Interaction between huntingtin exon 1 and HEAT repeat structure probed by chimeric model proteins

Huntington disease (HD) is associated with aggregation of huntingtin (HTT) protein containing over 35 continuous Q residues within the N-terminal exon 1 encoded region. The C-terminal of the HTT protein consists mainly of HEAT repeat structure which serves as a scaffold for multiple cellular activities. Structural and biochemical analysis of the intact HTT protein has been hampered by its huge size (~300 kDa) and most in vitro studies to date have focused on the properties of the exon 1 region. To explore the interaction between HTT exon 1 and the HEAT repeat structure, we constructed chimeric proteins containing the N-terminal HTT exon 1 region and the HEAT repeat protein PR65/A. The results indicate that HTT exon 1 slightly destabilizes the downstream HEAT repeat structure and endows the HEAT repeat structure with more conformational flexibility. Wild-type and pathological lengths of polyQ did not show differences in the interaction between HTT exon 1 and the HEAT repeats. With the C-terminal fusion of PR65/A, HTT exon 1 containing pathological lengths of polyQ could still form amyloid fibrils, but the higher-order architecture of fibrils and kinetics of fibril formation were affected by the C-terminal fusion of HEAT repeats. This indicates that interaction between HTT exon 1 and HEAT repeat structure is compatible with both normal function of HTT protein and the pathogenesis of HD, and this study provides a potential model for further exploration.

Methylene blue accelerates liquid-to-gel transition of tau condensates impacting tau function and pathology

Preventing tau aggregation is a potential therapeutic strategy in Alzheimer's disease and other tauopathies. Recently, liquid-liquid phase separation has been found to facilitate the formation of pathogenic tau conformations and fibrillar aggregates, although many aspects of the conformational transitions of tau during the phase transition process remain unknown. Here, we demonstrate that the tau aggregation inhibitor methylene blue promotes tau liquid-liquid phase separation and accelerates the liquid-to-gel transition of tau droplets independent of the redox activity of methylene blue. We further show that methylene blue inhibits the conversion of tau droplets into fibrils and reduces the cytotoxicity of tau aggregates. Although gelation slows down the mobility of tau and tubulin, it does not impair microtubule assembly within tau droplets. These findings suggest that methylene blue inhibits tau amyloid fibrillization and accelerates tau droplet gelation via distinct mechanisms, thus providing insights into the activity of tau aggregation inhibitors in the context of phase transition.

Reversible Redox-Dependent Conformational Switch of the C-Terminal α-Helical Lid of Human Hsp70 Observed by In-Cell NMR

Glutathionylation of human stress-inducible Hsp70 (hHsp70) under oxidative stress conditions has been suggested to act as an on/off switch of hHsp70 chaperone activity and thus transfer redox signals to hHsp70 clients through a change in conformation. The mechanism of this switch involves unfolding of the C-terminal α-helical lid, SBDα, upon glutathionylation, which then binds to and blocks the hHsp70 substrate-binding site. This process is reversible and redox-regulated and has been demonstrated for purified protein in solution. Here, we found that this redox-regulated reversible process also occurs in the cellular environment. Using Escherichia coli as a model system, in-cell NMR data clearly indicate that hHsp70 SBDα undergoes a conformational transition from ordered to disordered after diamide stimulation. The disordered SBDα could spontaneously recover back to the helix bundle conformation over time. This oxidative-stress induced process also occurred in cell lysate, with a similar unfolding rate as in cells, but the refolding rate was significantly slower in cell lysate. Increased temperature accelerates this process. Under heat stress alone, unfolding of the SBDα could not be detected in cells. Our in-cell NMR results provide direct support for the molecular switch model of hHsp70 redox regulation and also demonstrate the power of in-cell NMR for real-time study of protein structures during biological processes in living cells.

Effect of evolution of the C-terminal region on chaperone activity of Hsp70

Dynamic interdomain interactions within the Hsp70 protein is the basis for the allosteric and functional properties of Hsp70s. While Hsp70s are generally conserved in terms of structure, allosteric behavior, and some overlapping functions, Hsp70s also contain variable sequence regions which are related to distinct functions. In the Hsp70 sequence, the part with the greatest sequence variation is the C-terminal α-helical lid subdomain of substrate-binding domain (SBDα) together with the intrinsically disordered region. Dynamic interactions between the SBDα and β-sandwich substrate-binding subdomain (SBDβ) contribute to the chaperone functions of Hsp70s by tuning kinetics of substrate binding. To investigate how the C-terminal region of Hsp70 has evolved from prokaryotic to eukaryotic organisms, we tested whether this region can be exchanged among different Hsp70 members to support basic chaperone functions. We found that this region from eukaryotic Hsp70 members cannot substitute for the same region in Escherichia coli DnaK to facilitate normal chaperone activity of DnaK. In contrast, this region from E. coli DnaK and Saccharomyces cerevisiae Hsp70 (Ssa1 and Ssa4) can partially support some roles of human stress inducible Hsp70 (hHsp70) and human cognate Hsp70 (hHsc70). Our results indicate that the C-terminal region from eukaryotic Hsp70 members cannot properly support SBDα-SBDβ interactions in DnaK, but this region from DnaK/Ssa1/Ssa4 can still form some SBDα-SBDβ interactions in hHsp70 or hHsc70, which suggests that the mode for SBDα-SBDβ interactions is different in prokaryotic and eukaryotic Hsp70 members. This study provides new insight in the divergency among different Hsp70 homologs and the evolution of Hsp70s.

