The small heat shock protein αB-crystallin (αB) contributes to cellular protection against stress. For decades, high-resolution structural studies on oligomeric αB have been confounded by its polydisperse nature. Here, we present a structural basis of oligomer assembly and activation of the chaperone using solid-state NMR and small-angle X-ray scattering (SAXS). The basic building block is a curved dimer, with an angle of ~121° between the planes of the β-sandwich formed by α-crystallin domains. The highly conserved IXI motif covers a substrate binding site at pH 7.5. We observe a pH-dependent modulation of the interaction of the IXI motif with β4 and β8, consistent with a pHdependent regulation of the chaperone function. N-terminal region residues Ser59-Trp60-Phe61 are involved in intermolecular interaction with β3. Intermolecular restraints from NMR and volumetric restraints from SAXS were combined to calculate a model of a 24-subunit αB oligomer with tetrahedral symmetry.Small heat shock proteins (sHSPs) help to maintain protein homeostasis by interacting with unfolded, aggregated or misfolded proteins to prevent cell damage [1][2][3] . The ATP-independent chaperone αB-crystallin (αB, 20 kDa, 175 residues) is a paradigm example 4 . αB was originally found in the eye-lens as the B-subunit of α-crystallin, a protein essential for maintaining eyelens transparency. In recent years, the list of biological roles for αB has grown substantially, including involvement in the regulation of the ubiquitin-proteasome pathway as well as AUTHOR CONTRIBUTIONSS.J. contributed to all aspects of the manuscript; P.R. performed solution NMR experiments and helped to write the manuscript; B.B. performed structure calculations; S.M. did solid-state NMR and SAXS measurements as well as data analysis; R.K. contributed to modeling of C-terminal intermolecular interactions; J.R.S. prepared samples; V.A.H. contributed to assignment strategies, was involved in structure calculations and helped write the manuscript; R.E.K. contributed to the interpretation of results and wrote the manuscript; B.J.v.R. contributed to solid-state NMR measurements, discussed the results and helped to write the manuscript; H.O. designed experimental strategies, contributed to the interpretation of results and wrote the manuscript. COMPETING FINANCIAL INTERESTSThe authors declare no competing financial interests.Reprints and permissions information is available online at http://npg.nature.com/reprintsandpermissions/. [6][7][8][9][10][11] . In the brain of patients with Alexander's disease, the insoluble cell fraction contains protein fibers (Rosenthal fibers) coprecipitated with αB phosphorylated at Ser59, whereas unphosphorylated αB remains in the soluble fraction 7 . A missense mutation, R120G, in αB is associated with desmin-related cardiomyopathy 8,9 . The mutations D140N and Q151X are associated with congenital cataracts and myopathy, respectively 10,11 . A decreased concentration of αB in the cerebrospinal fluid was found to be associated with ...

