Proline isomerization in the C-terminal region of HSP27

Capturing the Conformational Heterogeneity of HSPB1 Chaperone Oligomers at Atomic Resolution

Small heat shock proteins (sHSPs), including HSPB1, are essential regulators of cellular proteostasis that interact with unfolded and partially folded proteins to prevent aberrant misfolding and aggregation. These proteins fulfill a similar role in biological condensates, where they interact with intrinsically disordered proteins to modulate their liquid-liquid and liquid-to-solid phase transitions. Characterizing the sHSP structure, dynamics, and client interactions is challenging due to their partially disordered nature, their tendency to form polydisperse oligomers, and their diverse range of clients. In this work, we leverage various biophysical methods, including fast 1H-based magic angle spinning (MAS) NMR spectroscopy, molecular dynamics (MD) simulations, and modeling, to shed new light on the structure and dynamics of HSPB1 oligomers. Using split-intein-mediated segmental labeling, we provide unambiguous evidence that in the oligomer context, the N-terminal domain (NTD) of HSPB1 is rigid and adopts an ensemble of heterogeneous conformations, the α-Crystallin domain (ACD) forms dimers and experiences multiple distinct local environments, while the C-terminal domain (CTD) remains highly dynamic. Our computational models suggest that the NTDs participate in extensive NTD-NTD and NTD-ACD interactions and are sequestered within the oligomer interior. We further demonstrate that HSPB1 higher order oligomers disassemble into smaller oligomeric species in the presence of a client protein and that an accessible NTD is essential for HSPB1 partitioning into condensates and interactions with client proteins. Our integrated approach provides a high-resolution view of the complex oligomeric landscape of HSPB1 and sheds light on the elusive network of interactions that underlies the function of HSPB1 in biological condensates.

Targeting WDPF domain of Hsp27 achieves a broad spectrum of antiviral

Enterovirus A71 (EV-A71) is a positive-sense single-stranded RNA virus, which hijacks host proteins to benefit viral internal ribosome entry site (IRES)-dependent protein translation and further propagation. We demonstrated that serine 78 (S78) phosphorylation of Hsp27 is critical for Hsp27/hnRNP A1 relocalization upon EV-A71 infection. Here, we report that the deletion of WDPF and ACD domains disturbs subcellular localization of Hsp27, resulting in partial nuclear translocation. The domain deletion-induced Hsp27 nuclear translocation fails to direct hnRNP A1 translocation. The 2Apro-induced IRES activity and viral replication are suppressed by the deletion of WDPF or ACD domain. Surprisingly, a peptide (WDPF) dramatically inhibits S78 phosphorylation. Therefore, hnRNP A1 translocation, viral IRES activity, and viral protein translation and propagation are all strongly suppressed by the WDPF peptide, but not by peptide without WDPFR sequence (ΔWDPF). Moreover, the WDPF peptide also has potent antiviral activity on other RNA virus (e.g., coronavirus HCoV-OC43) and DNA virus (e.g., HSV-1 and HBV). Peptide treatment with kinase inhibitor Sorafenib brings an additional inhibitory effect on HCoV-OC43 and HSV-1. Taken together, we uncover a crucial role of WDPF domain in S78 phosphorylation for EV-A71-induced hnRNP A1 nuclear translocation, IRES-dependent viral protein translation, and EV-A71 propagation. Our results explore a new path for target-based pan-antiviral strategy.

Small Heat Shock Proteins: Protein Aggregation Amelioration and Neuro- and Age-Protective Roles

Protein misfolding, aggregation, and aberrant aggregate accumulation play a central role in neurodegenerative disease progression. The proteotoxic factors also govern the aging process to a large extent. Molecular chaperones modulate proteostasis and thereby impact aberrant-protein-induced proteotoxicity. These chaperones have a diverse functional spectrum, including nascent protein folding, misfolded protein sequestration, refolding, or degradation. Small heat shock proteins (sHsps) possess an ATP-independent chaperone-like activity that prevents protein aggregation by keeping target proteins in a folding-competent state to be refolded by ATP-dependent chaperones. Due to their near-universal upregulation and presence in sites of proteotoxic stress like diseased brains, sHsps were considered pathological. However, gene knockdown and overexpression studies have established their protective functions. This review provides an updated overview of the sHsp role in protein aggregation amelioration and highlights evidence for sHsp modulation of neurodegenerative disease-related protein aggregation and aging.