Exploration of the cysteine reactivity of human inducible Hsp70 and cognate Hsc70

Hsp70s are multifunctional proteins and serve as the central hub of the protein quality control network. Hsp70s are also related to a number of diseases and have been established as drug targets. Human HspA1A (hHsp70) and HspA8 (hHsc70) are the major cytosolic Hsp70s, and they have both overlapping and distinct functions. hHsp70 contains five cysteine residues, and hHsc70 contains four cysteine residues. Previous studies have shown these cysteine residues can undergo different cysteine modifications such as oxidation or reaction with electrophiles to regulate their function, and hHsp70 and hHsc70 have different cysteine reactivity. To address the mechanism of the differences in cysteine reactivity between hHsp70 and hHsc70, we studied the factors that determine this reactivity by Ellman assay for the quantification of accessible free thiols and NMR analysis for the assessment of structural dynamics. We found the lower cysteine reactivity of hHsc70 is probably due to its lower structural dynamics and the stronger inhibition effect of interaction between the α-helical lid subdomain of the substrate-binding domain (SBDα) and the β-sheet substrate-binding subdomain (SBDβ) on cysteine reactivity of hHsc70. We determined that Gly557 in hHsp70 contributes significantly to the higher structural dynamics and cysteine reactivity of hHsp70 SBDα. Exploring the cysteine reactivity of hHsp70 and hHsc70 facilitates an understanding of the effects of redox reactions and electrophiles on their chaperone activity and regulation mechanisms, and how these differences allow them to undertake distinct cellular roles.

Distinct lipid membrane-mediated pathways of Tau assembly revealed by single-molecule analysis

The conversion of intrinsically disordered Tau to highly ordered amyloid aggregates is associated with a wide range of neurodegenerative diseases termed tauopathies. The presence of lipid bilayer membranes is a critical factor that accelerates the abnormal aggregation of Tau protein. However, the lipid membrane-induced conformational changes of Tau and the mechanism for the accelerated fibrillation remain elusive. In this study, single-molecule Förster resonance energy transfer (smFRET) and fluorescence correlation spectroscopy (FCS) were applied to detect the conformational changes and intermolecular interactions of full-length Tau in the presence of different concentrations of 1,2-dimyristoyl-sn-glycero-3-phosphatidylserine (DMPS) vesicles. The results show that the conformation of Tau becomes expanded with opening of the N-terminal and C-terminal domains of Tau upon binding to DMPS. At low DMPS concentrations, Tau forms oligomers with a partially extended conformation which facilitates the amyloid fibrillization process. At high DMPS concentrations, Tau monomer binds to lipid membranes in a fully expanded conformation at low density thus inhibiting intermolecular aggregation. Our study reveals the underlying mechanisms by which lipid membranes influence amyloid formation of Tau, providing a foundation for further understanding of the pathogenesis and physiology of the interplay between Tau protein and lipid membranes.

PES derivative PESA is a potent tool to globally profile cellular targets of PES

PES (2-phenylethynesulfonamide, pifithrin-μ, PFTμ) is an electrophilic compound that exhibits anticancer properties, protects against chemotherapy-induced peripheral neuropathy in chemotherapy, and shows immunomodulatory, anti-inflammatory and anti-viral activities. PES generally shows higher cytotoxicity towards tumor cells than non-tumor cells. The mechanism of action of PES is unclear but may involve the covalent modification of proteins as PES has been found to be a covalent inhibitor of Hsp70. We developed a new PES derivative PESA with a terminal alkynyl group to perform click-reaction-assisted activity-based protein profiling (click-reaction ABPP) and used this to screen for cellular targets of PES. We found PES and its derivatives PES-Cl and PESA have comparable ability to undergo a Michael addition reaction with GSH and Hsp70, and showed similar cytotoxicity. By fluorescence imaging and proteomics studies we identified over 300 PESA-attached proteins in DOHH2 cells. Some proteins involved in cancer-related redox processes, such as peroxiredoxin 1 (PRDX1), showed higher frequency and abundance in mass spectrometry detection. Our results suggest that cytotoxicity of PES and its derivatives may be related to attack of protein thiols and cellular GSH resulting in breakdown of cellular redox homeostasis. This study provides a powerful new tool compound within the PES class of bioactive compounds and gives insight into the working mechanisms of PES and its derivatives.

Conformational Expansion of Tau in Condensates Promotes Irreversible Aggregation

Liquid-liquid phase separation (LLPS) of proteins into biomolecular condensates has emerged as a fundamental principle underpinning cellular function and malfunction. Indeed, many human pathologies, including protein misfolding diseases, are linked to aberrant liquid-to-solid phase transitions, and disease-associated protein aggregates often nucleate through phase separation. The molecular level determinants that promote pathological phase transitions remain, however, poorly understood. Here we study LLPS of the microtubule-associated protein Tau, whose aberrant aggregation is associated with a number of neurodegenerative diseases, including Alzheimer's disease. Using single molecule spectroscopy, we probe directly the conformational changes that the protein undergoes as a result of LLPS. We perform single-molecule FRET and fluorescence correlation spectroscopy experiments to monitor the intra- and intermolecular changes and demonstrate that the N- and C-terminal regions of Tau become extended, thus exposing the microtubule-binding region. These changes facilitate intermolecular interactions and allow for the formation of nanoscale clusters of Tau. Our results suggest that these clusters can promote the fibrillization of Tau, which can be dramatically accelerated by disease-related mutations P301L and P301S. Our findings thus provide important molecular insights into the mechanism of protein phase separation and the conversion of protein condensates from functional liquid assemblies to pathological aggregates.

Structural basis for the DNA-binding activity of human ARID4B Tudor domain

Human ARID4A and ARID4B are homologous proteins that are important in controlling gene expression and epigenetic regulation but have distinct functions. Previous studies have shown that the N-terminal domain of ARID4A is an unusual interdigitated double Tudor domain with DNA-binding activity. However, how the Tudor domain of ARID4B differs from that of ARID4A remains unknown. Here, we found that the ARID4B Tudor domain has significantly weaker DNA affinity than the ARID4A Tudor domain despite sharing more than 80% sequence identity. Structure determination and DNA titration analysis indicated that the ARID4B Tudor domain is also an interdigitated double Tudor domain with a DNA-binding surface similar to ARID4A. We identified a residue close to the DNA-binding site of the Tudor domain that differs between ARID4A and ARID4B. The Leu50 in ARID4A is Glu50 in ARID4B, and the latter forms salt bridges with two lysine residues at the DNA-binding surface. This causes a decrease in the strength of positive charge, thus reducing DNA-binding affinity while significantly increasing protein stability. We also found that a C-terminal extension region enhances the DNA-binding affinity of the ARID4B Tudor domain. This C-terminal extension is disordered and contains a positively charged RGR motif, providing an additional DNA-binding site. Finally, sequence and phylogenetic analyses indicated that the residue differences and the presence of the RGR extension region are conserved. These results provide new insight into the functional differences between ARID4A and ARID4B proteins, as well as elucidating the function of the disordered regions in these proteins.

