The molecular chaperone Ssb from Saccharomyces cerevisiae is a component of the ribosome-nascent chain complex

Chaperone-Assisted Folding of Newly Synthesized Proteins in the Cytosol

The way in which a newly synthesized polypeptide chain folds into its unique three-dimensional structure remains one of the fundamental questions in molecular biology. Protein folding in the cell is a problematic process and, in many cases, requires the assistance of a network of molecular chaperones to support productive protein foldingin vivo. During protein biosynthesis, ribosome-associated chaperones guide the folding of the nascent polypeptide emerging from the ribosomal tunnel. In this review we summarize the basic principles of the protein-folding process and the involved chaperones, and focus on the role of ribosome-associated chaperones. Our discussion emphasizes the bacterial Trigger Factor, which is the best studied chaperone of this type. Recent advances have determined the atomic structure of the Trigger Factor, providing new, exciting insights into the role of ribosome-associated chaperones in co-translational protein folding.

The ribosome-bound Hsp70 homolog Ssb of Saccharomyces cerevisiae

The Hsp70 homolog Ssb directly binds to the ribosome and contacts a variety of newly synthesized polypeptide chains as soon as they emerge from the ribosomal exit tunnel. For this reason a general role of Ssb in the de novo folding of newly synthesized proteins is highly suggestive. However, for more than a decade client proteins which require Ssb for proper folding have remained elusive. It was therefore speculated that Ssb, despite its ability to interact with a large variety of nascent polypeptides, may assist the folding of only a small and specific subset. Alternatively, it has been suggested that Ssb's function may be limited to the protection of nascent polypeptides from aggregation until downstream chaperones take over and actively fold their substrates. There is also evidence that Ssb, in parallel to a classical chaperone function, is involved in the regulation of cellular signaling processes. Here we aim to summarize what is currently known about Ssb's multiple functions and what remains to be ascertained by future research.

The chaperone-like protein HYPK acts together with NatA in cotranslational N-terminal acetylation and prevention of Huntingtin aggregation

The human NatA protein N(alpha)-terminal-acetyltransferase complex is responsible for cotranslational N-terminal acetylation of proteins with Ser, Ala, Thr, Gly, and Val N termini. The NatA complex is composed of the catalytic subunit hNaa10p (hArd1) and the auxiliary subunit hNaa15p (hNat1/NATH). Using immunoprecipitation coupled with mass spectrometry, we identified endogenous HYPK, a Huntingtin (Htt)-interacting protein, as a novel stable interactor of NatA. HYPK has chaperone-like properties preventing Htt aggregation. HYPK, hNaa10p, and hNaa15p were associated with polysome fractions, indicating a function of HYPK associated with the NatA complex during protein translation. Knockdown of both hNAA10 and hNAA15 decreased HYPK protein levels, possibly indicating that NatA is required for the stability of HYPK. The biological importance of HYPK was evident from HYPK-knockdown HeLa cells displaying apoptosis and cell cycle arrest in the G(0)/G(1) phase. Knockdown of HYPK or hNAA10 resulted in increased aggregation of an Htt-enhanced green fluorescent protein (Htt-EGFP) fusion with expanded polyglutamine stretches, suggesting that both HYPK and NatA prevent Htt aggregation. Furthermore, we demonstrated that HYPK is required for N-terminal acetylation of the known in vivo NatA substrate protein PCNP. Taken together, the data indicate that the physical interaction between HYPK and NatA seems to be of functional importance both for Htt aggregation and for N-terminal acetylation.

The Hsp70 homolog Ssb is essential for glucose sensing via the SNF1 kinase network

Yeast senses the availability of external energy sources via multiple interconnected signaling networks. One of the central components is SNF1, the homolog of mammalian AMP-activated protein kinase, which in yeast is essential for the expression of glucose-repressed genes. When glucose is available hyperphosphorylated SNF1 is rendered inactive by the type 1 protein phosphatase Glc7. Dephosphorylation requires Reg1, which physically targets Glc7 to SNF1. Here we show that the chaperone Ssb is required to keep SNF1 in the nonphosphorylated state in the presence of glucose. Using a proteome approach we found that the Deltassb1Deltassb2 strain displays alterations in protein expression and suffers from phenotypic characteristics reminiscent of glucose repression mutants. Microarray analysis revealed a correlation between deregulation on the protein and on the transcript level. Supporting studies uncovered that SSB1 was an effective multicopy suppressor of severe growth defects caused by the Deltareg1 mutation. Suppression of Deltareg1 by high levels of Ssb was coupled to a reduction of Snf1 hyperphosphorylation back to the wild-type phosphorylation level. The data are consistent with a model in which Ssb is crucial for efficient regulation within the SNF1 signaling network, thereby allowing an appropriate response to changing glucose levels.

Hsp70 structure, function, regulation and influence on yeast prions

Heat shock proteins protect cells from various conditions of stress. Hsp70, the most ubiquitous and highly conserved Hsp, helps proteins adopt native conformation or regain function after misfolding. Various co-chaperones specify Hsp70 function and broaden its substrate range. We discuss Hsp70 structure and function, regulation by co-factors and influence on propagation of yeast prions.

Cotranslational processing mechanisms: towards a dynamic 3D model

Recent major advances have been made in understanding how cotranslational events are achieved in the course of protein biosynthesis. Specifically, several studies have shed light into the dynamic process of how nascent chains emerging from the ribosome are supported by protein biogenesis factors to ensure both processing and folding mechanisms. To take into account the awareness that coordination is needed, a new 'concerted model' recently proposed simultaneous action of both processes on the ribosome. In the model, any emerging nascent chain is first encountered by the chaperone trigger factor (TF), which forms an open cradle underneath the ribosomal exit tunnel. This cradle serves as a passive router that channels the nascent chains to the first cotranslational event, the proteolysis event performed by the N-terminal methionine excision machinery. Although fascinating, this model clearly raises more questions than it answers. Does the data used to develop this model stand up to scrutiny and, if not, what are the alternative mechanisms that the data suggest?