UnidecNMR: automatic peak detection for NMR spectra in 1-4 dimensions

To extract information from NMR experiments, users need to identify the number of resonances in the spectrum, together with characteristic features such as chemical shifts and intensities. In many applications, particularly those involving biomolecules, this procedure is typically a manual and laborious process. While many algorithms are available to tackle this problem, their performance tends to be inferior to that of an experienced user. Here, we introduce UnidecNMR, which identifies resonances in NMR spectra using deconvolution. We demonstrate its favourable performance on 1 and 2D simulated spectra, strongly overlapped 1D spectra of oligosaccharides and 2D HSQC, 3D HNCO, 3D HNCA and 3/4D methyl-methyl NOE experimental spectra from a range of proteins. UnidecNMR outperforms a number of freely available algorithms and provides results comparable to those generated manually. Introducing additional restraints, such as a 2D peak list when analysing 3 and 4D data and incorporating reflection symmetry in NOE analysis further improves the results. UnidecNMR outputs a back-calculated spectrum and a peak list, both of which can be easily examined using the supplied GUI. The software allows interactive processing using nmrPipe, allowing users to go directly from raw data to processed spectra with picked peak lists.

An integrative characterization of proline cis and trans conformers in a disordered peptide

Intrinsically disordered proteins (IDPs) often contain proline residues that undergo cis/trans isomerization. While molecular dynamics (MD) simulations have the potential to fully characterize the proline cis and trans subensembles, they are limited by the slow timescales of isomerization and force field inaccuracies. NMR spectroscopy can report on ensemble-averaged observables for both the cis-proline and trans-proline states, but a full atomistic characterization of these conformers is challenging. Given the importance of proline cis/trans isomerization for influencing the conformational sampling of disordered proteins, we employed a combination of all-atom MD simulations with enhanced sampling (metadynamics), NMR, and small-angle x-ray scattering (SAXS) to characterize the two subensembles of the ORF6 C-terminal region (ORF6CTR) from SARS-CoV-2 corresponding to the proline-57 (P57) cis and trans states. We performed MD simulations in three distinct force fields: AMBER03ws, AMBER99SB-disp, and CHARMM36m, which are all optimized for disordered proteins. Each simulation was run for an accumulated time of 180-220 μs until convergence was reached, as assessed by blocking analysis. A good agreement between the cis-P57 populations predicted from metadynamic simulations in AMBER03ws was observed with populations obtained from experimental NMR data. Moreover, we observed good agreement between the radius of gyration predicted from the metadynamic simulations in AMBER03ws and that measured using SAXS. Our findings suggest that both the cis-P57 and trans-P57 conformations of ORF6CTR are extremely dynamic and that interdisciplinary approaches combining both multiscale computations and experiments offer avenues to explore highly dynamic states that cannot be reliably characterized by either approach in isolation.

Three-Dimensional Structure of the Antimicrobial Peptide Cecropin P1 in Dodecylphosphocholine Micelles and the Role of the C-Terminal Residues

Cecropin P1 (CP1) isolated from a large roundworm Ascaris suum, which is found in pig intestines, has been extensively studied as a model antimicrobial peptide (AMP). However, despite being a model AMP, its antibacterial mechanism is not well understood, particularly the function of its C-terminus. By using an Escherichia coli overexpression system with calmodulin as a fusion partner, we succeeded in the mass expression of recombinant peptides, avoiding toxicity to the host and degradation of CP1. The structure of the recombinant ¹⁵N- and ¹³C-labeled CP1 and its C-terminus truncated analogue in dodecylphosphocholine (DPC) micelles was determined by NMR. In this membrane-mimetic environment, CP1 formed an α-helix for almost its entire length, except for a short region at the C-terminus, and there was no evidence of a hinge, which is considered important for the expression of activity in other cecropins. Several NMR analyses showed that the entire length of CP1 was protected from water by micelles. Since the loss of the C-terminus of the analogue had little effect on the NMR structure or its interaction with the micelle, we investigated another role of the C-terminus of CP1 in its antimicrobial activity. The results showed that the C-terminal region affected the DNA-binding capacity of CP1, and this mechanism of action was also newly suggested that it contributed to the antimicrobial activity of CP1.

