A cytoplasmic chaperonin that catalyzes beta-actin folding

TRiC-P5, a novel TCP1-related protein, is localized in the cytoplasm and in the nuclear matrix

We have recently reported the cloning of a novel protein, TRiC-P5, with significant homology with protein 1 of the t-complex (TCP1). In the present study, the cellular localization of TRiC-P5 in Raji cells has been determined using an antiserum raised against a 18.5 kDa fusion protein. Results from cell fractionation and immunoblot studies indicate that TRiC-P5 is mainly localized in the cytoplasm. In addition, a significant part of TRiC-P5 is also found in the nucleus where it is attached to the nuclear matrix, a complex filament network involved in essential cellular functions such as DNA replication, and RNA transcription and maturation. Immunofluorescence experiments using the anti-TRiC-P5 antibodies confirm these results. We also provide evidence that, in the cytoplasm, TRiC-P5 is part of a large protein complex, most probably the TCP1-ring complex (TRiC), a hetero-oligomeric ring complex that plays a role of molecular chaperone in the folding of actin and tubulin.

A yeast TCP-1-like protein is required for actin function in vivo

We previously identified the ANC2 gene in a screen for mutations that enhance the defects caused by yeast actin mutations. Here we report that ANC2 is an essential gene that encodes a member of the TCP-1 family. TCP-1-related proteins are subunits of cytosolic heteromeric protein complexes referred to as chaperonins. These complexes can bind to newly synthesized actin and tubulin in vitro and can convert these proteins into an assembly-competent state. We show that anc2-1 mutants contain abnormal and disorganized actin structures, are defective in cellular morphogenesis, and are hypersensitive to the microtubule inhibitor benomyl. Furthermore, overexpression of wild-type Anc2p ameliorates defects in actin organization and cell growth caused by actin overproduction. Mutations in BIN2 and BIN3, two other genes that encode TCP-1-like proteins, also enhance the phenotypes of actin mutants. Taken together, these findings demonstrate that TCP-1-like proteins are required for actin and tubulin function in vivo.

Two yeast genes with similarity to TCP-1 are required for microtubule and actin function in vivo

We have isolated cold-sensitive mutations in two genes of the yeast Saccharomyces cerevisiae, BIN2 and BIN3, that cause aberrant chromosome segregation in vivo. BIN2 and BIN3 encode essential proteins that are similar to each other and to TCP-1. TCP-1 and TCP-1-like proteins are components of the eukaryotic cytoplasmic chaperonin that facilitates folding of tubulins and actin in vitro. Mutations in BIN2 and BIN3 cause defects in microtubule and actin assembly in vivo and confer supersensitivity to the microtubule-destabilizing drug benomyl. Overexpression of TCP1, BIN2, BIN3, or ANC2, a fourth member of the TCP-1 family in yeast, does not complement mutations in the other genes, indicating that the proteins have distinct functions. However, all double-mutant combinations are inviable; this synthetic lethality suggests that the proteins act in a common process. These results indicate that Bin2p and Bin3p are components of a yeast cytoplasmic chaperonin complex that is required for assembly of microtubules and actin in vivo.

Molecular chaperones in cellular protein folding

The discovery of 'molecular chaperones' has dramatically changed our concept of cellular protein folding. Rather than folding spontaneously, most newly synthesized polypeptide chains seem to acquire their native conformation in a reaction mediated by these versatile helper proteins. Understanding the structure and function of molecular chaperones is likely to yield useful applications for medicine and biotechnology in the future.

GroEL-mediated protein folding proceeds by multiple rounds of binding and release of nonnative forms

The chaperonin GroEL is a ribosome-sized double-ring structure that assists in folding a diverse set of polypeptides. We have examined the fate of a polypeptide during a chaperonin-mediated folding reaction. Strikingly, we find that, upon addition of ATP and the cochaperonin GroES, polypeptide is released rapidly from GroEL in a predominantly nonnative conformation that can be trapped by mutant forms of GroEL that are capable of binding but not releasing substrate. Released polypeptide undergoes kinetic partitioning: a fraction completes folding while the remainder is rebound rapidly by other GroEL molecules. Folding appears to occur in an all-or-none manner, as proteolysis and tryptophan fluorescence indicate that after rebinding, polypeptide has the same structure as in the original complex. These observations suggest that GroEL functions by carrying out multiple rounds of binding aggregation-prone or kinetically trapped intermediates, maintaining them in an unfolded state, and releasing them to attempt to fold in solution.