The ribosome as a platform for co-translational processing, folding and targeting of newly synthesized proteins

The early events in the life of newly synthesized proteins in the cellular environment are remarkably complex. Concurrently with their synthesis by the ribosome, nascent polypeptides are subjected to enzymatic processing, chaperone-assisted folding or targeting to translocation pores at membranes. The ribosome itself has a key role in these different tasks and governs the interplay between the various factors involved. Indeed, the ribosome serves as a platform for the spatially and temporally regulated association of enzymes, targeting factors and chaperones that act upon the nascent polypeptides emerging from the exit tunnel. Furthermore, the ribosome provides opportunities to coordinate the protein-synthesis activity of its peptidyl transferase center with the protein targeting and folding processes. Here we review the early co-translational events involving the ribosome that guide cytosolic proteins to their native state.

A novel human NatA Nalpha-terminal acetyltransferase complex: hNaa16p-hNaa10p (hNat2-hArd1)

BACKGROUND

Protein acetylation is among the most common protein modifications. The two major types are post-translational Nepsilon-lysine acetylation catalyzed by KATs (Lysine acetyltransferases, previously named HATs (histone acetyltransferases) and co-translational Nalpha-terminal acetylation catalyzed by NATs (N-terminal acetyltransferases). The major NAT complex in yeast, NatA, is composed of the catalytic subunit Naa10p (N alpha acetyltransferase 10 protein) (Ard1p) and the auxiliary subunit Naa15p (Nat1p). The NatA complex potentially acetylates Ser-, Ala-, Thr-, Gly-, Val- and Cys- N-termini after Met-cleavage. In humans, the homologues hNaa15p (hNat1) and hNaa10p (hArd1) were demonstrated to form a stable ribosome associated NAT complex acetylating NatA type N-termini in vitro and in vivo.

RESULTS

We here describe a novel human protein, hNaa16p (hNat2), with 70% sequence identity to hNaa15p (hNat1). The gene encoding hNaa16p originates from an early vertebrate duplication event from the common ancestor of hNAA15 and hNAA16. Immunoprecipitation coupled to mass spectrometry identified both endogenous hNaa15p and hNaa16p as distinct interaction partners of hNaa10p in HEK293 cells, thus demonstrating the presence of both hNaa15p-hNaa10p and hNaa16p-hNaa10p complexes. The hNaa16p-hNaa10p complex acetylates NatA type N-termini in vitro. hNaa16p is ribosome associated, supporting its potential role in cotranslational Nalpha-terminal acetylation. hNAA16 is expressed in a variety of human cell lines, but is generally less abundant as compared to hNAA15. Specific knockdown of hNAA16 induces cell death, suggesting an essential role for hNaa16p in human cells.

CONCLUSION

At least two distinct NatA protein Nalpha-terminal acetyltransferases coexist in human cells potentially creating a more complex and flexible system for Nalpha-terminal acetylation as compared to lower eukaryotes.

Knockdown of human N alpha-terminal acetyltransferase complex C leads to p53-dependent apoptosis and aberrant human Arl8b localization

Protein N(alpha)-terminal acetylation is one of the most common protein modifications in eukaryotic cells. In yeast, three major complexes, NatA, NatB, and NatC, catalyze nearly all N-terminal acetylation, acetylating specific subsets of protein N termini. In human cells, only the NatA and NatB complexes have been described. We here identify and characterize the human NatC (hNatC) complex, containing the catalytic subunit hMak3 and the auxiliary subunits hMak10 and hMak31. This complex associates with ribosomes, and hMak3 acetylates Met-Leu protein N termini in vitro, suggesting a model in which the human NatC complex functions in cotranslational N-terminal acetylation. Small interfering RNA-mediated knockdown of NatC subunits results in p53-dependent cell death and reduced growth of human cell lines. As a consequence of hMAK3 knockdown, p53 is stabilized and phosphorylated and there is a significant transcriptional activation of proapoptotic genes downstream of p53. Knockdown of hMAK3 alters the subcellular localization of the Arf-like GTPase hArl8b, supporting that hArl8b is a hMak3 substrate in vivo. Taken together, hNatC-mediated N-terminal acetylation is important for maintenance of protein function and cell viability in human cells.

A signal-anchor sequence stimulates signal recognition particle binding to ribosomes from inside the exit tunnel

Sorting of eukaryotic membrane and secretory proteins depends on recognition of ribosome-bound nascent chain signal sequences by the signal recognition particle (SRP). The current model suggests that the SRP cycle is initiated when a signal sequence emerges from the ribosomal tunnel and binds to SRP. Then elongation is slowed until the SRP-bound ribosome-nascent chain complex (RNC) is targeted to the SRP receptor in the endoplasmic reticulum (ER) membrane. The RNC is then transferred to the translocon, SRP is released, and translation resumes. Because RNCs do not target to the translocon efficiently if nascent chains become too long, the window for SRP to identify its substrates is short. We now show that a transmembrane signal-anchor sequence (SA) significantly enhances binding of SRP to RNCs even before the SA emerges from the ribosomal tunnel. In this mode, SRP does not contact the SA directly but is in close proximity to the portion of the nascent polypeptide that has already left the ribosomal tunnel. Early recruitment of SRP provides a mechanism to expand the window for substrate identification. We suggest that the dynamics of the SRP-ribosome interaction is affected not only by the direct binding of SRP to an exposed signal sequence but also by properties of the translating ribosome that are triggered from within the tunnel.

Import of nuclear-encoded mitochondrial proteins: a cotranslational perspective

A growing amount of evidence suggests that the cytosolic translation of nuclear-encoded mitochondrial proteins and their subsequent import into mitochondria are tightly coupled in a process termed cotranslational import. In addition to the original posttranslational view of mitochondrial protein import, early literature also provides both in vitro and in vivo experimental evidence supporting the simultaneous existence of a cotranslational protein-import mechanism in mitochondria. Recent investigations have started to reveal the cotranslational import mechanism which is initiated by transporting either a translation complex or a translationally competent mRNA encoding a mitochondrial protein to the mitochondrial surface. The intracellular localization of mRNA to the mitochondrial surface has emerged as the latest addition to our understanding of mitochondrial biogenesis. It is mediated by targeting elements within the mRNA molecule in association with potential mRNA-binding proteins.