Specific phosphorylation of microtubule-associated protein 2c by extracellular signal-regulated kinase reduces interactions at its Pro-rich regions

Microtubule-associated protein 2 (MAP2) is an important neuronal target of extracellular signal-regulated kinase 2 (ERK2) involved in Raf signaling pathways, but mechanistic details of MAP2 phosphorylation are unclear. Here, we used NMR spectroscopy to quantitatively describe the kinetics of phosphorylation of individual serines and threonines in the embryonic MAP2 variant MAP2c. We carried out real-time monitoring of phosphorylation to discover major phosphorylation sites that were not identified in previous studies relying on specific antibodies. Our comparison with phosphorylation of MAP2c by a model cyclin-dependent kinase CDK2 and with phosphorylation of the MAP2c homolog Tau revealed differences in phosphorylation profiles that explain specificity of regulation of biological functions of MAP2c and Tau. To probe the molecular basis of the regulatory effect of ERK2, we investigated the interactions of phosphorylated and unphosphorylated MAP2c by NMR with single-residue resolution. As ERK2 phosphorylates mostly outside the regions binding microtubules, we studied the binding of proteins other than tubulin, namely regulatory subunit RIIα of cAMP-dependent protein kinase (PKA), adaptor protein Grb2, Src homology domain 3 of tyrosine kinases Fyn and Abl, and ERK2 itself. We found ERK2 phosphorylation interfered mostly with binding to proline-rich regions of MAP2c. Furthermore, our NMR experiments in SH-SY5Y neuroblastoma cell lysates showed that the kinetics of dephosphorylation are compatible with in-cell NMR studies and that residues targeted by ERK2 and PKA are efficiently phosphorylated in the cell lysates. Taken together, our results provide a deeper characterization of MAP2c phosphorylation and its effects on interactions with other proteins.

Systematic Review on Post-Mortem Protein Alterations: Analysis of Experimental Models and Evaluation of Potential Biomarkers of Time of Death

Estimating the post-mortem interval (PMI) is a very complex issue due to numerous variables that may affect the calculation. Several authors have investigated the quantitative and qualitative variations of protein expression on post-mortem biological samples in certain time intervals, both in animals and in humans. However, the literature data are very numerous and often inhomogeneous, with different models, tissues and proteins evaluated, such that the practical application of these methods is limited to date. The aim of this paper was to offer an organic view of the state of the art about post-mortem protein alterations for the calculation of PMI through the analysis of the various experimental models proposed. The purpose was to investigate the validity of some proteins as “molecular clocks” candidates, focusing on the evidence obtained in the early, intermediate and late post-mortem interval. This study demonstrates how the study of post-mortem protein alterations may be useful for estimating the PMI, although there are still technical limits, especially in the experimental models performed on humans. We suggest a protocol to homogenize the study of future experimental models, with a view to the next concrete application of these methods also at the crime scene.

Assignment of Ala, Ile, LeuproS, Met, and ValproS methyl groups of the protruding domain of murine norovirus capsid protein VP1 using methyl-methyl NOEs, site directed mutagenesis, and pseudocontact shifts

The protruding domain (P-domain) of the murine norovirus (MNV) capsid protein VP1 is essential for infection. It mediates receptor binding and attachment of neutralizing antibodies. Protein NMR studies into interactions of the P-domain with ligands will yield insights not easily available from other biophysical techniques and will extend our understanding of MNV attachment to host cells. Such studies require at least partial NMR assignments. Here, we describe the assignment of about 70% of the Ala, Ile, LeuproS, Met, and ValproS methyl groups. An unfavorable distribution of methyl group resonance signals prevents complete assignment based exclusively on 4D HMQC-NOESY-HMQC experiments, yielding assignment of only 55 out of 100 methyl groups. Therefore, we created point mutants and measured pseudo contact shifts, extending and validating assignments based on methyl-methyl NOEs. Of note, the P-domains are present in two different forms caused by an approximate equal distribution of trans- and cis-configured proline residues in position 361.

