains of each of the two proteins with the oligomeric protein prefoldin (PFD) and their subsequent transfer to the cytosolic chaperonin CCT (chaperonin containing TCP-1). Here we show by electron microscopy that eukaryotic PFD, which has a similar structure to its archaeal counterpart, interacts with unfolded actin along the tips of its projecting arms. In its PFD-bound state, actin seems to acquire a conformation similar to that adopted when it is bound to CCT. Three-dimensional reconstruction of the CCT:PFD complex based on cryoelectron microscopy reveals that PFD binds to each of the CCT rings in a unique conformation through two specific CCT subunits that are placed in a 1,4 arrangement. This defines the phasing of the CCT rings and suggests a handoff mechanism for PFD.

The yeast ubiquitin protease, Ubp3p, promotes protein stability

Stu1p is a microtubule-associated protein required for spindle assembly. In this article we show that the temperature-sensitive stu1-5 allele is synthetically lethal in combination with ubp3, gim1-gim5, and kem1 mutations. The primary focus of this article is on the stu1-5 ubp3 interaction. Ubp3 is a deubiquitination enzyme and a member of a large family of cysteine proteases that cleave ubiquitin moieties from protein substrates. UBP3 is the only one of 16 UBP genes in yeast whose loss is synthetically lethal with stu1-5. Stu1p levels in stu1-5 cells are several-fold lower than the levels in wild-type cells and the stu1-5 temperature sensitivity can be rescued by additional copies of stu1-5. These results indicate that the primary effect of the stu1-5 mutation is to make the protein less stable. The levels of Stu1p are even lower in ubp3Delta stu1-5 cells, suggesting that Ubp3p plays a role in promoting protein stability. We also found that ubp3Delta produces growth defects in combination with mutations in other genes that decrease protein stability. Overall, these data support the idea that Ubp3p has a general role in the reversal of protein ubiquitination.

Folding of newly translated proteins in vivo: the role of molecular chaperones

Recent years have witnessed dramatic advances in our understanding of how newly translated proteins fold in the cell and the contribution of molecular chaperones to this process. Folding in the cell must be achieved in a highly crowded macromolecular environment, in which release of nonnative polypeptides into the cytosolic solution might lead to formation of potentially toxic aggregates. Here I review the cellular mechanisms that ensure efficient folding of newly translated proteins in vivo. De novo protein folding appears to occur in a protected environment created by a highly processive chaperone machinery that is directly coupled to translation. Genetic and biochemical analysis shows that several distinct chaperone systems, including Hsp70 and the cylindrical chaperonins, assist the folding of proteins upon translation in the cytosol of both prokaryotic and eukaryotic cells. The cellular chaperone machinery is specifically recruited to bind to ribosomes and protects nascent chains and folding intermediates from nonproductive interactions. In addition, initiation of folding during translation appears to be important for efficient folding of multidomain proteins.

Molecular chaperones in the cytosol: from nascent chain to folded protein

Efficient folding of many newly synthesized proteins depends on assistance from molecular chaperones, which serve to prevent protein misfolding and aggregation in the crowded environment of the cell. Nascent chain--binding chaperones, including trigger factor, Hsp70, and prefoldin, stabilize elongating chains on ribosomes in a nonaggregated state. Folding in the cytosol is achieved either on controlled chain release from these factors or after transfer of newly synthesized proteins to downstream chaperones, such as the chaperonins. These are large, cylindrical complexes that provide a central compartment for a single protein chain to fold unimpaired by aggregation. Understanding how the thousands of different proteins synthesized in a cell use this chaperone machinery has profound implications for biotechnology and medicine.

Pyrococcus prefoldin stabilizes protein-folding intermediates and transfers them to chaperonins for correct folding

A molecular chaperone prefoldin/GimC from the hyperthermophilic archaeum Pyrococcus horikoshii OT3 was characterized. Pyrococcus prefoldin protected porcine heart citrate synthase from thermal aggregation whereas each subunit alone afforded little protection. It also arrested the spontaneous refolding of acid-denatured green fluorescent protein and then transferred it not only to a group II chaperonin from the hyperthermophilic archaeum Thermococcus sp. strain KS-1, but also to a group I chaperonin from the thermophilic bacterium Thermus thermophilus HB8 for subsequent ATP dependent refolding.