Access to ribosomal protein Rpl25p by the signal recognition particle is required for efficient cotranslational translocation

Targeting of proteins to the endoplasmic reticulum (ER) occurs cotranslationally necessitating the interaction of the signal recognition particle (SRP) and the translocon with the ribosome. Biochemical and structural studies implicate ribosomal protein Rpl25p as a major ribosome interaction site for both these factors. Here we characterize an RPL25GFP fusion, which behaves as a dominant mutant leading to defects in co- but not posttranslational translocation in vivo. In these cells, ribosomes still interact with ER membrane and the translocon, but are defective in binding SRP. Overexpression of SRP can restore ribosome binding of SRP, but only partially rescues growth and translocation defects. Our results indicate that Rpl25p plays a critical role in the recruitment of SRP to the ribosome.

Fine-tuning of translation termination efficiency in Saccharomyces cerevisiae involves two factors in close proximity to the exit tunnel of the ribosome

In eukaryotes, release factors 1 and 3 (eRF1 and eRF3) are recruited to promote translation termination when a stop codon on the mRNA enters at the ribosomal A-site. However, their overexpression increases termination efficiency only moderately, suggesting that other factors might be involved in the termination process. To determine such unknown components, we performed a genetic screen in Saccharomyces cerevisiae that identified genes increasing termination efficiency when overexpressed. For this purpose, we constructed a dedicated reporter strain in which a leaky stop codon is inserted into the chromosomal copy of the ade2 gene. Twenty-five antisuppressor candidates were identified and characterized for their impact on readthrough. Among them, SSB1 and snR18, two factors close to the exit tunnel of the ribosome, directed the strongest antisuppression effects when overexpressed, showing that they may be involved in fine-tuning of the translation termination level.

Functional characterization of the atypical Hsp70 subunit of yeast ribosome-associated complex

Eukaryotic ribosomes carry a stable chaperone complex termed ribosome-associated complex consisting of the J-domain protein Zuo1 and the Hsp70 Ssz1. Zuo1 and Ssz1 together with the Hsp70 homolog Ssb1/2 form a functional triad involved in translation and early polypeptide folding processes. Strains lacking one of these components display slow growth, cold sensitivity, and defects in translational fidelity. Ssz1 diverges from canonical Hsp70s insofar that neither the ability to hydrolyze ATP nor binding to peptide substrates is essential in vivo. The exact role within the chaperone triad and whether or not Ssz1 can hydrolyze ATP has remained unclear. We now find that Ssz1 is not an ATPase in vitro, and even its ability to bind ATP is dispensable in vivo. Furthermore, Ssz1 function was independent of ribosome-associated complex formation, indicating that Ssz1 is not merely a structural scaffold for Zuo1. Finally, Ssz1 function in vivo was inactivated when both nucleotide binding and Zuo1 interaction via the C-terminal domain were disrupted in the same mutant. The two domains of this protein thus cooperate in a way that allows for severe interference in either but not in both of them.

New tricks for an old dog: the evolving world of Hsp70

The Hsp70 chaperone is arguably the most studied member of the heat shock protein family, a legacy traced back to the early days of phage genetics. However, much still remains to be learned about this essential protein-folding machine. Its involvement in a number of human pathologies, ranging from cancer to protein aggregation diseases, underscores the need for a comprehensive understanding of the myriad cellular roles Hsp70 plays and the outstanding open questions. This article will explore several exciting avenues of research into the function and biology of the chaperone. Analysis of the many eukaryotic Hsp70 isoforms has demonstrated distinct functional roles for some Hsp70 members, to the point of transition from a protein "foldase" to a chaperone cofactor. New insights gained from structural studies have unveiled a likely model for interdomain communication and thus regulation of substrate binding and processing. Advances in small molecule modulation of Hsp70 activity are likely to have significant clinical impact. There is also a growing realization that Hsp70 participates in distinct functional networks in partnership with other protein chaperones. The field is thus at an exciting time when the substantial successes of the past have provided a solid framework that will be used to fuel both discovery and application--Hsp70, from molecule to man.

Probing the Membrane Interface-Interacting Proteome Using Photoactivatable Lipid Cross-Linkers

To analyze proteins interacting at the membrane interface, a phospholipid analogue was used with a photoactivatable headgroup (ASA-DLPE, N-(4-azidosalicylamidyl)-1,2-dilauroyl-sn-glycero-3-phosphoethanolamine) for selective cross-linking. The peripheral membrane protein cytochrome c from the inner mitochondrial membrane was rendered carbonate wash-resistant by cross-linking to ASA-DLPE in a model membrane system, validating our approach. Cross-link products of cytochrome c and its precursor apocytochrome c were demonstrated by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and were specifically detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), taking advantage of the intrinsic UV absorbance of the cross-linker. Application of the method to inner mitochondrial membranes from Saccharomyces cerevisae revealed cross-link products of both exogenously added apocytochrome c and endogenous proteins with molecular weights around 34 and 72 kDa. Liquid chromatograpy (LC)-MS/MS was performed to identify these proteins, resulting in a list of candidate proteins potentially cross-linked at the membrane interface. The approach described here provides methodology for capturing phospholipid-protein interactions in their native environment of the biomembrane using modern proteomics techniques.

Proteomic changes inDebaryomyces hanseniiupon exposure to NaCl stress

The proteome of the highly NaCl-tolerant yeast Debaryomyces hansenii was investigated by two-dimensional polyacrylamide gel electrophoresis (2D PAGE), and 47 protein spots were identified by matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) followed by mass spectrometry (MS). The influence of NaCl on the D. hansenii proteome was investigated during the first 3 h of NaCl exposure. The rate of protein synthesis was strongly decreased by exposure to 8% and 12% (w/v) NaCl, as the average incorporation rates of l-[(35)S]methionine within the first 30 min after addition of NaCl were only 7% and 4% of the rate in medium without NaCl. In addition, the number of protein spots detected on 2D gels prepared from cells exposed to 8% and 12% (w/v) NaCl exceeded less than 28% of the number of protein spots detected on 2D gels prepared from cells without added NaCl. Several proteins were identified as being either induced or repressed upon NaCl exposure. The induced proteins were enzymes involved in glycerol synthesis/dissimilation and the upper part of glycolysis, whereas the repressed proteins were enzymes involved in the lower part of glycolysis, the route to the Krebs cycle, and the synthesis of amino acids. Furthermore, one heat shock protein (Ssa1p) was induced, whereas others (Ssb2p and Hsp60p) were repressed.

