Solid-state NMR and SAXS studies provide a structural basis for the activation of αB-crystallin oligomers

The small heat shock protein alpha B-crystallin (alpha B) contributes to cellular protection against stress. For decades, high-resolution structural studies on oligomeric alpha B 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 similar to 121 degrees between the planes of the beta-sandwich formed by alpha-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 beta 4 and beta 8, consistent with a pH-dependent regulation of the chaperone function. N-terminal region residues Ser59-Trp60-Phe61 are involved in intermolecular interaction with beta 3. Intermolecular restraints from NMR and volumetric restraints from SAXS were combined to calculate a model of a 24-subunit alpha B oligomer with tetrahedral symmetry.