Breaking the Deadlock of Molecular Chaperones

Protein folding in the cell requires ATP-driven chaperone machines. It is poorly understood, however, how these machines fold proteins. Here we propose that the conserved Hsp70 and Hsp90 chaperones support formation of the folding nucleus by providing a gradient of decreasing hydrophobicity. Early on the folding pathway Hsp70 uses its highly hydrophobic binding pocket to recover a stalled, unproductive folding intermediate. The aggressive nature of Hsp70 action, however, blocks productive folding by grabbing hydrophobic, core-forming segments. This precludes on-pathway nucleation at high, physiological Hsp70 levels. Transfer to the less hydrophobic Hsp90 enables the intermediate to resume forming its folding nucleus. Subsequently, the protein enters a spontaneous folding trajectory towards its native state, independent of the ATPase activities of both Hsp70 and Hsp90. Our findings provide a general mechanistic concept for chaperoned protein folding.