Mutational analysis of the Hsp70 substrate-binding domain: Correlating molecular-level changes with in vivo function

Hsp70 is an evolutionarily conserved chaperone involved in maintaining protein homeostasis during normal growth and upon exposure to stresses. Mutations in the β6/β7 region of the substrate-binding domain (SBD) disrupt the SBD hydrophobic core resulting in impairment of the heat-shock response and prion propagation in yeast. To elucidate the mechanisms behind Hsp70 loss of function due to disruption of the SBD, we undertook targeted mutational analysis of key residues in the β6/β7 region. We demonstrate the critical functional role of the F475 residue across yeast cytosolic Hsp70-Ssa family. We identify the size of the hydrophobic side chain at 475 as the key factor in maintaining SBD stability and functionality. The introduction of amino acid variants to either residue 475, or close neighbor 483, caused instability and cleavage of the Hsp70 SBD and subsequent degradation. Interestingly, we found that Hsp70-Ssa cleavage may occur through a vacuolar carboxypeptidase (Pep4)-dependent mechanism rather than proteasomal. Mutations at 475 and 483 result in compromised ATPase function, which reduces protein re-folding activity and contributes to depletion of cytosolic Hsp70 in vivo. The combination of reduced functionality and stability of Hsp70-Ssa results in yeast cells that are compromised in their stress response and cannot propagate the [PSI⁺ ] prion.

PES inhibits human-inducible Hsp70 by covalent targeting of cysteine residues in the substrate-binding domain

Hsp70 proteins are a family of ancient and conserved chaperones. They play important roles in vital cellular processes, such as protein quality control and the stress response. Hsp70 proteins are a potential drug target for treatment of disease, particularly cancer. PES (2-phenylethynesulfonamide or pifithrin-μ) has been reported to be an inhibitor of Hsp70. However, the mechanism of PES inhibition is still unclear. In this study we found that PES can undergo a Michael addition reaction with Cys-574 and Cys-603 in the SBDα of human HspA1A (hHsp70), resulting in covalent attachment of a PES molecule to each Cys residue. We previously showed that glutathionylation of Cys-574 and Cys-603 affects the structure and function of hHsp70. In this study, PES modification showed similar structural and functional effects on hHsp70 to glutathionylation. Further, we found that susceptibility to PES modification is influenced by changes in the conformational dynamics of the SBDα, such as are induced by interaction with adjacent domains, allosteric changes, and mutations. This study provides new avenues for development of covalent inhibitors of hHsp70.

The Five Nations model for prison health surveillance: lessons from practice across the UK and Republic of Ireland

Background

Prison populations experience an increased burden of physical, mental and social health needs compared to the community, further impacted by the prison environment. Surveillance systems to monitor health and well-being trends in prisons are lacking, presenting a challenge to services planners, and policy makers who often lack evidence to inform decisions.

Method

The Five Nations Health and Justice Collaboration, a body of experts on prison health across the UK and Republic of Ireland (ROI), met to share and discuss challenges and opportunities to developing robust prison health surveillance systems that could inform local provision, guide national policy and enable cross-border comparisons.

Results

Challenges to robust prison health surveillance systems were shared across the UK and ROI. Methods of surveillance differed across nations and included performance indicators and outcome measures as part of local or national programs. All nations had strong public health infectious disease notification systems.

Conclusions

The Five Nations Health and Justice Collaboration is proposing a new model for prison health surveillance, based on established guidelines for public health surveillance but with additional features that recognize the uniqueness of the prison environment and need for a whole prison approach, built on collaboration and sharing of data between health and justice sectors.

Discovery and mechanism of a pH-dependent dual-binding-site switch in the interaction of a pair of protein modules

Many important proteins undergo pH-dependent conformational changes resulting in "on-off" switches for protein function, which are essential for regulation of life processes and have wide application potential. Here, we report a pair of cellulosomal assembly modules, comprising a cohesin and a dockerin from Clostridium acetobutylicum, which interact together following a unique pH-dependent switch between two functional sites rather than on-off states. The two cohesin-binding sites on the dockerin are switched from one to the other at pH 4.8 and 7.5 with a 180° rotation of the bound dockerin. Combined analysis by nuclear magnetic resonance spectroscopy, crystal structure determination, mutagenesis, and isothermal titration calorimetry elucidates the chemical and structural mechanism of the pH-dependent switching of the binding sites. The pH-dependent dual-binding-site switch not only represents an elegant example of biological regulation but also provides a new approach for developing pH-dependent protein devices and biomaterials beyond an on-off switch for biotechnological applications.

Amelioration of aggregate cytotoxicity by catalytic conversion of protein oligomers into amyloid fibrils

The aggregation of peptides and proteins into amyloid fibrils is a molecular self-assembly phenomenon associated with both biological function and malfunction, notably in the context of neurodegenerative diseases. Oligomeric species formed early in the aggregation process are generally associated with cytotoxicity. Extrinsic molecules such as peptides have been found to influence amyloid formation kinetics and regulate this cellular process. Here, we use single-molecule FRET and bulk assays combined with global kinetic analysis to study quantitatively the effect of an 8-residue peptide (LQVNIGNR) on fibril formation by the yeast prion protein Ure2. This peptide, which is derived from a segment of the Ure2 prion domain, forms vesicular assemblies that accelerate fibril formation of Ure2 by promoting conformational conversion of oligomeric intermediates into fibrillar species in a catalytic manner. This reduces oligomer longevity and consequently ameliorates cytotoxicity. The LQVNIGNR peptide was found to accelerate fibril formation of unrelated proteins including Tau and α-Synuclein, suggesting a general ability to catalyse fibrillation. This study provides a general strategy for investigating the microscopic mechanism of extrinsic factors on amyloid aggregation. This approach can readily be applied to other amyloid systems and demonstrates that acceleration of oligomer conversion is a promising strategy to reduce amyloid toxicity.

