Substrate Binding Drives Large-Scale Conformational Changes in the Hsp90 Molecular Chaperone

Hsp90 is a ubiquitous molecular chaperone. Previous structural analysis demonstrated that Hsp90 can adopt a large number of structurally distinct conformations; however, the functional role of this flexibility is not understood. Here we investigate the structural consequences of substrate binding with a model system in which Hsp90 interacts with a partially folded protein (Delta 131 Delta), a well-studied fragment of staphylococcal nuclease. SAXS measurements reveal that under apo conditions, Hsp90 partially closes around Delta 131 Delta, and in the presence of AMPPNP, Delta 131 Delta binds with increased affinity to Hsp90's fully closed state. FRET measurements show that Delta 131 Delta accelerates the nucleotide-driven open/closed transition and stimulates ATP hydrolysis by Hsp90. NMR measurements reveal that Hsp90 binds to a specific, highly structured region of Delta 131 Delta. These results suggest that Hsp90 preferentially binds a locally structured region in a globally unfolded protein, and this binding drives functional changes in the chaperone by lowering a rate-limiting conformational barrier.