Folding of nascent polypeptide chains in a high molecular mass assembly with molecular chaperones

The folding of polypeptides emerging from ribosomes was analysed in a mammalian translation system using firefly luciferase as a model protein. The growing polypeptide interacts with a specific set of molecular chaperones, including Hsp70, the DnaJ homologue Hsp40 and the chaperonin TRiC. The ordered assembly of these components on the nascent chain forms a high molecular mass complex that allows the cotranslational formation of protein domains and the completion of folding once the chain is released from the ribosome.

Reversible interaction of beta-actin along the channel of the TCP-1 cytoplasmic chaperonin

The cytoplasm of eukaryotes contains a heteromeric toroidal chaperonin assembled from the t-complex TCP-1 and several other related polypeptides. The structure of the TCP-1 cytoplasmic chaperonin and that of the binary complex formed between this chaperonin and unfolded beta-actin have been studied using electron microscopy and image processing techniques. Two-dimensional averaging of front views reveals a circular stain-excluding mass surrounding a central stain-penetrating region in which the stain is excluded upon actin binding. Sections of a three-dimensional reconstruction of the chaperonin show that the inner core is an empty channel that becomes filled upon binary complex formation with unfolded beta-actin. Upon incubation with Mg-ATP, the beta-actin:chaperonin complex discharges the actin such that the chaperonin central cavity reappears. Side views from different forms of TCP-1 reveals that upon Mg-ATP binding, the cytoplasmic chaperonin undergoes a structural rearrangement that is confirmed using a new classification method.

Role of 300 kDa complexes as intermediates in tubulin folding and dimerization: characterization of a 25 kDa cytosolic protein involved in the GTP-dependent release of monomeric tubulin

beta-Tubulin synthesized in vitro in rabbit reticulocyte lysate is found associated with 900 kDa complexes (C900) containing T Complex Polypeptide 1 (TCP1), heat-shock protein (hsp) 70 and other unidentified proteins, with smaller 300 kDa complexes (C300) of unknown nature, in dimeric association with reticulocyte alpha-tubulin and in monomeric forms. Pulse-chase experiments indicated that production of fully functional beta-tubulin was preceded by its association with C900 and C300 multimolecular complexes and by the appearance of beta-monomers. The high-molecular-mass forms appeared as intermediate products in the process leading to fully functional dimerizable beta-tubulin. C300-associated tubulin can be released as beta-monomer by addition of a cofactor present in reticulocyte lysate. Here a 25 kDa protein which releases tubulin monomers from C300 has been identified and characterized. The protein specifically released monomers from C300, but not from C900, in a process favoured by GTP.

Functional role of a consensus peptide which is common to alpha-, beta-, and gamma-tubulin, to actin and centractin, to phytochrome A, and to the TCP1 alpha chaperonin protein

The TRiC (TCP1 Ring Complex) chaperonin complex participates in the functional folding of actin, centractin, alpha-, beta-, gamma-tubulin, and phytochrome. Each of the cytoskeletal proteins contain a peptide, RK(A,C,T)F/KRAF, located towards the C-terminus, which is homologous to a TCP1 alpha peptide, while the equivalent phytochrome peptide (RLKAF in certain isoforms) is very similar to the KLRAF peptide of TCP1 alpha. We propose that this TCP1 alpha peptide binds to the nascent polypeptides as they emerge from the ribosome, that this binding restricts the folding pathway, and that the TCP1 alpha peptide is subsequently displaced by the synthesis of the consensus peptide. This hypothesis is strongly supported by the crystallographic structure of actin.

A novel cochaperonin that modulates the ATPase activity of cytoplasmic chaperonin

The folding of alpha- and beta-tubulin requires three proteins: the heteromeric TCP-1-containing cytoplasmic chaperonin and two additional protein cofactors (A and B). We show that these cofactors participate in the folding process and do not merely trigger release, since in the presence of Mg-ATP alone, alpha- and beta-tubulin target proteins are discharged from cytoplasmic chaperonin in a nonnative form. Like the prokaryotic cochaperonin GroES, which interacts with the prototypical Escherichia coli chaperonin GroEL and regulates its ATPase activity, cofactor A modulates the ATPase activity of its cognate chaperonin. However, the sequence of cofactor A derived from a cloned cDNA defines a 13-kD polypeptide with no significant homology to other known proteins. Moreover, while GroES functions as a heptameric ring, cofactor A behaves as a dimer. Thus, cofactor A is a novel cochaperonin that is structurally unrelated to GroES.