Association of protein biogenesis factors at the yeast ribosomal tunnel exit is affected by the translational status and nascent polypeptide sequence

Ribosome-associated protein biogenesis factors (RPBs) act during a short but critical period of protein biogenesis. The action of RPBs starts as soon as a nascent polypeptide becomes accessible from the outside of the ribosome and ends upon termination of translation. In yeast, RPBs include the chaperones Ssb1/2 and ribosome-associated complex, signal recognition particle, nascent polypeptide-associated complex (NAC), the aminopeptidases Map1 and Map2, and the Nalpha-terminal acetyltransferase NatA. Here, we provide the first comprehensive analysis of RPB binding at the yeast ribosomal tunnel exit as a function of translational status and polypeptide sequence. We measured the ratios of RPBs to ribosomes in yeast cells and determined RPB occupation of translating and non-translating ribosomes. The combined results imply a requirement for dynamic and coordinated interactions at the tunnel exit. Exclusively, NAC was associated with the majority of ribosomes regardless of their translational status. All other RPBs occupied only ribosomal subpopulations, binding with increased apparent affinity to randomly translating ribosomes as compared with non-translating ones. Analysis of RPB interaction with homogenous ribosome populations engaged in the translation of specific nascent polypeptides revealed that the affinities of Ssb1/2, NAC, and, as expected, signal recognition particle, were influenced by the amino acid sequence of the nascent polypeptide. Complementary cross-linking data suggest that not only affinity of RPBs to the ribosome but also positioning can be influenced in a nascent polypeptide-dependent manner.

The cytoplasmic Hsp70 chaperone machinery subjects misfolded and endoplasmic reticulum import-incompetent proteins to degradation via the ubiquitin-proteasome system

The mechanism of protein quality control and elimination of misfolded proteins in the cytoplasm is poorly understood. We studied the involvement of cytoplasmic factors required for degradation of two endoplasmic reticulum (ER)-import-defective mutated derivatives of carboxypeptidase yscY (DeltassCPY* and DeltassCPY*-GFP) and also examined the requirements for degradation of the corresponding wild-type enzyme made ER-import incompetent by removal of its signal sequence (DeltassCPY). All these protein species are rapidly degraded via the ubiquitin-proteasome system. Degradation requires the ubiquitin-conjugating enzymes Ubc4p and Ubc5p, the cytoplasmic Hsp70 Ssa chaperone machinery, and the Hsp70 cochaperone Ydj1p. Neither the Hsp90 chaperones nor Hsp104 or the small heat-shock proteins Hsp26 and Hsp42 are involved in the degradation process. Elimination of a GFP fusion (GFP-cODC), containing the C-terminal 37 amino acids of ornithine decarboxylase (cODC) directing this enzyme to the proteasome, is independent of Ssa1p function. Fusion of DeltassCPY* to GFP-cODC to form DeltassCPY*-GFP-cODC reimposes a dependency on the Ssa1p chaperone for degradation. Evidently, the misfolded protein domain dictates the route of protein elimination. These data and our further results give evidence that the Ssa1p-Ydj1p machinery recognizes misfolded protein domains, keeps misfolded proteins soluble, solubilizes precipitated protein material, and escorts and delivers misfolded proteins in the ubiquitinated state to the proteasome for degradation.

Fes1p acts as a nucleotide exchange factor for the ribosome-associated molecular chaperone Ssb1p

The HspBP1 homolog Fes1p was recently identified as a nucleotide exchange factor (NEF) of Ssa1p, a canonical Hsp70 molecular chaperone in the cytosol of Saccharomyces cerevisiae. Besides the Ssa-type Hsp70s, the yeast cytosol contains three additional classes of Hsp70, termed Ssb, Sse and Ssz. Here, we show that Fes1p also functions as NEF for the ribosome-bound Ssb Hsp70s. Sequence analysis indicated that residues important for interaction with Fes1p are highly conserved in Ssa1p and Ssb1p, but not in Sse1p and Ssz1p. Indeed, Fes1p interacts with Ssa1p and Ssb1p with similar affinity, but does not form a complex with Sse1p. Functional analysis showed that Fes1p accelerates the release of the nucleotide analog MABA-ADP from Ssb1p by a factor of 35. In contrast to the interaction between mammalian HspBP1 and Hsp70, however, addition of ATP only moderately decreases the affinity of Fes1p for Ssb1p. Point mutations in Fes1p abolishing complex formation with Ssa1p also prevent the interaction with Ssb1p. The ATPase activity of Ssb1p is stimulated by the ribosome-associated complex of Zuotin and Ssz1p (RAC). Interestingly, Fes1p inhibits the stimulation of Ssb1p ATPase by RAC, suggesting a complex regulatory role of Fes1p in modulating the function of Ssb Hsp70s in co-translational protein folding.

Why molecular chaperones buffer mutational damage: a case study with a yeast Hsp40/70 system

The malfunctioning of molecular chaperones may result in uncovering genetic variation. The molecular basis of this phenomenon remains largely unknown. Chaperones rescue proteins unfolded by environmental stresses and therefore they might also help to stabilize mutated proteins and thus mask damages. To test this hypothesis, we carried out a genomewide mutagenesis followed by a screen for mutations that were synthetically harmful when the RAC-Ssb1/2 cytosolic chaperones were inactive. Mutants with such a phenotype were found and mapped to single nucleotide substitutions. However, neither the genes identified nor the nature of genetic lesions implied that folding of the mutated proteins was being supported by the chaperones. In a second screen, we identified temperature-sensitive (ts) mutants, a phenotype indicative of structural instability of proteins. We tested these for an association with sensitivity to loss of chaperone activity but found no such correlation as might have been expected if the chaperones assisted the folding of mutant proteins. Thus, molecular chaperones can mask the negative effects of mutations but the mechanism of such buffering need not be direct. A plausible role of chaperones is to stabilize genetic networks, thus making them more tolerant to malfunctioning of their constituents.