S-Glutathionylation of human inducible Hsp70 reveals a regulatory mechanism involving the C-terminal α-helical lid

Heat shock protein 70 (Hsp70) proteins are a family of ancient and conserved chaperones. Cysteine modifications have been widely detected among different Hsp70 family members in vivo, but their effects on Hsp70 structure and function are unclear. Here, we treated HeLa cells with diamide, which typically induces disulfide bond formation except in the presence of excess GSH, when glutathionylated cysteines predominate. We show that in these cells, HspA1A (hHsp70) undergoes reversible cysteine modifications, including glutathionylation, potentially at all five cysteine residues. In vitro experiments revealed that modification of cysteines in the nucleotide-binding domain of hHsp70 is prevented by nucleotide binding but that Cys-574 and Cys-603, located in the C-terminal α-helical lid of the substrate-binding domain, can undergo glutathionylation in both the presence and absence of nucleotide. We found that glutathionylation of these cysteine residues results in unfolding of the α-helical lid structure. The unfolded region mimics substrate by binding to and blocking the substrate-binding site, thereby promoting intrinsic ATPase activity and competing with binding of external substrates, including heat shock transcription factor 1 (Hsf1). Thus, post-translational modification can alter the structure and regulate the function of hHsp70.

Distinct microscopic mechanisms for the accelerated aggregation of pathogenic Tau mutants revealed by kinetic analysis

Relating chemical kinetics and physical properties to pathogenicity of disease-related Tau mutants.

Rapid deacetylation of yeast Hsp70 mediates the cellular response to heat stress

Hsp70 is a highly conserved molecular chaperone critical for the folding of new and denatured proteins. While traditional models state that cells respond to stress by upregulating inducible HSPs, this response is relatively slow and is limited by transcriptional and translational machinery. Recent studies have identified a number of post-translational modifications (PTMs) on Hsp70 that act to fine-tune its function. We utilized mass spectrometry to determine whether yeast Hsp70 (Ssa1) is differentially modified upon heat shock. We uncovered four lysine residues on Ssa1, K86, K185, K354 and K562 that are deacetylated in response to heat shock. Mutation of these sites cause a substantial remodeling of the Hsp70 interaction network of co-chaperone partners and client proteins while preserving essential chaperone function. Acetylation/deacetylation at these residues alter expression of other heat-shock induced chaperones as well as directly influencing Hsf1 activity. Taken together our data suggest that cells may have the ability to respond to heat stress quickly though Hsp70 deacetylation, followed by a slower, more traditional transcriptional response.

Resonance assignments for the tandem PWWP-ARID domains of human RBBP1

Retinoblastoma-binding protein 1 (RBBP1), also known as AT-rich interaction domain 4A (ARID4A), is a tumour suppressor involved in the regulation of the epigenetic programming in leukemia and Prader-Willi/Angelman syndromes. The ARID domain of RBBP1 binds to DNA non-specifically and has gene suppression activity. However, no structural data has been obtained for the human RBBP1 ARID domain so far. Here we report the near-complete ¹H, ¹³C, ¹⁵N backbone and side-chain NMR assignment of a 27 kDa tandem PWWP-ARID domain construct that spans residues 171-414 with the removal of a short disordered region between the two domains. The predicted secondary structure based on the assigned chemical shifts is consistent with the structures of the isolated PWWP domain of human RBBP1 previously solved and the homologous ARID domains of other proteins.

The C-terminal GGAP motif of Hsp70 mediates substrate recognition and stress response in yeast

The allosteric coupling of the highly conserved nucleotide- and substrate-binding domains of Hsp70 has been studied intensively. In contrast, the role of the disordered, highly variable C-terminal region of Hsp70 remains unclear. In many eukaryotic Hsp70s, the extreme C-terminal EEVD motif binds to the tetratricopeptide-repeat domains of Hsp70 co-chaperones. Here, we discovered that the TVEEVD sequence of Saccharomyces cerevisiae cytoplasmic Hsp70 (Ssa1) functions as a SUMO-interacting motif. A second C-terminal motif of ∼15 amino acids between the α-helical lid and the extreme C terminus, previously identified in bacterial and eukaryotic organellar Hsp70s, is known to enhance chaperone function by transiently interacting with folding clients. Using structural analysis, interaction studies, fibril formation assays, and in vivo functional assays, we investigated the individual contributions of the α-helical bundle and the C-terminal disordered region of Ssa1 in the inhibition of fibril formation of the prion protein Ure2. Our results revealed that although the α-helical bundle of the Ssa1 substrate-binding domain (SBDα) does not directly bind to Ure2, the SBDα enhances the ability of Hsp70 to inhibit fibril formation. We found that a 20-residue C-terminal motif in Ssa1, containing GGAP and GGAP-like tetrapeptide repeats, can directly bind to Ure2, the Hsp40 co-chaperone Ydj1, and α-synuclein, but not to the SUMO-like protein SMT3 or BSA. Deletion or substitution of the Ssa1 GGAP motif impaired yeast cell tolerance to temperature and cell-wall damage stress. This study highlights that the C-terminal GGAP motif of Hsp70 is important for substrate recognition and mediation of the heat shock response.