Diverse effects of mutations in exon II of the von Hippel-Lindau (VHL) tumor suppressor gene on the interaction of pVHL with the cytosolic chaperonin and pVHL-dependent ubiquitin ligase activity

We examined the biogenesis of the von Hippel-Lindau (VHL) tumor suppressor protein (pVHL) in vitro and in vivo. pVHL formed a complex with the cytosolic chaperonin containing TCP-1 (CCT or TRiC) en route to assembly with elongin B/C and the subsequent formation of the VCB-Cul2 ubiquitin ligase. Blocking the interaction of pVHL with elongin B/C resulted in accumulation of pVHL within the CCT complex. pVHL present in purified VHL-CCT complexes, when added to rabbit reticulocyte lysate, proceeded to form VCB and VCB-Cul2. Thus, CCT likely functions, at least in part, by retaining VHL chains pending the availability of elongin B/C for final folding and/or assembly. Tumor-associated mutations within exon II of the VHL syndrome had diverse effects upon the stability and/or function of pVHL-containing complexes. First, a pVHL mutant lacking the entire region encoded by exon II did not bind to CCT and yet could still assemble into complexes with elongin B/C and elongin B/C-Cul2. Second, a number of tumor-derived missense mutations in exon II did not decrease CCT binding, and most had no detectable effect upon VCB-Cul2 assembly. Many exon II mutants, however, were found to be defective in the binding to and subsequent ubiquitination of hypoxia-inducible factor 1alpha (HIF-1alpha), a substrate of the VCB-Cul2 ubiquitin ligase. We conclude that the selection pressure to mutate VHL exon II during tumorigenesis does not relate to loss of CCT binding but may reflect quantitative or qualitative defects in HIF binding and/or in pVHL-dependent ubiquitin ligase activity.

MM-1, a c-Myc-binding protein, is a candidate for a tumor suppressor in leukemia/lymphoma and tongue cancer

The c-myc oncogene product (c-Myc) is a transcription factor that dimerizes with Max and recognizes the E-box sequence, and it plays key functions in cell proliferation, differentiation, and apoptosis. We previously showed that MM-1 bound to myc box II within the transactivation domain of c-Myc and repressed the E-box-dependent transcriptional activity of c-Myc. Here we report that MM-1 showed features of a tumor suppressor. In an EST data base search for cDNAs homologous to MM-1, we found a frequent substitution of amino acid 157 of MM-1, from alanine to arginine (A157R), and the substitution was observed more in tumor cells than in normal cells. A survey of the A157R mutation of MM-1 in 57 cultured cancer cells and 90 tissues from cancer patients showed that the A157R was present in about 50-60% of leukemia/lymphoma cells and in more than 75% of squamous cell carcinoma of tongue cancer. Although both the A157R and the wild-type MM-1 bound to c-Myc, only A157R lost the activities to repress both the E-box-dependent transcriptional activity of c-Myc and the myc/ras cooperative transforming activity in rat 3Y1 cells. Furthermore, the wild-type MM-1, but not A157R, arrested the growth of 3Y1 cells. The human MM-1 gene was mapped at chromosome 12q12-12q13, where many chromosome abnormalities in cancer cells have been reported. The results suggest that MM-1 is a novel candidate for a tumor suppressor that controls the transcriptional activity of c-Myc.

Prefoldin recognition motifs in the nonhomologous proteins of the actin and tubulin families

Nascent actin and tubulin molecules undergo a series of complex interactions with chaperones and are thereby guided to their native conformation. These cytoskeletal proteins have the initial part of the pathway in common: both interact with prefoldin and with the cytosolic chaperonin containing tailless complex polypeptide 1. Little is understood with regard to how these chaperones and, in particular, prefoldin recognize the non-native forms of these target proteins. Using mutagenesis, we provide evidence that beta-actin and alpha-tubulin each have two prefoldin interaction sites. The most amino-terminally located site of both proteins shows striking sequence similarity, although these proteins are nonhomologous. Very similar motifs are present in beta- and gamma-tubulin and in the newly identified prefoldin target protein actin-related protein 1. Actin-related proteins 2 and 3 have related motifs, but these have altered charge properties. The latter two proteins do not bind prefoldin, although we identify them here as target proteins for the cytosolic chaperonin. Actin fragments containing the two prefoldin interaction regions compete efficiently with actin for prefoldin binding. In addition, they also compete with tubulins, suggesting that these target proteins contact similar prefoldin subunits.