The small heat shock protein (sHSP) αB-crystallin (αB) plays a key role in the cellular protection system against stress. For decades, high-resolution structural studies on heterogeneous sHSPs have been confounded by the polydisperse nature of αB oligomers. We present an atomic-level model of full-length αB as a symmetric 24-subunit multimer based on solid-state NMR, small-angle X-ray scattering (SAXS), and EM data. The model builds on our recently reported structure of the homodimeric α-crystallin domain (ACD) and C-terminal IXI motif in the context of the multimer. A hierarchy of interactions contributes to build multimers of varying sizes: Interactions between two ACDs define a dimer, three dimers connected by their C-terminal regions define a hexameric unit, and variable interactions involving the N-terminal region define higher-order multimers. Within a multimer, N-terminal regions exist in multiple environments, contributing to the heterogeneity observed by NMR. Analysis of SAXS data allows determination of a heterogeneity parameter for this type of system. A mechanism of multimerization into higher-order asymmetric oligomers via the addition of up to six dimeric units to a 24-mer is proposed. The proposed asymmetric multimers explain the homogeneous appearance of αB in negative-stain EM images and the known dynamic exchange of αB subunits. The model of αB provides a structural basis for understanding known disease-associated missense mutations and makes predictions concerning substrate binding and the reported fibrilogenesis of αB. S mall heat shock proteins (sHSPs) help to maintain protein homeostasis by interacting with partly folded substrates to prevent cell damage (1-3). The ATP-independent chaperone αB-crystallin (αB, 20 kDa, 175 residues) is an archetypal example (4). Discovered as a highly abundant protein in the eye lens that plays a critical role in maintenance of lens transparency, the known biological roles of αB continue to expand. The protein is expressed in many tissue types, notably muscle and brain, in a stress-inducible manner, where it presumably serves as a chaperone for misfolded cellular proteins. Consistent with such a role, αB is implicated in a growing number of diseases that includes cardiac myopathies and neurodegenerative diseases such as Alexander disease and Alzheimer's disease (5-7). Furthermore, αB has been shown to play a protective role and can reverse symptoms of multiple sclerosis (8). Thus, a full structural description of αB is an important step toward understanding its mode(s) of action. Past models of αB have been based on biochemical data (9); however, recent advances in structural biology of sHSPs (9-11) allow for a more detailed understanding of the assembly of αB multimers.As for all sHSPs, αB is organized in three domains (Fig. 1A): (i) an N-terminal domain of approximately 60 residues, (ii) a central α-crystallin domain (ACD) of about 90 residues involved in dimerization (Fig. S1A), and (iii) a C-terminal domain of 25 residues containing the IXI motif, usually...

SummaryAtomic level structural information on αB-Crystallin (αB), a prominent member of the small Heat Shock Protein (sHSP) family has been a challenge to obtain due its polydisperse, oligomeric nature. We show that magic-angle spinning solid-state NMR can be used to obtain high-resolution information on ∼ 580 kDa human αB assembled from 175-residue, 20 kDa subunits. An ∼100-residue α-crystallin domain is common to all sHSPs and solution-state NMR was performed on two different α-crystallin domain constructs isolated from αB. In vitro, the chaperone-like activities of full-length αB and the isolated α-crystallin domain are identical. Chemical shifts of the backbone and the C β resonances have been obtained for residues 64-162 (α-crystallin domain plus part of the C-terminus) in αB and the isolated α-crystallin domain by solid-and solution-state NMR, respectively. Both sets of data strongly predict six β-strands in the α-crystallin domain. A majority of residues in the α-crystallin domain have similar chemical shifts in both solid-and solution-state indicating a similar structure for the domain in its isolated and oligomeric forms. Sites of inter-subunit interaction are identified from chemical shift differences that cluster to specific regions of the α-crystallin domain. Multiple signals are observed for the resonances of M68 in the oligomer, identifying the region containing this residue as existing in heterogeneous environments within αB. Evidence for a novel dimerization motif in the human α-crystallin domain is obtained by a comparison of (i) solid-and solution-state chemical shift data and (ii) 1 H-15 N HSQC spectra as a function of pH. The isolated α-crystallin domain undergoes a dimer-monomer transition over the pH range of 7.5 to 6.8. This steep pH-dependent switch may be important for αB to function optimally, e.g., to preserve the filament integrity of cardiac muscle proteins such as actin and desmin during cardiac ischemia which is accompanied by acidosis.

We show that large protein complexes can be investigated in solution using magic-angle-spinning (MAS) NMR spectroscopy without the need for sample crystallization or precipitation. In order to efficiently average anisotropic interactions with MAS, the rotational diffusion of the molecule has to be suppressed. This can be readily achieved by lowering the sample temperature and by adding glycerol to the protein solution. The approach is demonstrated using the human small heat shock protein (sHSP) alphaB-Crystallin, which forms oligomeric assemblies of approximately 600 kDa. We suggest this scheme as an approach for overcoming size limitations imposed by overall tumbling in solution-state NMR investigations of large protein complexes.