The same but different: the role of Hsp70 in heat shock response and prion propagation

The Hsp70 chaperone machinery is a key component of the heat-shock response and a modulator of prion propagation in yeast. A major factor in optimizing Hsp70 function is the highly coordinated activities of the nucleotide-binding and substrate-binding domains of the protein. Hsp70 inter-domain communication occurs through a bidirectional allosteric interaction network between the two domains. Recent findings identified the β6/β7 region of the substrate-binding domain as playing a critical role in optimizing Hsp70 function in both the stress response and prion propagation and highlighted the allosteric interaction interface between the domains. Importantly, while functional changes in Hsp70 can result in phenotypic consequences for both the stress response and prion propagation, there can be significant differences in the levels of phenotypic impact that such changes illicit.

The β6/β7 region of the Hsp70 substrate-binding domain mediates heat-shock response and prion propagation

Hsp70 is a highly conserved chaperone that in addition to providing essential cellular functions and aiding in cell survival following exposure to a variety of stresses is also a key modulator of prion propagation. Hsp70 is composed of a nucleotide-binding domain (NBD) and substrate-binding domain (SBD). The key functions of Hsp70 are tightly regulated through an allosteric communication network that coordinates ATPase activity with substrate-binding activity. How Hsp70 conformational changes relate to functional change that results in heat shock and prion-related phenotypes is poorly understood. Here, we utilised the yeast [PSI +] system, coupled with SBD-targeted mutagenesis, to investigate how allosteric changes within key structural regions of the Hsp70 SBD result in functional changes in the protein that translate to phenotypic defects in prion propagation and ability to grow at elevated temperatures. We find that variants mutated within the β6 and β7 region of the SBD are defective in prion propagation and heat-shock phenotypes, due to conformational changes within the SBD. Structural analysis of the mutants identifies a potential NBD:SBD interface and key residues that may play important roles in signal transduction between domains. As a consequence of disrupting the β6/β7 region and the SBD overall, Hsp70 exhibits a variety of functional changes including dysregulation of ATPase activity, reduction in ability to refold proteins and changes to interaction affinity with specific co-chaperones and protein substrates. Our findings relate specific structural changes in Hsp70 to specific changes in functional properties that underpin important phenotypic changes in vivo. A thorough understanding of the molecular mechanisms of Hsp70 regulation and how specific modifications result in phenotypic change is essential for the development of new drugs targeting Hsp70 for therapeutic purposes.

A co-expression strategy to achieve labeling of individual subunits within a dimeric protein for single molecule analysis

A generic co-expression strategy for site-specific incorporation of a single donor-acceptor dye pair into any position in a dimeric protein, allowing single molecule FRET study of proteins previously inaccessible to this technique, such as the intrinsically disordered prion N-domain of Ure2 in the context of its globular dimeric C-domain.

Direct Observation of Oligomerization by Single Molecule Fluorescence Reveals a Multistep Aggregation Mechanism for the Yeast Prion Protein Ure2

The self-assembly of polypeptides into amyloid structures is associated with a range of increasingly prevalent neurodegenerative diseases as well as with a select set of functional processes in biology. The phenomenon of self-assembly results in species with dramatically different sizes, from small oligomers to large fibrils; however, the kinetic relationship between these species is challenging to characterize. In the case of prion aggregates, these structures can self-replicate and act as infectious agents. Here we use single molecule spectroscopy to obtain quantitative information on the oligomer populations formed during aggregation of the yeast prion protein Ure2. Global analysis of the aggregation kinetics reveals the molecular mechanism underlying oligomer formation and depletion. Quantitative characterization indicates that the majority of Ure2 oligomers are relatively short-lived, and their rate of dissociation is much higher than their rate of conversion into growing fibrils. We identify an initial metastable oligomer, which can subsequently convert into a structurally distinct oligomer, which in turn converts into growing fibrils. We also show that fragmentation is responsible for the autocatalytic self-replication of Ure2 fibrils, but that preformed fibrils do not promote oligomer formation, indicating that secondary nucleation of the type observed for peptides and proteins associated with neurodegenerative disease does not occur at a significant rate for Ure2. These results establish a framework for elucidating the temporal and causal relationship between oligomers and larger fibrillar species in amyloid forming systems, and provide insights into why functional amyloid systems are not toxic to their host organisms.

Glutathionylation of the Bacterial Hsp70 Chaperone DnaK Provides a Link between Oxidative Stress and the Heat Shock Response

DnaK is the major bacterial Hsp70, participating in DNA replication, protein folding, and the stress response. DnaK cooperates with the Hsp40 co-chaperone DnaJ and the nucleotide exchange factor GrpE. Under non-stress conditions, DnaK binds to the heat shock transcription factor σ(32)and facilitates its degradation. Oxidative stress results in temporary inactivation of DnaK due to depletion of cellular ATP and thiol modifications such as glutathionylation until normal cellular ATP levels and a reducing environment are restored. However, the biological significance of DnaK glutathionylation remains unknown, and the mechanisms by which glutathionylation may regulate the activity of DnaK are also unclear. We investigated the conditions under which Escherichia coli DnaK undergoesS-glutathionylation. We observed glutathionylation of DnaK in lysates of E. coli cells that had been subjected to oxidative stress. We also obtained homogeneously glutathionylated DnaK using purified DnaK in the apo state. We found that glutathionylation of DnaK reversibly changes the secondary structure and tertiary conformation, leading to reduced nucleotide and peptide binding ability. The chaperone activity of DnaK was reversibly down-regulated by glutathionylation, accompanying the structural changes. We found that interaction of DnaK with DnaJ, GrpE, or σ(32)becomes weaker when DnaK is glutathionylated, and the interaction is restored upon deglutathionylation. This study confirms that glutathionylation down-regulates the functions of DnaK under oxidizing conditions, and this down-regulation may facilitate release of σ(32)from its interaction with DnaK, thus triggering the heat shock response. Such a mechanism provides a link between oxidative stress and the heat shock response in bacteria.