Cytosolic chaperonin-containing t-complex polypeptide 1 changes the content of a particular subunit species concomitant with substrate binding and folding activities during the cell cycle

The chaperonin-containing t-complex polypeptide 1 (CCT) is a cytosolic molecular chaperone composed of eight subunits that assists in the folding of actin, tubulin and other cytosolic proteins. We show here that the content of particular subunits of CCT within mammalian cells decreases concomitantly with the reduction of chaperone activity during cell cycle arrest at M phase. CCT recovers chaperone activity upon resumption of these subunits after release from M phase arrest or during arrest at S phase. The levels of alpha, delta and zeta-1 subunits decreased more rapidly than the other subunits during M phase arrest by colcemid treatment and recovered after release from the arrest. Gel filtration chromatography or native (nondenaturing) PAGE analysis followed by immunoblotting indicated that the alpha and delta subunit content in the 700- to 900-kDa CCT complex was appreciably lower in the M phase cells than in asynchronous cells. In vivo, the CCT complex of M-phase-arrested cells was found to bind lower amounts of tubulin than that of asynchronous cells. In vitro, the CCT complex of M phase-arrested cells was less active in binding and folding denatured actin than that of asynchronous cells. On the other hand, the CCT complex of asynchronous cells (a mixture of various phases of cell cycle) exhibited lower alpha and delta subunit content and lower chaperone activity than that of S-phase-arrested cells obtained by excess thymidine treatment. In addition, turnover (synthesis and degradation) rates of the alpha and delta subunits in vivo were more rapid than those of most other subunits. These results suggest that the content of alpha and delta subunits of CCT reduces from the complete active complex in S phase cells to incomplete inactive complex in M phase cells.

The Chaperones of the archaeon Thermoplasma acidophilum

Chaperonesare an essential component of a cell's ability to respond to environmental challenges. Chaperones have been studied primarily in bacteria, but in recent years it has become apparent that some classes of chaperones either are very divergent in bacteria relative to archaea and eukaryotes or are missing entirely. In contrast, a high degree of similarity was found between the chaperonins of archaea and those of the eukaryotic cytosol, which has led to the establishment of archaeal model systems. The archaeon most extensively used for such studies is Thermoplasma acidophilum, which thrives at 59 degrees C and pH 2. Here we review information on its chaperone complement in light of the recently determined genome sequence.

Gene duplication and the evolution of group II chaperonins: implications for structure and function

Chaperonins are multisubunit protein-folding assemblies. They are composed of two distinct structural classes, which also have a characteristic phylogenetic distribution. Group I chaperonins (called GroEL/cpn60/hsp60) are present in Bacteria and eukaryotic organelles while group II chaperonins are found in Archaea (called the thermosome or TF55) and the cytoplasm of eukaryotes (called CCT or TriC). Gene duplication has been an important force in the evolution of group II chaperonins: Archaea possess one, two, or three homologous chaperonin subunit-encoding genes, and eight distinct CCT gene families (paralogs) have been described in eukaryotes. Phylogenetic analyses indicate that while the duplications in archaeal chaperonin genes have occurred numerous times independently in a lineage-specific fashion, the eight different CCT subunits found in eukaryotes are the products of duplications that occurred early and very likely only once in the evolution of the eukaryotic nuclear genome. Analyses of CCT sequences from diverse eukaryotic species reveal that each of the CCT subunits possesses a suite of invariant subunit-specific amino acid residues ("signatures"). When mapped onto the crystal structure of the archaeal chaperonin from Thermoplasma acidophilum, these signatures are located in the apical, intermediate, and equatorial domains. Regions that were found to be variable in length and/or amino acid sequence were localized primarily to the exterior of the molecule and, significantly, to the extreme tip of the apical domain (the "helical protrusion"). In light of recent biochemical and electron microscopic data describing specific CCT-substrate interactions, our results have implications for the evolution of subunit-specific functions in CCT.

