A distribution with the source code of ARIA 2.1 is freely available at http://www.pasteur.fr/recherche/unites/Binfs/aria2.

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 ...

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