Insights Into the Role of Heat Shock Protein 27 in the Development of Neurodegeneration

Small heat shock protein 27 is a critically important chaperone, that plays a key role in several essential and varied physiological processes. These include thermotolerance, apoptosis, cytoskeletal dynamics, cell differentiation, protein folding, among others. Despite its relatively small size and intrinsically disordered termini, it forms large and polydisperse oligomers that are in equilibrium with dimers. This equilibrium is driven by transient interactions between the N-terminal region, the α-crystallin domain, and the C-terminal region. The continuous redistribution of binding partners results in a conformationally dynamic protein that allows it to adapt to different functions where substrate capture is required. However, the intrinsic disorder of the amino and carboxy terminal regions and subsequent conformational variability has made structural investigations challenging. Because heat shock protein 27 is critical for so many key cellular functions, it is not surprising that it also has been linked to human disease. Charcot-Marie-Tooth and distal hereditary motor neuropathy are examples of neurodegenerative disorders that arise from single point mutations in heat shock protein 27. The development of possible treatments, however, depends on our understanding of its normal function at the molecular level so we might be able to understand how mutations manifest as disease. This review will summarize recent reports describing investigations into the structurally elusive regions of Hsp27. Recent insights begin to provide the required context to explain the relationship between a mutation and the resulting loss or gain of function that leads to Charcot-Marie Tooth disease and distal hereditary motor neuropathy.

A weakened interface in the P182L variant of HSP27 associated with severe Charcot-Marie-Tooth neuropathy causes aberrant binding to interacting proteins

HSP27 is a human molecular chaperone that forms large, dynamic oligomers and functions in many aspects of cellular homeostasis. Mutations in HSP27 cause Charcot-Marie-Tooth (CMT) disease, the most common inherited disorder of the peripheral nervous system. A particularly severe form of CMT disease is triggered by the P182L mutation in the highly conserved IxI/V motif of the disordered C-terminal region, which interacts weakly with the structured core domain of HSP27. Here, we observed that the P182L mutation disrupts the chaperone activity and significantly increases the size of HSP27 oligomers formed in vivo, including in motor neurons differentiated from CMT patient-derived stem cells. Using NMR spectroscopy, we determined that the P182L mutation decreases the affinity of the HSP27 IxI/V motif for its own core domain, leaving this binding site more accessible for other IxI/V-containing proteins. We identified multiple IxI/V-bearing proteins that bind with higher affinity to the P182L variant due to the increased availability of the IxI/V-binding site. Our results provide a mechanistic basis for the impact of the P182L mutation on HSP27 and suggest that the IxI/V motif plays an important, regulatory role in modulating protein-protein interactions.

Protein yoga: Conformational versatility of the Hemolysin II C-terminal domain detailed by NMR structures for multiple states

The C-terminal domain of Bacillus cereus hemolysin II (HlyIIC), stabilizes the trans-membrane-pore formed by the HlyII toxin and may aid in target cell recognition. Initial efforts to determine the NMR structure of HlyIIC were hampered by cis/trans isomerization about the single proline at position 405 that leads to doubling of NMR resonances. We used the mutant P405M-HlyIIC that eliminates the cis proline to determine the NMR structure of the domain, which revealed a novel fold. Here, we extend earlier studies to the NMR structure determination of the cis and trans states of WT-HlyIIC that exist simultaneously in solution. The primary structural differences between the cis and trans states are in the loop that contains P405, and structurally adjacent loops. Thermodynamic linkage analysis shows that at 25 C the cis proline, which already has a large fraction of 20% in the unfolded protein, increases to 50% in the folded state due to coupling with the global stability of the domain. The P405M or P405A substitutions eliminate heterogeneity due to proline isomerization but lead to the formation of a new dimeric species. The NMR structure of the dimer shows that it is formed through domain-swapping of strand β5, the last segment of secondary structure following P405. The presence of P405 in WT-HlyIIC strongly disfavors the dimer compared to the P405M-HlyIIC or P405A-HlyIIC mutants. The WT proline may thus act as a "gatekeeper," warding off aggregative misfolding.

Hydration-Induced Structural Transitions in Biomimetic Tandem Repeat Proteins

A major challenge in developing biomimetic, high-performance, and sustainable products is the accurate replication of the biological materials' striking properties, such as high strength, self-repair, and stimuli-responsiveness. The rationalization of such features on the microscopic scale, together with the rational design of synthetic materials, is currently hindered by our limited understanding of the sequence-structure-property relationship. Here, employing state-of-the-art nuclear magnetic resonance (NMR) spectroscopy, we link the atomistic structural and dynamic properties of an artificial bioinspired tandem repeat protein TR(1,11) to its stunning macroscopic properties including high elasticity, self-healing capabilities, and record-holding proton conductivity among biological materials. We show that the hydration-induced structural rearrangement of the amorphous Gly-rich soft segment and the ordered Ala-rich hard segment is the key to the material's outstanding physical properties. We found that in the hydrated state both the Ala-rich ordered and Gly-rich disordered parts contribute to the formation of the nanoconfined β-sheets, thereby enhancing the strength and toughness of the material. This restructuring is accompanied by fast proline ring puckering and backbone cis-trans isomerization at the water-protein interface, which in turn enhances the elasticity and the thermal conductivity of the hydrated films. Our in-depth characterization provides a solid ground for the development of next-generation materials with improved properties.