The formation of symmetrical GroEL-GroES complexes in the presence of ATP

The incubation of chaperonins cpn60 (GroEL) and cpn10 (GroES) from E. coli in the presence of Mg-ATP and KCl generates the formation, as revealed by electron microscopy, of GroEL-GroES complexes with a symmetrical shape in which one toroidal GroES oligomer is bound to each end of the tetradecameric GroEL aggregate (1:2 GroEL:GroES oligomer molar ratio). The symmetrical complexes are not observed in the presence of ADP or the non-hydrolyzable ATP analog, ATP gamma S, where only asymmetrical complexes (1:1 GroEL:GroES oligomer molar ratio) are formed. These results suggest that ATP hydrolysis is required for the formation of symmetrical complexes.

Facilitated folding of actins and tubulins occurs via a nucleotide-dependent interaction between cytoplasmic chaperonin and distinctive folding intermediates

In the cytoplasm of eukaryotes, the folding of actins and tubulins is facilitated via interaction with a heteromeric toroidal complex (cytoplasmic chaperonin). The folding reaction consists of the formation of a binary complex between the unfolded target protein and the chaperonin, followed by the ultimate release of the native polypeptide in an ATP-dependent reaction. Here we show that the mitochondrial chaperonin (cpn60) and the cytoplasmic chaperonin both recognize a range of target proteins with different relative affinities; however, the cytoplasmic chaperonin shows the highest affinity for intermediates derived from unfolded tubulins and actins. These high-affinity actin and tubulin folding intermediates are distinct from the "molten globule" intermediates formed by noncytoskeletal target proteins in that they form relatively slowly. We show that the interaction between cytoplasmic chaperonin and unfolded target proteins depends on the chaperonin being in its ADP-bound state and that the release of the target protein occurs after a transition of the chaperonin to the ATP-bound state. Our data suggest a model in which ATP hydrolysis acts as a switch between conformational forms of the cytoplasmic chaperonin that interact either strongly or weakly with unfolded substrates.

Facilitated folding of actins and tubulins occurs via a nucleotide-dependent interaction between cytoplasmic chaperonin and distinctive folding intermediates

In the cytoplasm of eukaryotes, the folding of actins and tubulins is facilitated via interaction with a heteromeric toroidal complex (cytoplasmic chaperonin). The folding reaction consists of the formation of a binary complex between the unfolded target protein and the chaperonin, followed by the ultimate release of the native polypeptide in an ATP-dependent reaction. Here we show that the mitochondrial chaperonin (cpn60) and the cytoplasmic chaperonin both recognize a range of target proteins with different relative affinities; however, the cytoplasmic chaperonin shows the highest affinity for intermediates derived from unfolded tubulins and actins. These high-affinity actin and tubulin folding intermediates are distinct from the "molten globule" intermediates formed by noncytoskeletal target proteins in that they form relatively slowly. We show that the interaction between cytoplasmic chaperonin and unfolded target proteins depends on the chaperonin being in its ADP-bound state and that the release of the target protein occurs after a transition of the chaperonin to the ATP-bound state. Our data suggest a model in which ATP hydrolysis acts as a switch between conformational forms of the cytoplasmic chaperonin that interact either strongly or weakly with unfolded substrates.

A eukaryotic cytosolic chaperonin is associated with a high molecular weight intermediate in the assembly of hepatitis B virus capsid, a multimeric particle

We have established a system for assembly of hepatitis B virus capsid, a homomultimer of the viral core polypeptide, using cell-free transcription-linked translation. The mature particles that are produced are indistinguishable from authentic viral capsids by four criteria: velocity sedimentation, buoyant density, protease resistance, and electron microscopic appearance. Production of unassembled core polypeptides can be uncoupled from production of capsid particles by decreasing core mRNA concentration. Addition of excess unlabeled core polypeptides allows the chase of the unassembled polypeptides into mature capsids. Using this cell-free system, we demonstrate that assembly of capsids proceeds by way of a novel high molecular weight intermediate. Upon isolation, the high molecular weight intermediate is productive of mature capsids when energy substrates are manipulated. A 60-kD protein related to the chaperonin t-complex polypeptide 1 (TCP-1) is found in association with core polypeptides in two different assembly intermediates, but is not associated with either the initial unassembled polypeptides or with the final mature capsid product. These findings implicate TCP-1 or a related chaperonin in viral assembly and raise the possibility that eukaryotic cytosolic chaperonins may play a distinctive role in multimer assembly apart from their involvement in assisting monomer folding.