The 'sequential allosteric ring' mechanism in the eukaryotic chaperonin-assisted folding of actin and tubulin

Folding to completion of actin and tubulin in the eukaryotic cytosol requires their interaction with cytosolic chaperonin CCT [chaperonin containing tailless complex polypeptide 1 (TCP-1)]. Three-dimensional reconstructions of nucleotide-free CCT complexed to either actin or tubulin show that CCT stabilizes both cytoskeletal proteins in open and quasi-folded conformations mediated through interactions that are both subunit specific and geometry dependent. Here we find that upon ATP binding, mimicked by the non-hydrolysable analog AMP-PNP (5'-adenylyl-imido-diphosphate), to both CCT-alpha-actin and CCT- beta-tubulin complexes, the chaperonin component undergoes concerted movements of the apical domains, resulting in the cavity being closed off by the helical protrusions of the eight apical domains. However, in contrast to the GroE system, generation of this closed state does not induce the release of the substrate into the chaperonin cavity, and both cytoskeletal proteins remain bound to the chaperonin apical domains. Docking of the AMP-PNP-CCT-bound conformations of alpha-actin and beta-tubulin to their respective native atomic structures suggests that both proteins have progressed towards their native states.

Review: cellular substrates of the eukaryotic chaperonin TRiC/CCT

The TCP-1 ring complex (TRiC; also called CCT, for chaperonin containing TCP-1) is a large (approximately 900 kDa) multisubunit complex that mediates protein folding in the eukaryotic cytosol. The physiological substrate spectrum of TRiC is still poorly defined. Genetic and biochemical data show that it is required for the folding of the cytoskeletal proteins actin and tubulin. Recent years have witnessed a steady stream of reports that describe other proteins that require TRiC for proper folding. Furthermore, analysis of the transit of newly synthesized proteins through TRiC in intact cells suggests that the chaperonin contributes to the folding of a distinct subset of cellular proteins. Here we review the current understanding of a role for TRiC in the folding of newly synthesized polypeptides, with a focus on some of the individual proteins that require TRiC.

A genomic study of the bipolar bud site selection pattern in Saccharomyces cerevisiae

A genome-wide screen of 4168 homozygous diploid yeast deletion strains has been performed to identify nonessential genes that participate in the bipolar budding pattern. By examining bud scar patterns representing the sites of previous cell divisions, 127 mutants representing three different phenotypes were found: unipolar, axial-like, and random. From this screen, 11 functional classes of known genes were identified, including those involved in actin-cytoskeleton organization, general bud site selection, cell polarity, vesicular transport, cell wall synthesis, protein modification, transcription, nuclear function, translation, and other functions. Four characterized genes that were not known previously to participate in bud site selection were also found to be important for the haploid axial budding pattern. In addition to known genes, we found 22 novel genes (20 are designated BUD13-BUD32) important for bud site selection. Deletion of one resulted in unipolar budding exclusively from the proximal pole, suggesting that this gene plays an important role in diploid distal budding. Mutations in 20 other novel BUD genes produced a random budding phenotype and one produced an axial-like budding defect. Several of the novel Bud proteins were fused to green fluorescence protein; two proteins were found to localize to sites of polarized cell growth (i.e., the bud tip in small budded cells and the neck in cells undergoing cytokinesis), similar to that postulated for the bipolar signals and proteins that target cell division site tags to their proper location in the cell. Four others localized to the nucleus, suggesting that they play a role in gene expression. The bipolar distal marker Bud8 was localized in a number of mutants; many showed an altered Bud8-green fluorescence protein localization pattern. Through the genome-wide identification and analysis of different mutants involved in bipolar bud site selection, an integrated pathway for this process is presented in which proximal and distal bud site selection tags are synthesized and localized at their appropriate poles, thereby directing growth at those sites. Genome-wide screens of defined collections of mutants hold significant promise for dissecting many biological processes in yeast.

The Arabidopsis embryo mutant schlepperless has a defect in the chaperonin-60alpha gene

We identified a T-DNA-generated mutation in the chaperonin-60alpha gene of Arabidopsis that produces a defect in embryo development. The mutation, termed schlepperless (slp), causes retardation of embryo development before the heart stage, even though embryo morphology remains normal. Beyond the heart stage, the slp mutation results in defective embryos with highly reduced cotyledons. slp embryos exhibit a normal apical-basal pattern and radial tissue organization, but they are morphologically retarded. Even though slp embryos are competent to transcribe two late-maturation gene markers, this competence is acquired more slowly as compared with wild-type embryos. slp embryos also exhibit a defect in plastid development-they remain white during maturation in planta and in culture. Hence, the overall developmental phenotype of the slp mutant reflects a lesion in the chloroplast that affects embryo development. The slp phenotype highlights the importance of the chaperonin-60alpha protein for chloroplast development and subsequently for the proper development of the plant embryo and seedling.