A novel dominant mutation in CRYAB gene leading to a severe phenotype with childhood onset

Background

αB‐crystallin is a promiscuous protein involved in numerous cell functions. Mutations in CRYAB have been found in patients with different pathological phenotypes that are not properly understood. Patients can present different diseases like cataracts, muscle weakness, myopathy, cardiomyopathy, respiratory insufficiency or dysphagia, but also a variable combination of these pathologies has been found. These mutations can show either autosomal dominant or recessive mode of inheritance and variable penetrance and expressivity. This is the first report of congenital cataracts and myopathy described in childhood due to a CRYAB mutation with autosomal dominant mode of inheritance.

Methods

The whole exome sequence was subjected to phenotype‐driven analysis and a novel variant in CRYAB was detected: c.514delG, p.(Ala172ProfsTer14). The mutation was located in the C‐terminal domain of the protein, which is essential for chaperone activity. The deduced protein was analyzed searching for alterations of the relevant physico‐chemical properties described for this domain. A muscle biopsy was also tested for CRYAB with immunohistochemical and histoenzymatic techniques.

Results

CRYAB displayed a mild immunoreactivity in the subsarcolemmal compartment with no pathological sarcoplasmic accumulation. It agrees with an alteration of the physico‐chemical properties predicted for the C‐terminal domain: hydrophobicity, stiffness, and isomerization.

Conclusions

The described mutation leads to elongation of the protein at the carboxi‐terminal domain (CTD) with altered properties, which are essential for solubility and activity. It suggests that can be the cause of the severe conditions observed in this patient.

Conditional Disorder in Small Heat-shock Proteins

Small heat-shock proteins (sHSPs) are molecular chaperones that respond to cellular stresses to combat protein aggregation. HSP27 is a critical human sHSP that forms large, dynamic oligomers whose quaternary structures and chaperone activities depend on environmental factors. Upon exposure to cellular stresses, such as heat shock or acidosis, HSP27 oligomers can dissociate into dimers and monomers, which leads to significantly enhanced chaperone activity. The structured core of the protein, the α-crystallin domain (ACD), forms dimers and can prevent the aggregation of substrate proteins to a similar degree as the full-length protein. When the ACD dimer dissociates into monomers, it partially unfolds and exhibits enhanced activity. Here, we used solution-state NMR spectroscopy to characterize the structure and dynamics of the HSP27 ACD monomer. Web show that the monomer is stabilized at low pH and that its backbone chemical shifts, 15N relaxation rates, and 1H-15N residual dipolar couplings suggest structural changes and rapid motions in the region responsible for dimerization. By analyzing the solvent accessible and buried surface areas of sHSP structures in the context of a database of dimers that are known to dissociate into disordered monomers, we predict that ACD dimers from sHSPs across all kingdoms of life may partially unfold upon dissociation. We propose a general model in which conditional disorder-the partial unfolding of ACDs upon monomerization-is a common mechanism for sHSP activity.

Evidence of the Reduced Abundance of Proline cis Conformation in Protein Poly Proline Tracts

Proline is found in a cis conformation in proteins more often than other proteinogenic amino acids, where it influences structure and modulates function, being the focus of several high-resolution structural studies. However, until now, technical and methodological limitations have hampered the site-specific investigation of the conformational preferences of prolines present in poly proline (poly-P) homorepeats in their protein context. Here, we apply site-specific isotopic labeling to obtain high-resolution NMR data on the cis/trans equilibrium of prolines within the poly-P repeats of huntingtin exon 1, the causative agent of Huntington's disease. Screening prolines in different positions in long (poly-P₁₁) and short (poly-P₃) poly-P tracts, we found that, while the first proline of poly-P tracts adopts similar levels of cis conformation as isolated prolines, a length-dependent reduced abundance of cis conformers is observed for terminal prolines. Interestingly, the cis isomer could not be detected in inner prolines, in line with percentages derived from a large database of proline-centered tripeptides extracted from crystallographic structures. These results suggest a strong cooperative effect within poly-Ps that enhances their stiffness by diminishing the stability of the cis conformation. This rigidity is key to rationalizing the protection toward aggregation that the poly-P tract confers to huntingtin. Furthermore, the study provides new avenues to probe the structural properties of poly-P tracts in protein design as scaffolds or nanoscale rulers.