Primary structure and function of a second essential member of the heterooligomeric TCP1 chaperonin complex of yeast, TCP1 beta

A role for heterooligomeric TCP1 complex as a chaperonin in the eukaryotic cytosol has recently been suggested both by structural similarities with other chaperonins and by in vitro experiments showing it to mediate ATP-dependent folding of actin, tubulin, and luciferase. Here we present the primary structure of a second subunit of the complex and present genetic and functional analyses. The TCP1 beta amino acid sequence, predicted from the cloned gene, bears 35% identity to TCP1, termed here TCP1 alpha, containing the same highly conserved residues found in the collective sequence of chaperonins. The predicted product was identified as the fastest-migrating species of the TCP1 complex purified from soluble extracts of yeast. The TCP1 beta gene, like TCP1 alpha, is essential. Strains containing lethal disruptions of either gene could not be rescued by additional copies of the other. Spores bearing disruption of either gene germinated as single, large-budded cells. Similarly, large-budded cells were observed following shift to 37 degrees C of strains carrying temperature-sensitive mutations in either TCP1 alpha or TCP1 beta. The arrested cells contained replicated DNA present in single nuclear masses, associated with abnormal tubulin staining patterns, supporting the assertion that mitotic spindle formation and function are impaired. We conclude that TCP1 beta supplies an essential function that partially overlaps with that of TCP1 alpha in acting as a molecular chaperone in tubulin and spindle biogenesis.

cDNA encoding a novel TCP1-related protein

We report the cloning of a mouse cDNA encoding a novel protein which has significant homology with the t-complex protein 1b (TCP1b). In addition, this protein has high sequence identity with tryptic peptides from the bovine P5 subunit of the TCP1-ring complex. We named this novel protein mTRiC-P5 for mouse TCP1-Ring Complex Protein #5. Results indicate that mTRiC-P5 is a new member of the TCP1-TF55 family and is part of the TCP1-ring complex.

Identification of six Tcp-1-related genes encoding divergent subunits of the TCP-1-containing chaperonin

BACKGROUND

TCP-1 is a 60 kD subunit of a cytosolic hetero-oligomeric chaperone that is known to be involved in the folding of actin and tubulin. This protein is a member of the chaperonin family, which includes Escherichia coli GroEL, the mitochondrial heat-shock protein Hsp60, the plastid Rubisco-subunit-binding protein and the archaebacterial protein TF55. These chaperonins assist the folding of proteins upon ATP hydrolysis.

RESULTS

Using two-dimensional gel analysis, we have identified nine different subunits of TCP-1-containing chaperonin complexes from mammalian testis and seven different subunits of such complexes from mouse F9 cells. We have isolated full-length mouse cDNAs encoding six novel TCP-1-related polypeptides and show that these cDNAs encode subunits of the TCP-1-containing cytosolic chaperonin. These subunits are between 531 and 545 residues in length. Their sequences are 25-36% identical to one another, 27-35% identical to that of TCP-1 and 32-39% identical to that of the archaebacterial chaperonin, TF55. We have named these genes, Cctb, Cctg, Cctd, Ccte, Cctz and Ccth, which encode the CCT beta, CCT gamma, CCT delta, CCT epsilon, CCT zeta and CCT eta subunits, respectively, of the 'Chaperonin Containing TCP-1' (CCT). All the CCT subunits contain motifs that are also shared by all other known chaperonins of prokaryotes and eukaryotic organelles, and that probably relate to their common ATPase function.

CONCLUSION

It is likely that each CCT subunit has a specific, independent function, as they are highly diverged from each other but conserved from mammals to yeast. We suggest that the expansion in the number of types of CCT subunit, compared with other chaperonins, has allowed CCT to carry out the more complex functions that are required for the folding and assembly of highly evolved eukaryotic proteins.

Heat-shock proteins as molecular chaperones

Functional proteins within cells are normally present in their native, completely folded form. However, vital processes of protein biogenesis such as protein synthesis and translocation of proteins into intracellular compartments require the protein to exist temporarily in an unfolded or partially folded conformation. As a consequence, regions buried when a polypeptide is in its native conformation become exposed and interact with other proteins causing protein aggregation which is deleterious to the cell. To prevent aggregation as proteins become unfolded, heat-shock proteins protect these interactive surfaces by binding to them and facilitating the folding of unfolded or nascent polypeptides. In other instances the binding of heat-shock proteins to interactive surfaces of completely folded proteins is a crucial part of their regulation. As heat shock and other stress conditions cause cellular proteins to become partially unfolded, the ability of heat-shock proteins to protect cells against the adverse effects of stress becomes a logical extension of their normal function as molecular chaperones.