Proteasome-dependent degradation of cytosolic chaperonin CCT

The chaperonin containing t-complex polypeptide 1 (CCT) is a heterooligomeric molecular chaperone that assists in the folding of actin, tubulin, and other cytosolic proteins. We show here that degradation of CCT in mammalian cells is inhibited by a proteasome-specific inhibitor, lactacystin. When CCT synthesis was inhibited by growth arrest of cells, the decrease in CCT levels was much slower in the presence of lactacystin than in its absence. Pulse-chase experiments indicated that degradation of CCT is inhibited 2- to 2.5-fold by addition of lactacystin. In addition, CCT degradation rate in ts85 cells that produce thermolabile ubiquitin-activating enzyme E1 was reduced 3-fold at the nonpermissive temperature compared to the degradation at the permissive temperature. These results indicate that the ubiquitin-proteasome system is involved in CCT degradation.

Observation of the noncovalent assembly and disassembly pathways of the chaperone complex MtGimC by mass spectrometry

We have analyzed a newly described archaeal GimC/prefoldin homologue, termed MtGimC, by using nanoflow electrospray coupled with time-of-flight MS. The molecular weight of the complex from Methanobacterium thermoautotrophicum corresponds to a well-defined hexamer of two alpha subunits and four beta subunits. Dissociation of the complex within the gas phase reveals a quaternary arrangement of two central subunits, both alpha, and four peripheral beta subunits. By constructing a thermally controlled nanoflow device, we have monitored the thermal stability of the complex by MS. The results of these experiments demonstrate that a significant proportion of the MtGimC hexamer remains intact under low-salt conditions at elevated temperatures. This finding is supported by data from CD spectroscopy, which show that at physiological salt concentrations, the complex remains stable at temperatures above 65 degrees C. Mass spectrometric methods were developed to monitor in real time the assembly of the MtGimC hexamer from its component subunits. By using this methodology, the mass spectra recorded throughout the time course of the experiment showed the absence of any significantly populated intermediates, demonstrating that the assembly process is highly cooperative. Taken together, these data show that the complex is stable under the elevated temperatures that are appropriate for its hyperthermophile host and demonstrate that the assembly pathway leads exclusively to the hexamer, which is likely to be a structural unit in vivo.

Structure of the molecular chaperone prefoldin: unique interaction of multiple coiled coil tentacles with unfolded proteins

Prefoldin (GimC) is a hexameric molecular chaperone complex built from two related classes of subunits and present in all eukaryotes and archaea. Prefoldin interacts with nascent polypeptide chains and, in vitro, can functionally substitute for the Hsp70 chaperone system in stabilizing non-native proteins for subsequent folding in the central cavity of a chaperonin. Here, we present the crystal structure and characterization of the prefoldin hexamer from the archaeum Methanobacterium thermoautotrophicum. Prefoldin has the appearance of a jellyfish: its body consists of a double beta barrel assembly with six long tentacle-like coiled coils protruding from it. The distal regions of the coiled coils expose hydrophobic patches and are required for multivalent binding of nonnative proteins.

Origin and evolution of eukaryotic chaperonins: phylogenetic evidence for ancient duplications in CCT genes

Chaperonins are oligomeric protein-folding complexes which are divided into two distantly related structural classes. Group I chaperonins (called GroEL/cpn60/hsp60) are found in bacteria and eukaryotic organelles, while group II chaperonins are present in archaea and the cytoplasm of eukaryotes (called CCT/TriC). While archaea possess one to three chaperonin subunit-encoding genes, eight distinct CCT gene families (paralogs) have been characterized in eukaryotes. We are interested in determining when during eukaryotic evolution the multiple gene duplications producing the CCT subunits occurred. We describe the sequence and phylogenetic analysis of five CCT genes from TRICHOMONAS: vaginalis and seven from GIARDIA: lamblia, representatives of amitochondriate protist lineages thought to have diverged early from other eukaryotes. Our data show that the gene duplications producing the eight CCT paralogs took place prior to the organismal divergence of TRICHOMONAS: and GIARDIA: from other eukaryotes. Thus, these divergent protists likely possess completely hetero-oligomeric CCT complexes like those in yeast and mammalian cells. No close phylogenetic relationship between the archaeal chaperonins and specific CCT subunits was observed, suggesting that none of the CCT gene duplications predate the divergence of archaea and eukaryotes. The duplications producing the CCTdelta and CCTepsilon subunits, as well as CCTalpha, CCTbeta, and CCTeta, are the most recent in the CCT gene family. Our analyses show significant differences in the rates of evolution of archaeal chaperonins compared with the eukaryotic CCTs, as well as among the different CCT subunits themselves. We discuss these results in light of current views on the origin, evolution, and function of CCT complexes.