Cross-Talk between RNA and Prions

As concepts evolve in mammalian and yeast prion biology, rather preliminary research investigating the interplay between prion and RNA processes are gaining momentum. The yeast prion [PSI+] represents an aggregated state of the translation termination factor Sup35 resulting in the tendency of ribosomes to readthrough stop codons. This "nonsense suppression" activity is investigated for its possible physiological role to engender on Saccharomyces cerevisiae the ability to respond to stress or variable growth conditions and thereby act as a capacitor to evolve. The interaction between prion and RNA is a two way street--the cell may have adopted RNA processes in translation to govern the presence of prions and the [PSI+] prion's nonsense suppressor phenotype may exhibit different growth phenotypes by its control of translation termination. RNA processes in the mammalian cell also effect and are affected by prions.

Polypeptide chain termination and stop codon readthrough on eukaryotic ribosomes

During protein translation, a variety of quality control checks ensure that the resulting polypeptides deviate minimally from their genetic encoding template. Translational fidelity is central in order to preserve the function and integrity of each cell. Correct termination is an important aspect of translational fidelity, and a multitude of mechanisms and players participate in this exquisitely regulated process. This review explores our current understanding of eukaryotic termination by highlighting the roles of the different ribosomal components as well as termination factors and ribosome-associated proteins, such as chaperones.

Systems Analyses Reveal Two Chaperone Networks with Distinct Functions in Eukaryotic Cells

Molecular chaperones assist the folding of newly translated and stress-denatured proteins. In prokaryotes, overlapping sets of chaperones mediate both processes. In contrast, we find that eukaryotes evolved distinct chaperone networks to carry out these functions. Genomic and functional analyses indicate that in addition to stress-inducible chaperones that protect the cellular proteome from stress, eukaryotes contain a stress-repressed chaperone network that is dedicated to protein biogenesis. These stress-repressed chaperones are transcriptionally, functionally, and physically linked to the translational apparatus and associate with nascent polypeptides emerging from the ribosome. Consistent with a function in de novo protein folding, impairment of the translation-linked chaperone network renders cells sensitive to misfolding in the context of protein synthesis but not in the context of environmental stress. The emergence of a translation-linked chaperone network likely underlies the elaborate cotranslational folding process necessary for the evolution of larger multidomain proteins characteristic of eukaryotic cells.

Glycosylphosphatidylinositol-anchored proteins are required for the transport of detergent-resistant microdomain-associated membrane proteins Tat2p and Fur4p

In eukaryotic cells many cell surface proteins are attached to the membrane via the glycosylphosphatidylinositol (GPI) moiety. In yeast, GPI also plays important roles in the production of mannoprotein in the cell wall. We previously isolated gwt1 mutants and found that GWT1 is required for inositol acylation in the GPI biosynthetic pathway. In this study we isolated a new gwt1 mutant allele, gwt1-10, that shows not only high temperature sensitivity but also low temperature sensitivity. The gwt1-10 cells show impaired acyltransferase activity and attachment of GPI to proteins even at the permissive temperature. We identified TAT2, which encodes a high affinity tryptophan permease, as a multicopy suppressor of cold sensitivity in gwt1-10 cells. The gwt1-10 cells were also defective in the import of tryptophan, and a lack of tryptophan caused low temperature sensitivity. Microscopic observation revealed that Tat2p is not transported to the plasma membrane but is retained in the endoplasmic reticulum in gwt1-10 cells grown under tryptophan-poor conditions. We found that Tat2p was not associated with detergent-resistant membranes (DRMs), which are required for the recruitment of Tat2p to the plasma membrane. A similar result was obtained for Fur4p, a uracil permease localized in the DRMs of the plasma membrane. These results indicate that GPI-anchored proteins are required for the recruitment of membrane proteins Tat2p and Fur4p to the plasma membrane via DRMs, suggesting that some membrane proteins are redistributed in the cell in response to environmental and nutritional conditions due to an association with DRMs that is dependent on GPI-anchored proteins.

Dissecting functional similarities of ribosome-associated chaperones from Saccharomyces cerevisiae and Escherichia coli

Ribosome-tethered chaperones that interact with nascent polypeptide chains have been identified in both prokaryotic and eukaryotic systems. However, these ribosome-associated chaperones share no sequence similarity: bacterial trigger factors (TF) form an independent protein family while the yeast machinery is Hsp70-based. The absence of any component of the yeast machinery results in slow growth at low temperatures and sensitivity to aminoglycoside protein synthesis inhibitors. After establishing that yeast ribosomal protein Rpl25 is able to recruit TF to ribosomes when expressed in place of its Escherichia coli homologue L23, the ribosomal TF tether, we tested whether such divergent ribosome-associated chaperones are functionally interchangeable. E. coli TF was expressed in yeast cells that lacked the endogenous ribosome-bound machinery. TF associated with yeast ribosomes, cross-linked to yeast nascent polypeptides and partially complemented the aminoglycoside sensitivity, demonstrating that ribosome-associated chaperones from divergent organisms share common functions, despite their lack of sequence similarity.

The Yeast Hsp110 Sse1 Functionally Interacts with the Hsp70 Chaperones Ssa and Ssb

There is growing evidence that members of the extended Hsp70 family of molecular chaperones, including the Hsp110 and Grp170 subgroups, collaborate in vivo to carry out essential cellular processes. However, relatively little is known regarding the interactions and cellular functions of Sse1, the yeast Hsp110 homolog. Through co-immunoprecipitation analysis, we found that Sse1 forms heterodimeric complexes with the abundant cytosolic Hsp70s Ssa and Ssb in vivo. Furthermore, these complexes can be efficiently reconstituted in vitro using purified proteins. Binding of Ssa or Ssb to Sse1 was mutually exclusive. The ATPase domain of Sse1 was found to be critical for interaction as inactivating point mutations severely reduced interaction with Ssa and Ssb. Sse1 stimulated Ssa1 ATPase activity synergistically with the co-chaperone Ydj1, and stimulation required complex formation. Ssa1 is required for post-translational translocation of the yeast mating pheromone alpha-factor into the endoplasmic reticulum. Like ssa mutants, we demonstrate that sse1delta cells accumulate prepro-alpha-factor, but not the co-translationally imported protein Kar2, indicating that interaction between Sse1 and Ssa is functionally significant in vivo. These data suggest that the Hsp110 chaperone operates in concert with Hsp70 in yeast and that this collaboration is required for cellular Hsp70 functions.

