Molecular Mechanism of J-Domain-Triggered ATP Hydrolysis by Hsp70 Chaperones

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Conformational heterogeneity in the Hsp70 chaperone‐substrate ensemble identified from analysis of NMR‐detected titration data

The Hsp70 chaperone system plays a critical role in cellular homeostasis by binding to client protein molecules. We have recently shown by methyl‐TROSY NMR methods that the Escherichia coli Hsp70, DnaK, can form multiple bound complexes with a small client protein, hTRF1. In an effort to characterize the interactions further we report here the results of an NMR‐based titration study of hTRF1 and DnaK, where both molecular components are monitored simultaneously, leading to a binding model. A central finding is the formation of a previously undetected 3:1 hTRF1‐DnaK complex, suggesting that under heat shock conditions, DnaK might be able to protect cytosolic proteins whose net concentrations would exceed that of the chaperone. Moreover, these results provide new insight into the heterogeneous ensemble of complexes formed by DnaK chaperones and further emphasize the unique role of NMR spectroscopy in obtaining information about individual events in a complex binding scheme by exploiting a large number of probes that report uniquely on distinct binding processes.     

A comparison of Förster resonance energy transfer analysis approaches for Nanodrop fluorometry

Ensemble Förster resonance energy transfer (FRET) results can be analyzed in a variety of ways. Due to experimental artifacts, the results obtained from different analysis approaches are not always the same. To determine the optimal analysis approach to use for Nanodrop fluorometry, we have performed both ensemble and single-molecule FRET studies on oligomers of double-stranded DNA. We compared the single-molecule FRET results with those obtained using various ensemble FRET analysis approaches. This comparison shows that for Nanodrop fluorometry, analyzing the increase of the acceptor fluorescence is less likely to introduce errors in estimates of FRET efficiencies compared with analyzing the fluorescence intensity of the donor in the absence and presence of the acceptor.     

Design Principles of FRET‐Based Dye‐Sensitized Solar Cells with Buried Quantum Dot Donors

In this article, the physics of FRET is demonstrated for an architecture of dye‐sensitized solar cells, in which the quantum dot “antennas” that serve as donors are incorporated into the solid titania electrode, providing isolation from electrolyte quenching, and potentially increased photostability. The energy transferred to the dye acceptor from the quantum dot donor, in addition to the direct light absorption by the dye, finally induce dye excitation and electron injection to the metal oxide semiconductor electrode. We use time‐resolved photoluminescence measurements to directly show achievement of FRET efficiencies of up to 70%, corresponding to over 80% internal quantum efficiency when considering radiative energy transfer as well. The various parameters governing the FRET efficiency and the requirements for high efficiency FRET‐based cells are discussed. Since both buried donors inside the electrode and donors solubilized in the electrolyte have both been shown to achieve high energy transfer efficiencies, and as the two methods take advantage of different available volumes of the electrode to introduce donors providing the excess absorption, synergy of the two methods is highly promising for achieving panchromatic absorption within a thin electrode.     

Effective cotranslational folding of firefly luciferase without chaperones of the Hsp70 family

Molecular chaperones of the Hsp70 family (bacterial DnaK, DnaJ, and GrpE) were shown to be strictly required for refolding of firefly luciferase from a denatured state and thus for effective restoration of its activity. At the same time the luciferase was found to be synthesized in an Escherichia coli cell-free translation system in a highly active state in the extract with no chaperone activity. The addition of the chaperones to the extract during translation did not raise the activity of the enzyme. The abrupt arrest of translation by the addition of a translational inhibitor led to immediate cessation of the enzyme activity accumulation, indicating the cotranslational character of luciferase folding. The results presented suggest that the chaperones of the Hsp70 family are not required for effective cotranslational folding of firefly luciferase.     