Resonance assignments for the substrate binding domain of Hsp70 chaperone Ssa1 from Saccharomyces cerevisiae

Hsp70 chaperone proteins play crucial roles in the cell. Extensive structural and functional studies have been performed for bacterial and mammalian Hsp70s. Ssa1 from Saccharomyces cerevisiae is a member of the Hsp70 family. In vivo and biochemical studies on Ssa1 have revealed that it regulates prion propagation and the cell cycle. However, no structural data has been obtained for Ssa1 up to now. Here we report the almost complete (96 %) (1)H, (13)C, (15)N backbone and side chain NMR assignment of the 18.8 kDa Ssa1 substrate binding domain. The construct includes residues 382-554, which corresponds to the entire substrate binding domain and two following α-helices in homologous structures. The secondary structure predicted from the assigned chemical shifts is consistent with that of homologous Hsp70 substrate binding domains.

Protein Neighbors and Proximity Proteomics

Within cells, proteins can co-assemble into functionally integrated and spatially restricted multicomponent complexes. Often, the affinities between individual proteins are relatively weak, and proteins within such clusters may interact only indirectly with many of their other protein neighbors. This makes proteomic characterization difficult using methods such as immunoprecipitation or cross-linking. Recently, several groups have described the use of enzyme-catalyzed proximity labeling reagents that covalently tag the neighbors of a targeted protein with a small molecule such as fluorescein or biotin. The modified proteins can then be isolated by standard pulldown methods and identified by mass spectrometry. Here we will describe the techniques as well as their similarities and differences. We discuss their applications both to study protein assemblies and to provide a new way for characterizing organelle proteomes. We stress the importance of proteomic quantitation and independent target validation in such experiments. Furthermore, we suggest that there are biophysical and cell-biological principles that dictate the appropriateness of enzyme-catalyzed proximity labeling methods to address particular biological questions of interest.

Enzymatically Active Microgels from Self-Assembling Protein Nanofibrils for Microflow Chemistry

Amyloid fibrils represent a generic class of protein structure associated with both pathological states and with naturally occurring functional materials. This class of protein nanostructure has recently also emerged as an excellent foundation for sophisticated functional biocompatible materials including scaffolds and carriers for biologically active molecules. Protein-based materials offer the potential advantage that additional functions can be directly incorporated via gene fusion producing a single chimeric polypeptide that will both self-assemble and display the desired activity. To succeed, a chimeric protein system must self-assemble without the need for harsh triggering conditions which would damage the appended functional protein molecule. However, the micrometer to nanoscale patterning and morphological control of protein-based nanomaterials has remained challenging. This study demonstrates a general approach for overcoming these limitations through the microfluidic generation of enzymatically active microgels that are stabilized by amyloid nanofibrils. The use of scaffolds formed from biomaterials that self-assemble under mild conditions enables the formation of catalytic microgels while maintaining the integrity of the encapsulated enzyme. The enzymatically active microgel particles show robust material properties and their porous architecture allows diffusion in and out of reactants and products. In combination with microfluidic droplet trapping approaches, enzymatically active microgels illustrate the potential of self-assembling materials for enzyme immobilization and recycling, and for biological flow-chemistry. These design principles can be adopted to create countless other bioactive amyloid-based materials with diverse functions.

Evolutionarily conserved binding of translationally controlled tumor protein to eukaryotic elongation factor 1B

Translationally controlled tumor protein (TCTP) is an abundant protein that is highly conserved in eukaryotes. However, its primary function is still not clear. Human TCTP interacts with the metazoan-specific eukaryotic elongation factor 1Bδ (eEF1Bδ) and inhibits its guanine nucleotide exchange factor (GEF) activity, but the structural mechanism remains unknown. The interaction between TCTP and eEF1Bδ was investigated by NMR titration, structure determination, paramagnetic relaxation enhancement, site-directed mutagenesis, isothermal titration calorimetry, and HADDOCK docking. We first demonstrated that the catalytic GEF domain of eEF1Bδ is not responsible for binding to TCTP but rather a previously unnoticed central acidic region (CAR) domain in eEF1Bδ. The mutagenesis data and the structural model of the TCTP-eEF1Bδ CAR domain complex revealed the key binding residues. These residues are highly conserved in eukaryotic TCTPs and in eEF1B GEFs, including the eukaryotically conserved eEF1Bα, implying the interaction may be conserved in all eukaryotes. Interactions were confirmed between TCTP and the eEF1Bα CAR domain for human, fission yeast, and unicellular photosynthetic microalgal proteins, suggesting that involvement in protein translation through the conserved interaction with eEF1B represents a primary function of TCTP.

Self-Assembly of Amyloid Fibrils That Display Active Enzymes

Enzyme immobilization is an important strategy to enhance the stability and recoverability of enzymes and to facilitate the separation of enzymes from reaction products. However, enzyme purification followed by separate chemical steps to allow immobilization on a solid support reduces the efficiency and yield of the active enzyme. Here we describe polypeptide constructs that self-assemble spontaneously into nanofibrils with fused active enzyme subunits displayed on the amyloid fibril surface. We measured the steady-state kinetic parameters for the appended enzymes in situ within fibrils and compare these with the identical protein constructs in solution. Finally, we demonstrated that the fibrils can be recycled and reused in functional assays both in conventional batch processes and in a continuous-flow microreactor.

New insights into the DT40 B cell receptor cluster using a proteomic proximity labeling assay

In the vertebrate immune system, each B-lymphocyte expresses a surface IgM-class B cell receptor (BCR). When cross-linked by antigen or anti-IgM antibody, the BCR accumulates with other proteins into distinct surface clusters that activate cell signaling, division, or apoptosis. However, the molecular composition of these clusters is not well defined. Here we describe a quantitative assay we call selective proteomic proximity labeling using tyramide (SPPLAT). It allows proteins in the immediate vicinity of a target to be selectively biotinylated, and hence isolated for mass spectrometry analysis. Using the chicken B cell line DT40 as a model, we use SPPLAT to provide the first proteomic analysis of any BCR cluster using proximity labeling. We detect known components of the BCR cluster, including integrins, together with proteins not previously thought to be BCR-associated. In particular, we identify the chicken B-lymphocyte allotypic marker chB6. We show that chB6 moves to within about 30–40 nm of the BCR following BCR cross-linking, and we show that cross-linking chB6 activates cell binding to integrin substrates laminin and gelatin. Our work provides new insights into the nature and composition of the BCR cluster, and confirms SPPLAT as a useful research tool in molecular and cellular proteomics.