Long-oligomer microarray profiling in Neurospora crassa reveals the transcriptional program underlying biochemical and physiological events of conidial germination

To test the inferences of spotted microarray technology against a biochemically well-studied process, we performed transcriptional profiling of conidial germination in the filamentous fungus, Neurospora crassa. We first constructed a 70 base oligomer microarray that assays 3366 predicted genes. To estimate the relative gene expression levels and changes in gene expression during conidial germination, we analyzed a circuit design of competitive hybridizations throughout a time course using a Bayesian analysis of gene expression level. Remarkable consistency of mRNA profiles with previously published northern data was observed. Genes were hierarchically clustered into groups with respect to their expression profiles over the time course of conidial germination. A functional classification database was employed to characterize the global picture of gene expression. Consensus motif searches identified a putative regulatory component associated with genes involved in ribosomal biogenesis. Our transcriptional profiling data correlate well with biochemical and physiological processes associated with conidial germination and will facilitate functional predictions of novel genes in N.crassa and other filamentous ascomycete species. Furthermore, our dataset on conidial germination allowed comparisons to transcriptional mechanisms associated with germination processes of diverse propagules, such as teliospores of the phytopathogenic fungus Ustilago maydis and spores of the social amoeba Dictyostelium discoideum.

Hsp110 cooperates with different cytosolic HSP70 systems in a pathway for de novo folding

Molecular chaperones such as Hsp70 use ATP binding and hydrolysis to prevent aggregation and ensure the efficient folding of newly translated and stress-denatured polypeptides. Eukaryotic cells contain several cytosolic Hsp70 subfamilies. In yeast, these include the Hsp70s SSB and SSA as well as the Hsp110-like Sse1/2p. The cellular functions and interplay between these different Hsp70 systems remain ill-defined. Here we show that the different cytosolic Hsp70 systems functionally interact with Hsp110 to form a chaperone network that interacts with newly translated polypeptides during their biogenesis. Both SSB and SSA Hsp70s form stable complexes with the Hsp110 Sse1p. Pulse-chase analysis indicates that these Hsp70/Hsp110 teams, SSB/SSE and SSA/SSE, transiently associate with newly synthesized polypeptides with different kinetics. SSB Hsp70s bind cotranslationally to a large fraction of nascent chains, suggesting an early role in the stabilization of nascent chains. SSA Hsp70s bind mostly post-translationally to a more restricted subset of newly translated polypeptides, suggesting a downstream function in the folding pathway. Notably, loss of SSB dramatically enhances the cotranslational association of SSA with nascent chains, suggesting SSA can partially fulfill an SSB-like function. On the other hand, the absence of SSE1 enhances polypeptide binding to both SSB and SSA and impairs cell growth. It, thus, appears that Hsp110 is an important regulator of Hsp70-substrate interactions. Based on our data, we propose that Hsp110 cooperates with the SSB and SSA Hsp70 subfamilies, which act sequentially during de novo folding.

The chaperones MPP11 and Hsp70L1 form the mammalian ribosome-associated complex

Soluble Hsp70 homologs cotranslationally interact with nascent polypeptides in all kingdoms of life. In addition, fungi possess a specialized Hsp70 system attached to ribosomes, which in Saccharomyces cerevisiae consists of the Hsp70 homologs Ssb1/2p, Ssz1p, and the Hsp40 homolog zuotin. Ssz1p and zuotin are assembled into a unique heterodimeric complex termed ribosome-associated complex. So far, no such specialized chaperones have been identified on ribosomes of higher eukaryotes. However, a family of proteins characterized by an N-terminal zuotin-homology domain fused to a C-terminal two-repeat Myb domain is present in animals and plants. Members of this family, like human MPP11 and mouse MIDA1, have been implicated in the regulation of cell growth. Specific targets of MPP11/MIDA1, however, have remained elusive. Here, we report that MPP11 is localized to the cytosol and associates with ribosomes. Purification of MPP11 revealed that it forms a stable complex with Hsp70L1, a distantly related homolog of Ssz1p. Complementation experiments indicate that mammalian ribosome-associated complex is functional in yeast. We conclude that despite a low degree of homology on the amino acid level cooperation of ribosome-associated chaperones with the translational apparatus is well conserved in eukaryotic cells.

Broad sensitivity of Saccharomyces cerevisiae lacking ribosome-associated chaperone ssb or zuo1 to cations, including aminoglycosides

The Hsp70 Ssb and J protein Zuo1 of Saccharomyces cerevisiae are ribosome-associated molecular chaperones, proposed to be involved in the folding of newly synthesized polypeptide chains. Cells lacking Ssb and/or Zuo1 have been reported to be hypersensitive to cationic aminoglycoside protein synthesis inhibitors that affect translational fidelity and to NaCl. Since we found that Deltassb1 Deltassb2 (Deltassb1,2), Deltazuo1, and wild-type cells have very similar levels of translational misreading in the absence of aminoglycosides, we asked whether the sensitivities to aminoglycosides and NaCl represent a general increase in sensitivity to cations. We found that Deltassb1,2 and Deltazuo1 cells are hypersensitive to a wide range of cations. This broad sensitivity is similar to that of cells having lowered activity of major plasma membrane transporters, such as the major K+ transporters Trk1 and Trk2 or their regulators Hal4 and Hal5. Like Deltahal4,5 cells, Deltassb1,2 and Deltazuo1 cells have increased intracellular levels of Na+ and Li+ upon challenge with higher-than-normal levels of these cations, due to an increased rate of influx. In the presence of aminoglycosides, Deltassb1,2, Deltazuo1, and Deltahal 4,5 cells have similarly increased levels of translational misreading. We conclude that, in vivo, the major cause of the aminoglycoside sensitivity of cells lacking ribosome-associated molecular chaperones is a general increase in cation influx, perhaps due to altered maturation of membrane proteins.