The folding of nascent mitochondrial aspartate aminotransferase synthesized in a cell-free extract can be assisted by GroEL and GroES

At 30 degrees C, the precursor to mitochondrial aspartate aminotransferase (pmAspAT) cannot fold after synthesis in rabbit reticulocyte lysate (RRL), a model for studying intracellular protein folding. However, it folds rapidly once imported into mitochondria. Guanidinium chloride denatured pmAspAT likewise cannot refold at 30 degrees C in a defined in vitro system. However, it refolds rapidly and in good yield in the presence of the intramitochondrial chaperone homologues GroEL and GroES. In this report, we demonstrate that GroEL and GroES can also facilitate the folding of nascent pmAspAT in reticulocyte lysate under conditions where it otherwise would not. When added alone, GroEL arrests the slow folding of nascent pmAspAT and inhibits import into mitochondria. These effects are significantly reversed by adding GroES. These observations suggest that added GroEL participates in an equilibrium with endogenous chaperones in the cytosol which inhibit folding and promote import competence. Native gel electrophoresis suggests that nascent pmAspAT exists in RRL as a heterogeneous population of partially folded species, some of which bind to added GroEL more readily than others. The GroEL-trapped species appear to be among the productive pmAspAT folding intermediates formed in RRL or they at least appear to equilibrate with these intermediates, since they become import competent after GroES-stimulated release from GroEL.

Hsp70 and hsp40 chaperones can inhibit self-assembly of polyglutamine proteins into amyloid-like fibrils

The deposition of protein aggregates in neurons is a hallmark of neurodegenerative diseases caused by polyglutamine (polyQ) proteins. We analyzed the effects of the heat shock protein (Hsp) 70 chaperone system on the aggregation of fragments of huntingtin (htt) with expanded polyQ tracts. In vitro, Hsp70 and its cochaperone Hsp40 suppressed the assembly of htt into detergent-insoluble amyloid-like fibrils in an ATP-dependent manner and caused the formation of amorphous, detergent-soluble aggregates. The chaperones were most active in preventing fibrillization when added during the lag phase of the polymerization reaction. Similarly, coexpression of Hsp70 or Hsp40 with htt in yeast inhibited the formation of large, detergent-insoluble polyQ aggregates, resulting in the accumulation of detergent-soluble inclusions. Thus, the recently established potency of Hsp70 and Hsp40 to repress polyQ-induced neurodegeneration may be based on the ability of these chaperones to shield toxic forms of polyQ proteins and to direct them into nontoxic aggregates.

The interaction of the chaperonin tailless complex polypeptide 1 (TCP1) ring complex (TRiC) with ribosome-bound nascent chains examined using photo-cross-linking

The eukaryotic chaperonin tailless complex polypeptide 1 (TCP1) ring complex (TRiC) (also called chaperonin containing TCP1 [CCT]) is a hetero-oligomeric complex that facilitates the proper folding of many cellular proteins. To better understand the manner in which TRiC interacts with newly translated polypeptides, we examined its association with nascent chains using a photo-cross-linking approach. To this end, a series of ribosome-bound nascent chains of defined lengths was prepared using truncated mRNAs. Photoactivatable probes were incorporated into these (35)S- labeled nascent chains during translation. Upon photolysis, TRiC was cross-linked to ribosome-bound polypeptides exposing at least 50-90 amino acids outside the ribosomal exit channel, indicating that the chaperonin associates with much shorter nascent chains than indicated by previous studies. Cross-links were observed for nascent chains of the cytosolic proteins actin, luciferase, and enolase, but not to ribosome-bound preprolactin. The pattern of cross-links became more complex as the nascent chain increased in length. These results suggest a chain length-dependent increase in the number of TRiC subunits involved in the interaction that is consistent with the idea that the substrate participates in subunit-specific contacts with the chaperonin. Both ribosome isolation by centrifugation through sucrose cushions and immunoprecipitation with anti-puromycin antibodies demonstrated that the photoadducts form on ribosome-bound polypeptides. Our results indicate that TRiC/CCT associates with the translating polypeptide shortly after it emerges from the ribosome and suggest a close association between the chaperonin and the translational apparatus.