The structure and oxidation of the eye lens chaperone αA-crystallin

The small heat shock protein αA-crystallin is a molecular chaperone important for the optical properties of the vertebrate eye lens. It forms heterogeneous oligomeric ensembles. We determined the structures of human αA-crystallin oligomers by combining cryo-electron microscopy, cross-linking/mass spectrometry, NMR spectroscopy and molecular modeling. The different oligomers can be interconverted by the addition or subtraction of tetramers, leading to mainly 12-, 16- and 20-meric assemblies in which interactions between N-terminal regions are important. Cross-dimer domain-swapping of the C-terminal region is a determinant of αA-crystallin heterogeneity. Human αA-crystallin contains two cysteines, which can form an intramolecular disulfide in vivo. Oxidation in vitro requires conformational changes and oligomer dissociation. The oxidized oligomers, which are larger than reduced αA-crystallin and destabilized against unfolding, are active chaperones and can transfer the disulfide to destabilized substrate proteins. The insight into the structure and function of αA-crystallin provides a basis for understanding its role in the eye lens.

The Ambivalent Role of Proline Residues in an Intrinsically Disordered Protein: From Disorder Promoters to Compaction Facilitators

Intrinsically disordered proteins (IDPs) carry out many biological functions. They lack a stable three-dimensional structure, but rather adopt many different conformations in dynamic equilibrium. The interplay between local dynamics and global rearrangements is key for their function. In IDPs, proline residues are significantly enriched. Given their unique physicochemical and structural properties, a more detailed understanding of their potential role in stabilizing partially folded states in IDPs is highly desirable. Nuclear magnetic resonance (NMR) spectroscopy, and in particular ¹³C-detected NMR, is especially suitable to address these questions. We applied a ¹³C-detected strategy to study Osteopontin, a largely disordered IDP with a central compact region. By using the exquisite sensitivity and spectral resolution of these novel techniques, we gained unprecedented insight into cis-Pro populations, their local structural dynamics, and their role in mediating long-range contacts. Our findings clearly call for a reassessment of the structural and functional role of proline residues in IDPs. The emerging picture shows that proline residues have ambivalent structural roles. They are not simply disorder promoters but rather can, depending on the primary sequence context, act as nucleation sites for structural compaction in IDPs. These unexpected features provide a versatile mechanistic toolbox to enrich the conformational ensembles of IDPs with specific features for adapting to changing molecular and cellular environments.

Interplay of disordered and ordered regions of a human small heat shock protein yields an ensemble of 'quasi-ordered' states

Small heat shock proteins (sHSPs) are nature's 'first responders' to cellular stress, interacting with affected proteins to prevent their aggregation. Little is known about sHSP structure beyond its structured α-crystallin domain (ACD), which is flanked by disordered regions. In the human sHSP HSPB1, the disordered N-terminal region (NTR) represents nearly 50% of the sequence. Here, we present a hybrid approach involving NMR, hydrogen-deuterium exchange mass spectrometry, and modeling to provide the first residue-level characterization of the NTR. The results support a model in which multiple grooves on the ACD interact with specific NTR regions, creating an ensemble of 'quasi-ordered' NTR states that can give rise to the known heterogeneity and plasticity of HSPB1. Phosphorylation-dependent interactions inform a mechanism by which HSPB1 is activated under stress conditions. Additionally, we examine the effects of disease-associated NTR mutations on HSPB1 structure and dynamics, leveraging our emerging structural insights.