The Hsp70 Ssz1 modulates the function of the ribosome-associated J-protein Zuo1

J-proteins are obligate partners of Hsp70s, forming a ubiquitous class of molecular chaperone machinery. The ribosome-associated Hsp70 of yeast Ssb binds nascent polypeptides as they exit the ribosome. Here we report that the ribosome-associated J-protein Zuo1 is the partner of Ssb. However, Zuo1 efficiently stimulates the ATPase activity of Ssb only when in complex with another Hsp70, Ssz1. Ssz1 binds ATP, but none of the 11 different amino acid substitutions in the ATP-binding cleft affected Ssz1 function in vivo, suggesting that neither nucleotide binding nor hydrolysis is required. We propose that Ssz1's predominant function in the cell is to facilitate Zuo1's ability to function as a J-protein partner of Ssb on the ribosome, serving as an example of an Hsp70 family member that has evolved to carry out functions distinct from that of a chaperone.

Human Mpp11 J protein: ribosome-tethered molecular chaperones are ubiquitous

The existence of specialized molecular chaperones that interact directly with ribosomes is well established in microorganisms. Such proteins bind polypeptides exiting the ribosomal tunnel and provide a physical link between translation and protein folding. We report that ribosome-associated molecular chaperones have been maintained throughout eukaryotic evolution, as illustrated by Mpp11, the human ortholog of the yeast ribosome-associated J protein Zuo. When expressed in yeast, Mpp11 partially substituted for Zuo by partnering with the multipurpose Hsp70 Ssa, the homolog of mammalian Hsc70. We propose that in metazoans, ribosome-associated Mpp11 recruits the multifunctional soluble Hsc70 to nascent polypeptide chains as they exit the ribosome.

Distinct but overlapping functions of Hsp70, Hsp90, and an Hsp70 nucleotide exchange factor during protein biogenesis in yeast

Hsp70 and Hsp90 molecular chaperones play essential roles in protein expression and maturation, and while catalyzing protein folding they can "decide" to target mis-folded substrates for degradation. In this report, we show for the first time distinct but partially overlapping requirements for Hsp90, Hsp70, and an Hsp70 nucleotide exchange factor (NEF) at different steps during the biogenesis of a model substrate, firefly luciferase (FFLux), in yeast. By examining the inducible expression of FFLux in wild type cells and in specific yeast mutants, we find that the Fes1p NEF is required for efficient FFLux folding, whereas the Hsp70, Ssa1p, is required for both protein folding and stability, and to maintain maximal FFLux mRNA levels. In contrast, Hsp90 function was primarily necessary to express the FFLux-encoding gene from an inducible promoter. Together, these data indicate previously unknown roles for these proteins and point to the complexity with which chaperones and cochaperones function in the cell.

The ribosome-bound chaperones RAC and Ssb1/2p are required for accurate translation in Saccharomyces cerevisiae

The chaperone homologs RAC (ribosome-associated complex) and Ssb1/2p are anchored to ribosomes; Ssb1/2p directly interacts with nascent polypeptides. The absence of RAC or Ssb1/2p results in a similar set of phenotypes, including hypersensitivity against the aminoglycoside paromomycin, which binds to the small ribosomal subunit and compromises the fidelity of translation. In order to understand this phenomenon we measured the frequency of translation termination and misincorporation in vivo and in vitro with a novel reporter system. Translational fidelity was impaired in the absence of functional RAC or Ssb1/2p, and the effect was further enhanced by paromomycin. The mutant strains suffered primarily from a defect in translation termination, while misincorporation was compromised to a lesser extent. Consistently, a low level of soluble translation termination factor Sup35p enhanced growth defects in the mutant strains. Based on the combined data we conclude that RAC and Ssb1/2p are crucial in maintaining translational fidelity beyond their postulated role as chaperones for nascent polypeptides.

Activation of pleiotropic drug resistance by the J-protein and Hsp70-related proteins, Zuo1 and Ssz1

Ssz1 (Pdr13) and Zuo1, members of the Hsp70 and J-protein molecular chaperone families, respectively, form a heterodimer and function on the ribosome with the Hsp70, Ssb, presumably assisting folding of newly synthesized polypeptides. As it has also been reported that Ssz1 induces pleiotropic drug resistance (PDR) when overexpressed, a possible role for Zuo1 in PDR was investigated. The C-terminal domain of Zuo1, which is dispensable for Zuo1's chaperone function on the ribosome, is both necessary and sufficient for PDR induction by Zuo1. A single domain of Ssz1, the N-terminal ATPase domain, is sufficient for PDR induction as well, indicating that Ssz1 does not function as a chaperone in PDR. No role for Ssb was found in PDR; overexpression did not affect PDR, nor was its presence required for Ssz1's or Zuo1's effect on PDR. As our results also indicate that Ssz1 and Zuo1 must be free of ribosomes to induce PDR, we propose that Ssz1's and Zuo1's function in PDR is distinct from their role as ribosome-associated co-chaperones and may be regulatory in nature.

Hsp70 chaperones as modulators of prion life cycle: novel effects of Ssa and Ssb on the Saccharomyces cerevisiae prion [PSI+]

[PSI(+)] is a prion isoform of the yeast release factor Sup35. In some assays, the cytosolic chaperones Ssa1 and Ssb1/2 of the Hsp70 family were previously shown to exhibit "pro-[PSI(+)]" and "anti-[PSI(+)]" effects, respectively. Here, it is demonstrated for the first time that excess Ssa1 increases de novo formation of [PSI(+)] and that pro-[PSI(+)] effects of Ssa1 are shared by all other Ssa proteins. Experiments with chimeric constructs show that the peptide-binding domain is a major determinant of differences in the effects of Ssa and Ssb proteins on [PSI(+)]. Surprisingly, overproduction of either chaperone increases loss of [PSI(+)] when Sup35 is simultaneously overproduced. Excess Ssa increases both the average size of prion polymers and the proportion of monomeric Sup35 protein. Both in vivo and in vitro experiments uncover direct physical interactions between Sup35 and Hsp70 proteins. The proposed model postulates that Ssa stimulates prion formation and polymer growth by stabilizing misfolded proteins, which serve as substrates for prion conversion. In the case of very large prion aggregates, further increase in size may lead to the loss of prion activity. In contrast, Ssb either stimulates refolding into nonprion conformation or targets misfolded proteins for degradation, in this way counteracting prion formation and propagation.