Molecular chaperones in protein folding: the art of avoiding sticky situations

Molecular chaperones are a class of proteins that interact with the non-native conformations of other proteins. The major role of chaperones of the Hsp70 and Hsp60 families is to prevent aggregation of newly synthesized polypeptides and then to mediate their folding to the native state. As a result of functional studies of these proteins, there has been a revision of the long-held view that protein folding in the cell is a spontaneous process.

Genetically engineering mammalian cell lines for increased viability and productivity

The generation of new host cell lines for the production of foreign proteins can be achieved by cell engineering. This approach can be used to enhance the cell's ability to produce proteins that are properly processed and secreted at elevated levels and consequently can increase the overall productivity of an expression system. One potential target for cell engineering is the modification of the cell's protein folding capacity. The appropriate folding, assembly, localization and secretion of newly synthesized proteins is dependent upon the action of a group of proteins known as molecular chaperones. Improving the host cell's chaperoning capacity might increase the yield of properly folded recombinant proteins by preventing the formation of insoluble aggregates. Another potentially beneficial cell engineering goal is the inhibition of physiological cell death. The productivity of genetically engineered cells is dependent upon the maintenance of high levels of cell viability throughout the bioprocess period. Fluctuations in a cell's environment can trigger a deliberate form of cell death known as apoptosis. The proteins that mediate this self-destruction are currently being characterized. Regulating the expression of these death genes by cellular engineering could limit the loss of productivity that results from the physiological death of the recombinant cell line.

Heat-inducible proteins that react with antibodies to chaperonin60 are localized in the nucleus of a fish cell line

We report in the present paper that proteins which react with a polyclonal antibody (pAb) raised against the heat-shock protein chaperonin60 (cpn60) were revealed by indirect immunofluorescence in the nucleus of a fish (fathead minnow, Pimephales promelas) cell line after heat-shock. This immunoreactive cpn60 associated with the nucleolus and with discrete foci. An increased abundance of two nuclear proteins of approx. 57 and 42 kDa, present in approximately equal amounts, was detected by Western blotting using an anti-cpn60 pAb as a probe during the same time period that cpn60 was revealed in the nucleus. These proteins also reacted with a monoclonal antibody (mAb) against human cpn60 but did not react with an mAb against the cytoplasmic chaperonin, TCP1. The kinetics of translocation and pattern of nuclear localization of this immunoreactive cpn60 differed from that of stress70, another major family of heatshock proteins. We suggest that these nuclear immunoreactive cpn60 proteins are members of the cpn60 family and that they play a chaperone role in folding and assembly of proteins in the nucleus which is distinct from that of stress70.

Eukaryotic cytosolic chaperonin contains t-complex polypeptide 1 and seven related subunits

We have characterized the cytosolic chaperonin from both rabbit reticulocyte lysate and bovine testis. The heteromeric complex contains eight subunits. Partial amino acid sequence data reveal that one of these is t-complex polypeptide 1 (TCP-1), while the other seven are TCP-1-related polypeptides, implicating the existence of a multigene family of TCP-1 homologues. We provide evidence that TCP-1 ring complex from bovine testis can facilitate the folding of both actin and tubulin, although, as in the case of chaperonin from reticulocyte lysate, two cofactors are required for the generation of properly folded tubulin. An additional molecule of TCP-1 may associate with the chaperonin depending on the purification procedure used. We propose that a highly conserved region in these polypeptides and in other chaperonins of the cpn60 chaperone family participates in ATP binding.

The t-complex polypeptide 1 complex is a chaperonin for tubulin and actin in vivo

A role in folding newly translated cytoskeletal proteins in the cytosol of eukaryotes has been proposed for t-complex polypeptide 1 (TCP1). In this study, we investigated tubulin and actin biogenesis in Chinese hamster ovary (CHO) cells. When extracts of pulse-labeled cells were analyzed by anion-exchange and size-exclusion chromatography, newly synthesized alpha-tubulin, beta-tubulin, and actin were observed to enter a large molecular mass complex (approximately 900 kDa). These proteins were released from this complex capable, in the case of tubulin, of forming heterodimers. The large molecular mass complexes coeluted with TCP1 and could be immunoprecipitated by using an anti-TCP1 antibody. These findings demonstrate that there is a cytosolic pathway for folding tubulin and actin in vivo that involves the TCP1 complex.