Retinoblastoma-binding protein 1 has an interdigitated double Tudor domain with DNA binding activity

Retinoblastoma-binding protein 1 (RBBP1) is a tumor and leukemia suppressor that binds both methylated histone tails and DNA. Our previous studies indicated that RBBP1 possesses a Tudor domain, which cannot bind histone marks. In order to clarify the function of the Tudor domain, the solution structure of the RBBP1 Tudor domain was determined by NMR and is presented here. Although the proteins are unrelated, the RBBP1 Tudor domain forms an interdigitated double Tudor structure similar to the Tudor domain of JMJD2A, which is an epigenetic mark reader. This indicates the functional diversity of Tudor domains. The RBBP1 Tudor domain structure has a significant area of positively charged surface, which reveals a capability of the RBBP1 Tudor domain to bind nucleic acids. NMR titration and isothermal titration calorimetry experiments indicate that the RBBP1 Tudor domain binds both double- and single-stranded DNA with an affinity of 10-100 μM; no apparent DNA sequence specificity was detected. The DNA binding mode and key interaction residues were analyzed in detail based on a model structure of the Tudor domain-dsDNA complex, built by HADDOCK docking using the NMR data. Electrostatic interactions mediate the binding of the Tudor domain with DNA, which is consistent with NMR experiments performed at high salt concentration. The DNA-binding residues are conserved in Tudor domains of the RBBP1 protein family, resulting in conservation of the DNA-binding function in the RBBP1 Tudor domains. Our results provide further insights into the structure and function of RBBP1.

Complex energy landscape of a giant repeat protein

Here, we reveal a remarkable complexity in the unfolding of giant HEAT-repeat protein PR65/A, a molecular adaptor for the heterotrimeric PP2A phosphatases. The repeat array ruptures at multiple sites, leading to intermediate states with noncontiguous folded subdomains. There is a dominant sequence of unfolding, which reflects a nonuniform stability distribution across the repeat array and can be rationalized by theoretical models accounting for heterogeneous contact density in the folded structure. Unfolding of certain intermediates is, however, competitive, leading to parallel unfolding pathways. The low-stability, central repeats sample unfolded conformations under physiological conditions, suggesting how folding directs function: certain regions present rigid motifs for molecular recognition, whereas others have the flexibility with which to broaden the search area, as in the fly-casting mechanism. Partial unfolding of PR65/A also impacts catalysis by altering the proximity of bound catalytic subunit and substrate. Thus, the repeat array orchestrates the assembly and activity of PP2A.

Anti-apoptosis proteins Mcl-1 and Bcl-xL have different p53-binding profiles

One of the transcription-independent mechanisms of the tumor suppressor p53 discovered in recent years involves physical interaction between p53 and proteins of the Bcl-2 family. In this paper, significant differences between the interaction of p53 with Mcl-1 and Bcl-xL were demonstrated by NMR spectroscopy and isothermal titration calorimetry. Bcl-xL was found to bind strongly to the p53 DNA-binding domain (DBD) with a dissociation constant (Kd) of ~600 nM, whereas Mcl-1 binds to the p53 DBD weakly with a dissociation constant in the mM range. In contrast, the p53 transactivation domain (TAD) binds weakly to Bcl-xL with a Kd ~ 300-500 μM and strongly to Mcl-1 with a Kd ~ 10-20 μM. NMR titrations indicate that although the p53 TAD binds to the BH3-binding grooves of both Bcl-xL and Mcl-1, Bcl-xL prefers to bind to the first subdomain (TAD1) in the p53 TAD, and Mcl-1 prefers to bind to the second subdomain (TAD2). Therefore, Mcl-1 and Bcl-xL have different p53-binding profiles. This indicates that the detailed interaction mechanisms are different, although both Mcl-1 and Bcl-xL can mediate transcription-independent cytosolic roles of p53. The revealed differences in binding sites and binding affinities should be considered when BH3 mimetics are used in cancer therapy development.

Understanding the particokinetics of engineered nanomaterials for safe and effective therapeutic applications

Increasing numbers of engineered nanomaterials (ENMs) are being developed for therapeutic and diagnostic applications. However, the tunable and varied physicochemical properties of ENMs pose a new challenge for understanding their biological behavior, trafficking, and biodistribution. Herein the concept of "particokinetics" is introduced to address the dynamic biological behavior of ENMs at the molecular level (including gravitational sedimentation, dispersion, aggregation, and interaction with biomolecules in suspending media), cellular level (including cellular uptake, transport, biotransformation, and elimination), and whole-organism level (including absorption, distribution, metabolism, and excretion in vivo). Several mathematical modeling methods are introduced which guide a quantitative description of their biological behavior at different levels. Examples are also provided to delineate the impact of the physicochemical properties of ENMs on their particokinetics. A comprehensive understanding of the in vivo and in vitro particokinetics of ENMs will facilitate the design of tailor-made functional ENMs that act as highly effective and controllable drug-delivery systems with minimal side-effects.