Evidence that beta-tubulin induces a conformation change in the cytosolic chaperonin which stabilizes binding: implications for the mechanism of action

The class II chaperonin CCT facilitates protein folding by a process that is not well-understood. One striking feature of this chaperonin is its apparent selectivity in vivo, folding only actin, tubulin, and several other proteins. In contrast, the class I chaperonin GroEL is thought to facilitate the folding of many proteins within Escherichia coli. It has been proposed that this apparent selectivity is associated with certain regions of a substrate protein's primary structure. Using limiting amounts of beta-tubulin, beta-tubulin mutants, and beta-tubulin/ftsZ chimeras, we assessed the contribution of select regions of beta-tubulin to CCT binding. In a complementary study, we investigated inter-ring communication in CCT where we exploited polypeptide binding sensitivity to nucleotide to quantitate nucleotide binding. beta-Tubulin bound with a high apparent affinity to CCT in the absence of nucleotide (apparent K(D) approximately 3 nM; its apparent binding free energy, DeltaG, ca. -11.8 kcal/mol). Despite this, the interactions appear to be weak and distributed throughout much of the sequence, although certain sites ("hot spots") may interact somewhat more strongly with CCT. Globally averaged over the beta-tubulin sequence, these interactions appear to contribute ca. -9 to -11 cal/mol per residue, and to account for no more than 50-60% of the total binding free energy. We propose that a conformation change or deformation induced in CCT by substrate binding provides the missing free energy which stabilizes the binary complex. We suggest that by coupling CCT deformation with polypeptide binding, CCT avoids the need for high "intrinsic" affinities for its substrates. This strategy allows for dynamic interactions between chaperonin and bound substrate, which may facilitate folding on the interior surface of CCT in the absence of nucleotide and/or productive release of bound polypeptide into the central cavity upon subsequent MgATP binding. CCT displayed negative inter-ring cooperativity like GroEL. When ring 1 of CCT bound MgATP or beta-tubulin, the affinity of ring 2 for polypeptide or nucleotide was apparently reduced approximately 100-fold.

Protein folding: versatility of the cytosolic chaperonin TRiC/CCT

Efficient de novo folding of actins and tubulins requires two molecular chaperones, the chaperonin TRiC (or CCT) and its novel cofactor GimC (or prefoldin). Recent studies indicate that TRiC is exquisitely adapted for this task, yet has the ability to interact with and assist the folding of numerous other cellular proteins.

Specific chaperone-like activity of inhibitor of caspase-activated DNase for caspase-activated DNase

Caspase-activated DNase (CAD) is the enzyme that causes DNA fragmentation during apoptosis. CAD forms aggregates when it is synthesized in the absence of an inhibitor of CAD (ICAD). Here, using renaturation systems of chemically denatured CAD, we report that ICAD-L, a long form of ICAD, has a chaperone-like activity specific for CAD. Murine CAD carries 14 cysteines, most of which were found to be in reduced form. Reducing agents enhanced the production of the functional CAD in an in vitro translation system. The denatured CAD could be efficiently renatured under highly reducing conditions only in the presence of ICAD-L. This process was ATP-independent. In contrast, reticulocyte lysates stimulated ICAD-L- and ATP-dependent renaturation of denatured CAD without requiring a high concentration of reducing agents. These results indicate that ICAD-L works not only as a specific inhibitor but also as a specific chaperone for CAD.

Roles of molecular chaperones in cytoplasmic protein folding

Newly synthesized polypeptide chains must fold and assemble into unique three-dimensional structures in order to become functionally active. In many cases productive folding depends on assistance from molecular chaperones, which act in preventing off-pathway reactions during folding that lead to aggregation. The inherent tendency of incompletely folded polypeptide chains to aggregate is thought to be strongly enhanced$L in vivo *I$Lby the high macromolecular concentration of the cellular solution, resulting in crowding effects, and by the close proximity of nascent polypeptide chains during synthesis on polyribosomes. The major classes of chaperones acting in cytoplasmic protein folding are the Hsp70s and the chaperonins. Hsp70 chaperones shield the hydrophobic regions of nascent and incompletely folded chains, whereas the chaperonins provide a sequestered environment in which folding can proceed unimpaired by intermolecular interactions between non-native polypeptides. These two principles of chaperone action can function in a coordinated manner to ensure the efficient folding of a subset of cytoplasmic proteins.