Incorporation of tubulin subunits into dimers requires GTP hydrolysis

A toroid multisubunit complex of 800–900 kDa has been implicated in assisting protein folding of at least two cytoplasmic proteins, actin and tubulin. This process is dependent on the presence of magnesium ions and ATP hydrolysis. In vitro translation of cDNAs encoding different alpha- and beta-tubulin isotypes also gives rise to the formation of complexes of about 300 kDa. These complexes have been functionally implicated in the incorporation of tubulin monomers within the tubulin heterodimer. This work shows that, in addition to ATP hydrolysis, the incorporation of newly synthesized tubulin subunits into functional heterodimers requires GTP hydrolysis in the presence of magnesium ions. A two-step process is suggested, a first ATP-dependent step in which the 900 kDa complexes are implicated in a similar way to the step taking place in actin folding, and a second GTP-dependent step in which the 300 kDa complexes are involved in the assembly of the heterodimer.

ATP-dependent protein refolding activity in reticulocyte lysate. Evidence for the participation of different chaperone components

The protein folding capacity of rabbit reticulocyte cytosol was analyzed using the renaturation of firefly luciferase as a sensitive assay. In the absence of ATP, the aggregation of denatured luciferase diluted into reticulocyte lysate was prevented. Chaperone-stabilized luciferase was detected in high molecular weight complexes overlapping the distributions of Hsc70, Hsp90 and the chaperonin TRiC on gel filtration columns. The readdition of unfractionated cytosol and Mg-ATP was required for the efficient folding of these forms of luciferase to the active enzyme. We conclude that protein folding in the eukaryotic cytosol depends on the functional cooperation of different chaperone activities and cofactors in a complex, ATP-dependent process.

Folding in vivo of bacterial cytoplasmic proteins: role of GroEL

A general role for chaperonin ring structures in mediating folding of newly translated proteins has been suggested. Here we have directly examined the role of the E. coli chaperonin GroEL in the bacterial cytoplasm by production of temperature-sensitive lethal mutations in this essential gene. After shift to nonpermissive temperature, the rate of general translation in the mutant cells was reduced, but, more specifically, a defined group of cytoplasmic proteins--including citrate synthase, ketoglutarate dehydrogenase, and polynucleotide phosphorylase--were translated but failed to reach native form. Similarly, a monomeric test protein, maltose-binding protein, devoid of its signal domain, was translated but failed to fold to its native conformation. We conclude that GroEL indeed is a machine at the distal end of the pathway of transfer of genetic information, assisting a large and specific set of newly translated cytoplasmic proteins to reach their native tertiary structures.

Chaperonin-mediated folding of vertebrate actin-related protein and gamma-tubulin

The folding of actin and tubulin is mediated via interaction with a heteromeric toroidal complex (cytoplasmic chaperonin) that hydrolyzes ATP as part of the reaction whereby native proteins are ultimately released. Vertebrate actin-related protein (actin-RPV) (also termed centractin) and gamma-tubulin are two proteins that are distantly related to actin and tubulin, respectively: gamma-tubulin is exclusively located at the centrosome, while actin-RPV is conspicuously abundant at the same site. Here we show that actin-RPV and gamma-tubulin are both folded via interaction with the same chaperonin that mediates the folding of beta-actin and alpha- and beta-tubulin. In each case, the unfolded polypeptide forms a binary complex with cytoplasmic chaperonin and is released as a soluble, monomeric protein in the presence of Mg-ATP and the presence or absence of Mg-GTP. In contrast to alpha- and beta-tubulin, the folding of gamma-tubulin does not require the presence of cofactors in addition to chaperonin itself. Monomeric actin-RPV produced in in vitro folding reactions cocycles efficiently with native brain actin, while in vitro folded gamma-tubulin binds to polymerized microtubules in a manner consistent with interaction with microtubule ends. Both monomeric actin-RPV and gamma-tubulin bind to columns of immobilized nucleotide: monomeric actin-RPV has no marked preference for ATP or GTP, while gamma-tubulin shows some preference for GTP binding. We show that actin-RPV and gamma-tubulin compete with one another, and with beta-actin or alpha-tubulin, for binary complex formation with cytoplasmic chaperonin.