The disaggregation activity of the mitochondrial ClpB homolog Hsp78 maintains Hsp70 function during heat stress

Molecular chaperones are important components of mitochondrial protein biogenesis and are required to maintain the organellar function under normal and stress conditions. We addressed the functional role of the Hsp100/ClpB homolog Hsp78 during aggregation reactions and its functional cooperation with the main mitochondrial Hsp70, Ssc1, in mitochondria of the yeast Saccharomyces cerevisiae. By establishing an aggregation/disaggregation assay in intact mitochondria we demonstrated that Hsp78 is indispensable for the resolubilization of protein aggregates generated by heat stress under in vivo conditions. The ATP-dependent disaggregation activity of Hsp78 was capable of reversing the preprotein import defect of a destabilized mutant form of Ssc1. This role in disaggregation of Ssc1 is unique for Hsp78, since the recently identified, Hsp70-specific chaperone Zim17 had no effect on the resolubilization reaction. We observed only a minor effect of the second mitochondrial Hsp100 family member Mcx1 on protein disaggregation. A "holding" activity of the mitochondrial Hsp70 system was a prerequisite for a successful resolubilization of aggregated proteins. We conclude that the protective role of Hsp78 in thermotolerance is mainly based on maintaining the molecular chaperone Ssc1 in a soluble and functional state.     

Concerted release of substrate domains from GroEL by ATP is demonstrated with FRET

The chaperonin GroEL assists protein folding by undergoing ATP-induced conformational changes that are concerted within each of its two back-to-back stacked rings. Here we examined whether concerted allosteric switching gives rise to all-or-none release and folding of domains in a chimeric fluorescent protein substrate, CyPet-YPet. Using this substrate, it was possible to determine the folding yield of each domain from its intrinsic fluorescence and that of the entire chimera by measuring Förster resonance energy transfer between the two domains. Hence, it was possible to determine whether release of one domain is accompanied by release of the other domain (concerted mechanism), or whether their release is not coupled. Our results show that the chimera's release tends to be concerted when folding is assisted by a wild-type GroEL variant, but not when assisted by the F44W/D155A mutant that undergoes a sequential allosteric switch. A connection between the allosteric mechanism of this molecular machine and its biological function in assisting folding is thus established.     

Maintenance of structure and function of mitochondrial Hsp70 chaperones requires the chaperone Hep1

Hsp70 chaperones mediate folding of proteins and prevent their misfolding and aggregation. We report here on a new kind of Hsp70 interacting protein in mitochondria, Hep1. Hep1 is a highly conserved protein present in virtually all eukaryotes. Deletion of HEP1 results in a severe growth defect. Cells lacking Hep1 are deficient in processes that need the function of mitochondrial Hsp70s, such as preprotein import and biogenesis of proteins containing FeS clusters. In the mitochondria of these cells, Hsp70s, Ssc1 and Ssq1 accumulate as insoluble aggregates. We show that it is the nucleotide-free form of mtHsp70 that has a high tendency to self-aggregate. This process is efficiently counteracted by Hep1. We conclude that Hep1 acts as a chaperone that is necessary and sufficient to prevent self-aggregation and to thereby maintain the function of the mitochondrial Hsp70 chaperones.     

Quantitative time domain analysis of lifetime‐based Förster resonant energy transfer measurements with fluorescent proteins: Static random isotropic fluorophore orientation distributions

Förster resonant energy transfer (FRET) measurements are widely used to obtain information about molecular interactions and conformations through the dependence of FRET efficiency on the proximity of donor and acceptor fluorophores. Fluorescence lifetime measurements can provide quantitative analysis of FRET efficiency and interacting population fraction. Many FRET experiments exploit the highly specific labelling of genetically expressed fluorescent proteins, applicable in live cells and organisms. Unfortunately, the typical assumption of fast randomization of fluorophore orientations in the analysis of fluorescence lifetime‐based FRET readouts is not valid for fluorescent proteins due to their slow rotational mobility compared to their upper state lifetime. Here, previous analysis of effectively static isotropic distributions of fluorophore dipoles on FRET measurements is incorporated into new software for fitting donor emission decay profiles. Calculated FRET parameters, including molar population fractions, are compared for the analysis of simulated and experimental FRET data under the assumption of static and dynamic fluorophores and the intermediate regimes between fully dynamic and static fluorophores, and mixtures within FRET pairs, is explored. Finally, a method to correct the artefact resulting from fitting the emission from static FRET pairs with isotropic angular distributions to the (incorrect) typically assumed dynamic FRET decay model is presented.      