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 expression of the HSP70 gene in Moneuplotes crassus is controlled by a two-step process at the transcript level

Steady state levels of the HSP70 transcript were followed by Northern hybridization in Moneuplotes crassus in order to investigate the mechanisms of the short term and long term response to heat shock in a spirotrichous ciliate. The influence of inhibitors of transcription or translation on the transcript levels was also studied. The heat shock response could be dissected into two phases. An initial protein-dependent stabilization of the mRNA was followed by an increase of the steady state transcript level that was dependent on continued transcription. As expected, the half-life of the RNA was short. Western blot analysis then showed that the HSP70 protein accumulated only upon permanent heat shock. It is concluded that the regulation of the heat shock response is a two-step process that occurs at the transcript level.

The heat shock protein 70 cochaperone hip enhances functional maturation of glucocorticoid receptor

Multiple molecular chaperones interact with steroid receptors to promote functional maturation and stability of receptor complexes. The heat shock protein (Hsp)70 cochaperone Hip has been identified in conjunction with Hsp70, Hsp90, and the Hsp70/Hsp90 cochaperone Hop/Sti1p in receptor complexes during an intermediate stage of receptor assembly, but a functional requirement for Hip in the receptor assembly process has not been established. Because the budding yeast Saccharomyces cerevisiae contains orthologs for most of the receptor-associated chaperones yet lacks an orthologous Hip gene, we exploited the well-established yeast model for steroid receptor function to ask whether Hip can alter steroid receptor function in vivo. Introducing human Hip into yeast enhances hormone-dependent activation of a reporter gene by glucocorticoid receptor (GR). Because Hip does not similarly enhance signaling by mineralocorticoid, progesterone, or estrogen receptors, a general effect on transcription can be excluded. Instead, Hip promotes functional maturation of GR without increasing steady-state levels of GR protein. Unexpectedly, Hip binding to Hsp70 is not critical for boosting GR responsiveness to hormone. In conclusion, Hip functions by a previously unrecognized mechanism to promote the efficiency of GR maturation in cells.

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.

The yeast N(alpha)-acetyltransferase NatA is quantitatively anchored to the ribosome and interacts with nascent polypeptides

The majority of cytosolic proteins in eukaryotes contain a covalently linked acetyl moiety at their very N terminus. The mechanism by which the acetyl moiety is efficiently transferred to a large variety of nascent polypeptides is currently only poorly understood. Yeast N(alpha)-acetyltransferase NatA, consisting of the known subunits Nat1p and the catalytically active Ard1p, recognizes a wide range of sequences and is thought to act cotranslationally. We found that NatA was quantitatively bound to ribosomes via Nat1p and contained a previously unrecognized third subunit, the N(alpha)-acetyltransferase homologue Nat5p. Nat1p not only anchored Ard1p and Nat5p to the ribosome but also was in close proximity to nascent polypeptides, independent of whether they were substrates for N(alpha)-acetylation or not. Besides Nat1p, NAC (nascent polypeptide-associated complex) and the Hsp70 homologue Ssb1/2p interact with a variety of nascent polypeptides on the yeast ribosome. A direct comparison revealed that Nat1p required longer nascent polypeptides for interaction than NAC and Ssb1/2p. Delta nat1 or Delta ard1 deletion strains were temperature sensitive and showed derepression of silent mating type loci while Delta nat5 did not display any obvious phenotype. Temperature sensitivity and derepression of silent mating type loci caused by Delta nat1 or Delta ard1 were partially suppressed by overexpression of SSB1. The combination of data suggests that Nat1p presents the N termini of nascent polypeptides for acetylation and might serve additional roles during protein synthesis.

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.

Molecular characterization of a ribosome-associated Hsp70-homologous gene from Rhizopus nigricans

A ribosome-associated Hsp70-homologous gene (Rnssb-1) was isolated from the genomic library of the filamentous zygomycete fungus Rhizopus nigricans. The nucleotide sequence of a genomic clone encoded the N-terminal part of a protein with high similarity to the yeast SSB ribosome-associated chaperones. The missing 3' end of the gene was obtained by 3' RACE. The Northern blot analysis showed that the Rnssb-1 gene is constitutively expressed and is not induced upon heat shock at 37 degrees C. The primary structure analyses revealed that the coding region of the Rnssb-1 gene is interrupted by at least four introns. Their splicing was not inhibited by exposure of the organism to heat shock as proven by RT-PCR. A Southern blot analysis of R. nigricans genomic DNA confirmed the presence of two additional gene copies of ribosome-associated Hsp70 genes in the fungal genome.

Sti1 is a novel activator of the Ssa proteins

The molecular chaperones Hsp70 and Hsp90 are involved in the folding and maturation of key regulatory proteins in eukaryotes. Of specific importance in this context is a ternary multichaperone complex in which Hsp70 and Hsp90 are connected by Hop. In Saccharomyces cerevisiae two components of the complex, yeast Hsp90 (yHsp90) and Sti1, the yeast homologue of Hop, had already been identified, but it remained to be shown which of the 14 different yeast Hsp70s are part of the Sti1 complex and what were the functional consequences resulting from this interaction. With a two-hybrid approach and co-immunoprecipitations, we show here that Sti1 specifically interacts with the Ssa group of the cytosolic yeast Hsp70 proteins. Using purified components, we reconstituted the dimeric Ssa1-Sti1 complex and the ternary Ssa1-Sti1-yHsp90 complex in vitro. The dissociation constant between Sti1 and Ssa1 was determined to be 2 orders of magnitude weaker than the affinity of Sti1 for yHsp90. Surprisingly, binding of Sti1 activates the ATPase of Ssa1 by a factor of about 200, which is in contrast to the behavior of Hop in the mammalian Hsp70 system. Analysis of the underlying activation mechanism revealed that ATP hydrolysis is rate-limiting in the Ssa1 ATPase cycle and that this step is accelerated by Sti1. Thus, Sti1 is a potent novel effector for the Hsp70 ATPase.