Expression of chick and yeast beta-tubulin-encoding genes in insect cells

A chick cDNA encoding the beta 2 isotype of tubulin (beta 2Tub) was cloned into a baculovirus expression vector designed to produce unfused proteins, and several recombinant viruses (re-viruses) were isolated. Immunoblotting studies of homogenates of insect cells infected with re-virus showed a 50-kDa protein that reacted with antibodies specific for beta Tub. Cells infected with the re-virus appeared to contain much higher levels of beta Tub than uninfected control cells, perhaps as much as five- to tenfold higher. Isotype-specific antibody for beta 2Tub showed little reaction in uninfected cells or cells infected with wild-type virus; strong reaction was found with cells infected with re-virus. Analysis by gel filtration of extracts of cells infected with re-virus showed that almost all beta Tub eluted in the column void volume, suggesting that it was aggregated or associated with other cell proteins. Recombinant baculoviruses producing Saccharomyces cerevisiae beta Tub were also isolated. Immunoblotting studies using antibodies specific for yeast beta Tub showed a 50-kDa protein which was absent in uninfected cells or cells infected with wt virus. Immunofluorescence studies suggest that yeast beta Tub is incorporated poorly, if at all, into the insect cell cytoskeleton.

Identification of chaperonin particles in mammalian brain cytosol and of T-complex polypeptide 1 as one of their components

An approximately 950-kDa heteromeric particle was purified from guinea-pig and rat brain by sucrose gradient fractionation of post-mitochondrial supernatants. Further purification, by affinity chromatography on ATP-Sepharose and anion exchange FPLC on MonoQ, yielded a particle with typical chaperonin ultrastructure. One of the component polypeptides was recognized by a monoclonal antibody to murine T-complex polypeptide 1. Brain cytosolic chaperonin particles formed a binary complex with unfolded tubulin subunits. The polypeptide compositions of the cytosolic chaperonin particles appeared very similar between brain and testicular tissues of the same animal, but differed subtly between the guinea-pig and rat.

Heat shock proteins: molecular chaperones of protein biogenesis

Heat shock proteins (Hsps) were first identified as proteins whose synthesis was enhanced by stresses such as an increase in temperature. Recently, several of the major Hsps have been shown to be intimately involved in protein biogenesis through a direct interaction with a wide variety of proteins. As a reflection of this role, these Hsps have been referred to as molecular chaperones. Hsp70s interact with incompletely folded proteins, such as nascent chains on ribosomes and proteins in the process of translocation from the cytosol into mitochondria and the endoplasmic reticulum. Hsp60 also binds to unfolded proteins, preventing aggregation and facilitating protein folding. Although less well defined, other Hsps such as Hsp90 also play important roles in modulating the activity of a number of proteins. The function of the proteolytic system is intertwined with that of molecular chaperones. Several components of this system, encoded by heat-inducible genes, are responsible for the degradation of abnormal or misfolded proteins. The budding yeast Saccharomyces cerevisiae has proven very useful in the analysis of the role of molecular chaperones in protein maturation, translocation, and degradation. In this review, results of experiments are discussed within the context of experiments with other organisms in an attempt to describe the current state of understanding of these ubiquitous and important proteins.

ATP induces large quaternary rearrangements in a cage-like chaperonin structure

BACKGROUND

The chaperonins, a family of molecular chaperones, are large oligomeric proteins that bind nonnative intermediates of protein folding. They couple the release and correct folding of their ligands to the binding and hydrolysis of ATP. Chaperonin 60 (cpn60) is a decatetramer (14-mer) of 60 kD subunits. Folding of some ligands also requires the cooperation of cpn10, a heptamer of 10 kD subunits.

RESULTS

We have determined the three-dimensional arrangements of subunits in Rhodobacter sphaeroides cpn60 in the nucleotide-free and ATP-bound forms. Negative stain electron microscopy and tilt reconstruction show the cylindrical structure of the decatetramer comprising two rings of seven subunits. The decatetramer consists of two cages joined base-to-base without a continuous central channel. These cages appear to contain bound polypeptide with an asymmetric distribution between the two rings. The two major domains of each subunit are connected on the exterior of the cylinder by a narrower bridge of density that could be a hinge region. Binding of ATP to cpn60 causes a major rearrangement of the protein density, which is reversed upon the hydrolysis of the ATP. Cpn10 binds to only one end of the cpn60 structure and is visible as an additional layer of density forming a cap on one end of the cpn60 cylinder.

CONCLUSIONS

The observed rearrangement is consistent with an inward 5-10 degrees rotation of subunits, pivoting about the subunit contacts between the two heptamers, and thus bringing cpn60 domains towards the position occupied by the bound polypeptide. This change could explain the stimulation of ATPase activity by ligands, and the effects of ATP on lowering the affinity of cpn60 for ligands and on triggering the release of folding polypeptides.