Small heat shock proteins (sHSPs) play a central role in protein homeostasis under conditions of stress by binding partly unfolded, aggregate-prone proteins and keeping them soluble. Like many sHSPs, the widely expressed human sHSP, aB-crystallin ('aB'), forms large polydisperse multimeric assemblies. Molecular interactions involved in both sHSP function and oligomer formation remain to be delineated. A growing database of structural information reveals that a central conserved a-crystallin domain (ACD) forms dimeric building blocks, while flanking N-and C-termini direct the formation of larger sHSP oligomers. The most commonly observed inter-subunit interaction involves a highly conserved C-terminal 'IxI/V' motif and a groove in the ACD that is also implicated in client binding. To investigate the inherent properties of this interaction, peptides mimicking the IxI/V motif of aB and other human sHSPs were tested for binding to dimeric aB-ACD. IxI-mimicking peptides bind the isolated ACD at 221C in a manner similar to interactions observed in the oligomer at low temperature, confirming these interactions are likely to exist in functional aB oligomers.

In recent years, solid-state magic-angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) has been growing into an important technique to study the structure of membrane proteins, amyloid fibrils and other protein preparations which do not form crystals or are insoluble. Currently, a key bottleneck is the assignment process due to the absence of the resolving power of proton chemical shifts. Particularly for large proteins (approximately >150 residues) it is difficult to obtain a full set of resonance assignments. In order to address this problem, we present an assignment method based upon samples prepared using [1,3-13C]- and [2-13C]-glycerol as the sole carbon source in the bacterial growth medium (so-called selectively and extensively labelled protein). Such samples give rise to higher quality spectra than uniformly [13C]-labelled protein samples, and have previously been used to obtain long-range restraints for use in structure calculations. Our method exploits the characteristic cross-peak patterns observed for the different amino acid types in 13C-13C correlation and 3D NCACX and NCOCX spectra. An in-depth analysis of the patterns and how they can be used to aid assignment is presented, using spectra of the chicken alpha-spectrin SH3 domain (62 residues), alphaB-crystallin (175 residues) and outer membrane protein G (OmpG, 281 residues) as examples. Using this procedure, over 90% of the Calpha, Cbeta, C' and N resonances in the core domain of alphaB-crystallin and around 73% in the flanking domains could be assigned (excluding 24 residues at the extreme termini of the protein).

Fragment-based drug discovery (FBDD) relies on the premise that the fragment binding mode will be conserved on subsequent expansion to a larger ligand. However, no general condition has been established to explain when fragment binding modes will be conserved. We show that a remarkably simple condition can be developed in terms of how fragments coincide with binding energy hot spots—regions of the protein where interactions with a ligand contribute substantial binding free energy—the locations of which can easily be determined computationally. Because a substantial fraction of the free energy of ligand binding comes from interacting with the residues in the energetically most important hot spot, a ligand moiety that sufficiently overlaps with this region will retain its location even when other parts of the ligand are removed. This hypothesis is supported by eight case studies. The condition helps identify whether a protein is suitable for FBDD, predicts the size of fragments required for screening, and determines whether a fragment hit can be extended into a higher affinity ligand. Our results show that ligand binding sites can usefully be thought of in terms of an anchor site, which is the top-ranked hot spot and dominates the free energy of binding, surrounded by a number of weaker satellite sites that confer improved affinity and selectivity for a particular ligand and that it is the intrinsic binding potential of the protein surface that determines whether it can serve as a robust binding site for a suitably optimized ligand.

Narrow 1H NMR linewidths can be obtained for fully protonated protein samples in the solid state by using ultrafast magic‐angle spinning (60 kHz). Medium‐size microcrystalline and noncrystalline proteins can be analyzed without any need for deuteration of the protein sample. This approach provides assignments of the backbone 1H, 15N, 13Cα, and 13CO resonances and yields information about 1H–1H proximities.