Immunomodulatory activity of extracellular Hsp70 mediated via paired receptors Siglec‐5 and Siglec‐14

The intracellular chaperone heat‐shock protein 70 (Hsp70) can be secreted from cells, but its extracellular role is unclear, as the protein has been reported to both activate and suppress the innate immune response. Potential immunomodulatory receptors on myelomonocytic lineage cells that bind extracellular Hsp70 are not well defined. Siglecs are Ig‐superfamily lectins on mammalian leukocytes that recognize sialic acid‐bearing glycans and thereby modulate immune responses. Siglec‐5 and Siglec‐14, expressed on monocytes and neutrophils, share identical ligand‐binding domains but have opposing signaling functions. Based on phylogenetic analyses of these receptors, we predicted that endogenous sialic acid‐independent ligands should exist. An unbiased screen revealed Hsp70 as a ligand for Siglec‐5 and Siglec‐14. Hsp70 stimulation through Siglec‐5 delivers an anti‐inflammatory signal, while stimulation through Siglec‐14 is pro‐inflammatory. The functional consequences of this interaction are also addressed in relation to a SIGLEC14 polymorphism found in humans. Our results demonstrate that an endogenous non‐sialic acid‐bearing molecule can be either a danger‐associated or self‐associated signal through paired Siglecs, and may explain seemingly contradictory prior reports on extracellular Hsp70 action.     

Rapamycin conditionally inhibits Hsp90 but not Hsp70 mRNA translation in <Emphasis Type="Italic">Drosophila</Emphasis>: implications for the mechanisms of Hsp mRNA translation

Rapamycin inhibits the activity of the target of rapamycin (TOR)-dependent signaling pathway, which has been characterized as one dedicated to translational regulation through modulating cap-dependent translation, involving eIF4E binding protein (eIF4E-BP) or 4E-BP. Results show that rapamycin strongly inhibits global translation in Drosophila cells. However, Hsp70 mRNA translation is virtually unaffected by rapamycin treatment, whereas Hsp90 mRNA translation is strongly inhibited, at normal growth temperature. Intriguingly, during heat shock Hsp90 mRNA becomes significantly less sensitive to rapamycin-mediated inhibition, suggesting the pathway for Hsp90 mRNA translation is altered during heat shock. Reporter mRNAs containing the Hsp90 or Hsp70 mRNAs’ 5′ untranslated region recapitulate these rapamycin-dependent translational characteristics, indicating this region regulates rapamycin-dependent translational sensitivity as well as heat shock preferential translation. Surprisingly, rapamycin-mediated inhibition of Hsp90 mRNA translation at normal growth temperature is not caused by 4E-BP-mediated inhibition of cap-dependent translation. Indeed, no evidence for rapamycin-mediated impaired eIF4E function is observed. These results support the proposal that preferential translation of different Hsp mRNA utilizes distinct translation mechanisms, even within a single species.     

Hsp70核苷酸结合域突变体影响酶活的分子动力学模拟研究

分子伴侣蛋白Hsp70氮端核苷酸结合域（NBD, nucleotide-binding domain）的ATP酶活性变化对其行使分子伴侣功能具有重要作用。本文采用分子动力学模拟方法研究酵母分子伴侣Hsp70氮端NBD内残基A17,R23,G32和R167点突变对其ATP酶活性区域构象影响及功能关系。结果表明,突变体A17V,T23H,G32S的ATP结合口袋袋口的loopl（第一个转角,连接p1与p2）结构柔性增强,活性残基T11侧链明显向内移动,从而更加接近ATP的γ-磷酸基团,更容易使ATP水解。这可能蕞终导致ATP酶活性增强,从而引起分子伴侣功能的变化。     