Influence of specific HSP70 domains on fibril formation of the yeast prion protein Ure2

Ure2p is the protein determinant of the Saccharomyces cerevisiae prion state [URE3]. Constitutive overexpression of the HSP70 family member SSA1 cures cells of [URE3]. Here, we show that Ssa1p increases the lag time of Ure2p fibril formation in vitro in the presence or absence of nucleotide. The presence of the HSP40 co-chaperone Ydj1p has an additive effect on the inhibition of Ure2p fibril formation, whereas the Ydj1p H34Q mutant shows reduced inhibition alone and in combination with Ssa1p. In order to investigate the structural basis of these effects, we constructed and tested an Ssa1p mutant lacking the ATPase domain, as well as a series of C-terminal truncation mutants. The results indicate that Ssa1p can bind to Ure2p and delay fibril formation even in the absence of the ATPase domain, but interaction of Ure2p with the substrate-binding domain is strongly influenced by the C-terminal lid region. Dynamic light scattering, quartz crystal microbalance assays, pull-down assays and kinetic analysis indicate that Ssa1p interacts with both native Ure2p and fibril seeds, and reduces the rate of Ure2p fibril elongation in a concentration-dependent manner. These results provide new insights into the structural and mechanistic basis for inhibition of Ure2p fibril formation by Ssa1p and Ydj1p.

CDK-dependent Hsp70 Phosphorylation controls G1 cyclin abundance and cell-cycle progression

In budding yeast, the essential functions of Hsp70 chaperones Ssa1–4 are regulated through expression level, isoform specificity, and cochaperone activity. Suggesting a novel regulatory paradigm, we find that phosphorylation of Ssa1 T36 within a cyclin-dependent kinase (CDK) consensus site conserved among Hsp70 proteins alters cochaperone and client interactions. T36 phosphorylation triggers displacement of Ydj1, allowing Ssa1 to bind the G1 cyclin Cln3 and promote its degradation. The stress CDK Pho85 phosphorylates T36 upon nitrogen starvation or pheromone stimulation, destabilizing Cln3 to delay onset of S phase. In turn, the mitotic CDK Cdk1 phosphorylates T36 to block Cln3 accumulation in G2/M. Suggesting broad conservation from yeast to human, CDK-dependent phosphorylation of Hsc70 T38 similarly regulates Cyclin D1 binding and stability. These results establish an active role for Hsp70 chaperones as signal transducers mediating growth control of G1 cyclin abundance and activity.

Structural insight into recognition of methylated histone tails by retinoblastoma-binding protein 1

Retinoblastoma-binding protein 1 (RBBP1), also named AT-rich interaction domain containing 4A (ARID4A), is a tumor and leukemia suppressor involved in epigenetic regulation in leukemia and Prader-Willi/Angelman syndromes. Although the involvement in epigenetic regulation is proposed to involve its chromobarrel and/or Tudor domains because of their potential binding to methylated histone tails, the structures of these domains and their interactions with methylated histone tails are still uncharacterized. In this work, we first found that RBBP1 contains five domains by bioinformatics analysis. Three of the five domains, i.e. chromobarrel, Tudor, and PWWP domains, are Royal Family domains, which potentially bind to methylated histone tails. We further purified these domains and characterized their interaction with methylated histone tails by NMR titration experiments. Among the three Royal Family domains, only the chromobarrel domain could recognize trimethylated H4K20 (with an affinity of ∼3 mm), as well as recognizing trimethylated H3K9, H3K27, and H3K36 (with lower affinities). The affinity could be further enhanced up to 15-fold by the presence of DNA. The structure of the chromobarrel domain of RBBP1 determined by NMR spectroscopy has an aromatic cage. Mutagenesis analysis identified four aromatic residues of the cage as the key residues for methylated lysine recognition. Our studies indicate that the chromobarrel domain of RBBP1 is responsible for recognizing methylated histone tails in chromatin remodeling and epigenetic regulation, which presents a significant advance in our understanding of the mechanism and relationship between RBBP1-related gene suppression and epigenetic regulation.

The fibrils of Ure2p homologs from Saccharomyces cerevisiae and Saccharoymyces paradoxus have similar cross-β structure in both dried and hydrated forms

The ability to convert into amyloid fibrils is a common feature of prion proteins. However, not all amyloid-forming proteins act as prions. Here, we compared two homologs of the yeast prion protein Ure2 from Saccharomyces cerevisiae and Saccharomyces paradoxus, ScUre2p and SpUre2p, which have different prion propensities in vivo. We also addressed the controversial issue of whether hydrated fibrils of Ure2 show a fundamentally different X-ray diffraction pattern than dried samples. Using Fourier transform infrared spectrometry (FTIR) and wide angle X-ray scattering of dried and concentrated hydrated fibrils, we compared the fibril structure of ScUre2p and SpUre2p. The results show that fibrils of ScUre2p and SpUre2 have a similar cross-β core under dried and hydrated conditions, with the same inter-strand and inter-sheet spacings. Given the different prion propensity of the two Ure2p homologs, this suggests that the detailed organization of the cross-β core may play an important role in the efficiency of prion propagation.

Relationship between prion propensity and the rates of individual molecular steps of fibril assembly

Peptides and proteins possess an inherent propensity to self-assemble into generic fibrillar nanostructures known as amyloid fibrils, some of which are involved in medical conditions such as Alzheimer disease. In certain cases, such structures can self-propagate in living systems as prions and transmit characteristic traits to the host organism. The mechanisms that allow certain amyloid species but not others to function as prions are not fully understood. Much progress in understanding the prion phenomenon has been achieved through the study of prions in yeast as this system has proved to be experimentally highly tractable; but quantitative understanding of the biophysics and kinetics of the assembly process has remained challenging. Here, we explore the assembly of two closely related homologues of the Ure2p protein from Saccharomyces cerevisiae and Saccharomyces paradoxus, and by using a combination of kinetic theory with solution and biosensor assays, we are able to compare the rates of the individual microscopic steps of prion fibril assembly. We find that for these proteins the fragmentation rate is encoded in the structure of the seed fibrils, whereas the elongation rate is principally determined by the nature of the soluble precursor protein. Our results further reveal that fibrils that elongate faster but fracture less frequently can lose their ability to propagate as prions. These findings illuminate the connections between the in vitro aggregation of proteins and the in vivo proliferation of prions, and provide a framework for the quantitative understanding of the parameters governing the behavior of amyloid fibrils in normal and aberrant biological pathways.