Two cofactors and cytoplasmic chaperonin are required for the folding of alpha- and beta-tubulin

Though the chaperonins that mediate folding in prokaryotes, mitochondria, and chloroplasts have been relatively well characterized, the folding of proteins in the eukaryotic cytosol is much less well understood. We recently identified a cytoplasmic chaperonin as an 800-kDa multisubunit toroid which forms a binary complex with unfolded actin; the correctly folded polypeptide is released upon incubation with Mg-ATP (Y. Gao, J. O. Thomas, R. L. Chow, G.-H. Lee, and N. J. Cowan, Cell 69:1043-1050, 1992). Here we show that the same chaperonin also forms a binary complex with unfolded alpha- or beta-tubulin; however, there is no detectable release of the correctly folded product, irrespective of the concentration of added Mg-ATP and Mg-GTP or the presence of added carrier tubulin heterodimers with which newly folded alpha- or beta-tubulin polypeptides might exchange. Rather, two additional protein cofactors are required for the generation of properly folded alpha- or beta-tubulin, which is then competent for exchange into preexisting alpha/beta-tubulin heterodimers. We show that actin and tubulins compete efficiently with one another for association with cytoplasmic chaperonin complexes. These data imply that actin and alpha- and beta-tubulin interact with the same site(s) on chaperonin complexes.

Two cofactors and cytoplasmic chaperonin are required for the folding of alpha- and beta-tubulin

Though the chaperonins that mediate folding in prokaryotes, mitochondria, and chloroplasts have been relatively well characterized, the folding of proteins in the eukaryotic cytosol is much less well understood. We recently identified a cytoplasmic chaperonin as an 800-kDa multisubunit toroid which forms a binary complex with unfolded actin; the correctly folded polypeptide is released upon incubation with Mg-ATP (Y. Gao, J. O. Thomas, R. L. Chow, G.-H. Lee, and N. J. Cowan, Cell 69:1043-1050, 1992). Here we show that the same chaperonin also forms a binary complex with unfolded alpha- or beta-tubulin; however, there is no detectable release of the correctly folded product, irrespective of the concentration of added Mg-ATP and Mg-GTP or the presence of added carrier tubulin heterodimers with which newly folded alpha- or beta-tubulin polypeptides might exchange. Rather, two additional protein cofactors are required for the generation of properly folded alpha- or beta-tubulin, which is then competent for exchange into preexisting alpha/beta-tubulin heterodimers. We show that actin and tubulins compete efficiently with one another for association with cytoplasmic chaperonin complexes. These data imply that actin and alpha- and beta-tubulin interact with the same site(s) on chaperonin complexes.

Protein folding in the cell: functions of two families of molecular chaperone, hsp 60 and TF55-TCP1

Two families of molecular chaperone, the hsp 60-GroEL family and the TF55-TCP1 family, have been discovered in evolutionarily related cellular compartments. A member of one of these families, hsp 60, has been shown to play a global role in polypeptide chain folding in mitochondria. We review here studies of both hsp 60 and other family members, discussing their essential physiological roles and mechanism of action.

Heat shock proteins functioning as molecular chaperones: their roles in normal and stressed cells

In response to either elevated temperatures or several other metabolic insults, cells from all organisms respond by increasing the expression of so-called heat shock proteins (hsp or stress proteins). In general, the stress response appears to represent a universal cellular defence mechanism. The increased expression and accumulation of the stress proteins provides the cell with an added degree of protection. Studies over the past few years have revealed a role for some of the stress proteins as being intimately involved in protein maturation. Members of the hsp 70 family, distributed throughout various intracellular compartments, interact transiently with other proteins undergoing synthesis, translocation, or higher ordered assembly. Although not yet proven, it has been suggested that members of the hsp 70 family function to slow down or retard the premature folding of proteins in the course of synthesis and translocation. Yet another family of stress proteins, the hsp 60 or GroEL proteins (chaperonins), appear to function as catalysts of protein folding. Here I discuss the role of those stress proteins functioning as molecular chaperones, both within the normal cell and in the cell subjected to metabolic stress.

A structural model for the GroEL chaperonin

Individual particle analysis of end views from negatively stained specimens of purified GroEL from Escherichia coli showed the presence of two different particle populations, those with a six-fold symmetry and those with a seven-fold symmetry, when studied at pH 7.7 and 5.0. Image processing of particles from frozen-hydrated specimens revealed at both pH values a homogeneous population of particles with a strong seven-fold symmetry component and an average image with seven asymmetric units. Biochemical analysis of purified GroEL showed unequivocally the presence of a single polypeptide with the N-terminal sequence identical to that of GroEL. These results are compatible with a structural model of GroEL as an asymmetric aggregate built up by two rings of seven-fold and six-fold symmetries, respectively.