A molecular imaging analysis of Cx43 association with Cdo during skeletal myoblast differentiation

Cell‐to‐cell contacts are crucial for cell differentiation. The promyogenic cell surface protein, Cdo, functions as a component of multiprotein clusters to mediate cell adhesion signaling. Connexin 43, the main connexin forming gap junctions, also plays a key role in myogenesis. At least part of its effects is independent of the intercellular channel function, but the mechanisms underlying are unknown. Here, using multiple optical approaches, we provided the first evidence that Cx43 physically interacts with Cdo to form dynamic complexes during myoblast differentiation, offering clues for considering this interaction a structural basis of the channel‐independent function of Cx43. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermal adaptation of the yeast mitochondrial Hsp70 system is regulated by the reversible unfolding of its nucleotide exchange factor

The Hsp70 protein switches during its functional cycle from an ADP-bound state with a high affinity for substrates to a low-affinity, ATP-bound state, with concomitant release of the client protein. The rate of the chaperone cycle is regulated by co-chaperones such as nucleotide exchange factors that significantly accelerate the ADP/ATP exchange. Mge1p, a mitochondrial matrix protein with homology to bacterial GrpE, serves as the nucleotide exchange factor of mitochondrial Hsp70. Here, we analyze the influence of temperature on the structure and functional properties of Mge1p from the yeast Saccharomyces cerevisiae. Mge1p is a dimer in solution that undergoes a reversible thermal transition at heat-shock temperatures, i.e. above 37 degrees C, that involves protein unfolding and dimer dissociation. The thermally denatured protein is unable to interact stably with mitochondrial Hsp70, and therefore is unable to regulate its ATPase and chaperone cycle. Crosslinking of wild-type mitochondria reveals that Mge1p undergoes the same dimer to monomer temperature-dependent shift, and that the nucleotide exchange factor does not associate with its Hsp70 partner at stress temperatures (i.e. > or =45 degrees C). Once the stress conditions disappear, Mge1p refolds and recovers both structure and functional properties. Therefore, Mge1p can act as a thermosensor for the mitochondrial Hsp70 system, regulating the nucleotide exchange rates under heat shock, as has been described for two bacterial GrpE proteins. The thermosensor activity is conserved in the GrpE-like nucleotide exchange factors although, as discussed here, it is achieved through a different structural mechanism.     

Förster resonance energy transfer measurements of cofactor‐dependent effects on protein arginine N‐methyltransferase homodimerization

Protein arginine N‐methyltransferase (PRMT) dimerization is required for methyl group transfer from the cofactor S‐adenosyl‐L ‐methionine (AdoMet) to arginine residues in protein substrates, forming S‐adenosyl‐L ‐homocysteine (AdoHcy) and methylarginine residues. In this study, we use Förster resonance energy transfer (FRET) to determine dissociation constant (K_D) values for dimerization of PRMT1 and PRMT6. By attaching monomeric Cerulean and Citrine fluorescent proteins to their N‐termini, fluorescent PRMTs are formed that exhibit similar enzyme kinetics to unconjugated PRMTs. These fluorescent proteins are used in FRET‐based binding studies in a multi‐well format. In the presence of AdoMet, fluorescent PRMT1 and PRMT6 exhibit 4‐ and 6‐fold lower dimerization K_D values, respectively, than in the presence of AdoHcy, suggesting that AdoMet promotes PRMT homodimerization in contrast to AdoHcy. We also find that the dimerization K_D values for PRMT1 in the presence of AdoMet or AdoHcy are, respectively, 6‐ and 10‐fold lower than the corresponding values for PRMT6. Considering that the affinity of PRMT6 for AdoHcy is 10‐fold higher than for AdoMet, PRMT6 function may be subject to cofactor‐dependent regulation in cells where the methylation potential (i.e., ratio of AdoMet to AdoHcy) is low. Since PRMT1 affinity for AdoMet and AdoHcy is similar, however, a low methylation potential may not affect PRMT1 function.     