Local unfolding of the HSP27 monomer regulates chaperone activity

The small heat-shock protein HSP27 is a redox-sensitive molecular chaperone that is expressed throughout the human body. Here, we describe redox-induced changes to the structure, dynamics, and function of HSP27 and its conserved α-crystallin domain (ACD). While HSP27 assembles into oligomers, we show that the monomers formed upon reduction are highly active chaperones in vitro, but are susceptible to self-aggregation. By using relaxation dispersion and high-pressure nuclear magnetic resonance (NMR) spectroscopy, we observe that the pair of β-strands that mediate dimerisation partially unfold in the monomer. We note that numerous HSP27 mutations associated with inherited neuropathies cluster to this dynamic region. High levels of sequence conservation in ACDs from mammalian sHSPs suggest that the exposed, disordered interface present in free monomers or oligomeric subunits may be a general, functional feature of sHSPs.

The small heat-shock protein HSP27 occurs predominantly in oligomeric forms, which makes its structural characterisation challenging. Here the authors employ CPMG and high-pressure NMR with native mass spectrometry and biophysical assays to show that the active monomeric form of HSP27 is substantially disordered and highly chaperone-active.

Small heat shock proteins: Simplicity meets complexity

Small heat shock proteins (sHsps) are a ubiquitous and ancient family of ATP-independent molecular chaperones. A key characteristic of sHsps is that they exist in ensembles of iso-energetic oligomeric species differing in size. This property arises from a unique mode of assembly involving several parts of the subunits in a flexible manner. Current evidence suggests that smaller oligomers are more active chaperones. Thus, a shift in the equilibrium of the sHsp ensemble allows regulating the chaperone activity. Different mechanisms have been identified that reversibly change the oligomer equilibrium. The promiscuous interaction with non-native proteins generates complexes that can form aggregate-like structures from which native proteins are restored by ATP-dependent chaperones such as Hsp70 family members. In recent years, this basic paradigm has been expanded, and new roles and new cofactors, as well as variations in structure and regulation of sHsps, have emerged.

Terminal Regions Confer Plasticity to the Tetrameric Assembly of Human HspB2 and HspB3

Heterogeneity in small heat shock proteins (sHsps) spans multiple spatiotemporal regimes-from fast fluctuations of part of the protein, to conformational variability of tertiary structure, plasticity of the interfaces, and polydispersity of the inter-converting, and co-assembling oligomers. This heterogeneity and dynamic nature of sHsps has significantly hindered their structural characterization. Atomic coordinates are particularly lacking for vertebrate sHsps, where most available structures are of extensively truncated homomers. sHsps play important roles in maintaining protein levels in the cell and therefore in organismal health and disease. HspB2 and HspB3 are vertebrate sHsps that are found co-assembled in neuromuscular cells, and variants thereof are associated with disease. Here, we present the structure of human HspB2/B3, which crystallized as a hetero-tetramer in a 3:1 ratio. In the HspB2/B3 tetramer, the four α-crystallin domains (ACDs) assemble into a flattened tetrahedron which is pierced by two non-intersecting approximate dyads. Assembly is mediated by flexible "nuts and bolts" involving IXI/V motifs from terminal regions filling ACD pockets. Parts of the N-terminal region bind in an unfolded conformation into the anti-parallel shared ACD dimer grooves. Tracts of the terminal regions are not resolved, most likely due to their disorder in the crystal lattice. This first structure of a full-length human sHsp heteromer reveals the heterogeneous interactions of the terminal regions and suggests a plasticity that is important for the cytoprotective functions of sHsps.

Propensity for cis-Proline Formation in Unfolded Proteins

In unfolded proteins, peptide bonds involving Pro residues exist in equilibrium between the minor cis and major trans conformations. Folded proteins predominantly contain trans-Pro bonds, and slow cis-trans Pro isomerization in the unfolded state is often found to be a rate-limiting step in protein folding. Moreover, kinases and phosphatases that act upon Ser/Thr-Pro motifs exhibit preferential recognition of either the cis- or trans-Pro conformer. Here, NMR spectra obtained at both atmospheric and high pressures indicate that the population of cis-Pro falls well below previous estimates, an effect attributed to the use of short peptides with charged termini in most prior model studies. For the intrinsically disordered protein α-synuclein, cis-Pro populations at all of its five X-Pro bonds are less than 5 %, with only modest ionic strength dependence and no detectable effect of the previously demonstrated interaction between the N- and C-terminal halves of the protein. Comparison to small peptides with the same amino-acid sequence indicates that peptides, particularly those with unblocked, oppositely charged amino and carboxyl end groups, strongly overestimate the amount of cis-Pro.