Structure of a NADPH-dependent blue fluorescent protein revealed the unique role of Gly176 on the fluorescence enhancement

A NADPH-dependent blue fluorescent protein from Vibrio vulnificus CKM-1 (BFPvv) emits blue fluorescence under UV-exposure. Previously, the BFPvvD7 mutant generated by directed evolution displayed a fourfold enhancement in fluorescent intensity. Herein, a further increase in fluorescence in the new BFPvvD8 mutant, with three additional mutations from BFPvvD7, was made. To understand the underlying mechanism of the increased fluorescent intensity of BFPvv, we solved the BFPvvD8-NADPH complex structure. Accompanied with lifetime detection, we proposed that the enhanced intensity is related to the conformational change caused by a glycine residue (Gly176) mutated to other non-glycine residues at a turn close to the NADPH binding site. We also observed the F?rster resonance energy transfer (FRET) from our BFPvvD8 to each of the GFP-like fluorescent proteins, mTFP1 and EGFP, joined by an eight-residue linker between the N-terminal of BFPvvD8 and the C-terminal of GFPs. Taken together, with the newly solved BFPvvD8 structure, our results not only provide new considerations within the rational-based protein engineering of this NADPH-dependent BFP, but also suggest that BFPvvD8 could be a potential candidate in FRET-based biosensor techniques.     

Evolving paradigms on the interplay of mitochondrial Hsp70 chaperone system in cell survival and senescence

Abstract

The role of mitochondria within a cell has grown beyond being the prime source of cellular energy to one of the major signaling platforms. Recent evidence provides several insights into the crucial roles of mitochondrial chaperones in regulating the organellar response to external triggers. The mitochondrial Hsp70 (mtHsp70/Mortalin/Grp75) chaperone system plays a critical role in the maintenance of proteostasis balance in the organelle. Defects in mtHsp70 network result in attenuated protein transport and misfolding of polypeptides leading to mitochondrial dysfunction. The functions of Hsp70 are primarily governed by J-protein cochaperones. Although human mitochondria possess a single Hsp70, its multifunctionality is characterized by the presence of multiple specific J-proteins. Several studies have shown a potential association of Hsp70 and J-proteins with diverse pathological states that are not limited to their canonical role as chaperones. The role of mitochondrial Hsp70 and its co-chaperones in disease pathogenesis has not been critically reviewed in recent years. We evaluated some of the cellular interfaces where Hsp70 machinery associated with pathophysiological conditions, particularly in context of tumorigenesis and neurodegeneration. The mitochondrial Hsp70 machinery shows a variable localization and integrates multiple components of the cellular processes with varied phenotypic consequences. Although Hsp70 and J-proteins function synergistically in proteins folding, their precise involvement in pathological conditions is mainly idiosyncratic. This machinery is associated with a heterogeneous set of molecules during the progression of a disorder. However, the precise binding to the substrate for a specific physiological response under a disease subtype is still an undocumented area of analysis.     

The Hsp70 chaperones of the Tritryps are characterized by unusual features and novel members

Proteins belonging to the Hsp70 class of molecular chaperones are highly conserved and ubiquitous, performing an essential role in the maintenance of cellular homeostasis in almost all known organisms. Trypanosoma brucei, Trypanosoma cruzi and Leishmania major are human parasites collectively known as the Tritryps. The Tritryps undergo extensive morphological changes during their life cycles, largely triggered by the marked differences between conditions in their insect vector and human host. Hsp70s are synthesised in response to these marked changes in environment and are proposed to be required for these parasites to successfully transition between differentiation stages while remaining viable and infective. While the Tritryps Hsp70 complement consists of homologues of all the major eukaryotic Hsp70s, there are a number of novel members, and some unique structural features. This review critically evaluates the current knowledge on the Tritryps Hsp70 proteins with an emphasis on T. brucei, and highlights some novel and previously unstudied aspects of these multifaceted molecular chaperones.