Modulation of Cholera Toxin Structure/Function by Hsp90

Abstract

Cholera toxin (CT) is released into the extracellular environment, but the catalytic CTA1 subunit attacks its G protein target within the cytosol of an intoxicated cell. To access the cytosol, CT moves by vesicle carriers from the cell surface to the endoplasmic reticulum (ER). CTA1 then dissociates from the rest of the toxin and shifts to a disordered conformation that facilitates its passage into the cytosol through a pore in the ER membrane. We have found that CTA1 export to the cytosol requires the host cytosolic chaperone Hsp90. Loss of Hsp90 function trapped CTA1 in the ER, and Hsp90 was sufficient for in vitro export of CTA1 from the ER. Structural studies demonstrated Hsp90 will refold disordered CTA1. ATP hydrolysis by Hsp90 was required for both CTA1 refolding and CTA1 extraction from the ER, which suggests a ratchet mechanism for the chaperone-driven movement of CTA1 to the cytosol: the refolding of CTA1 as it emerges at the cytosolic face of the ER membrane would prevent the toxin from sliding back into the translocon pore and would thereby ensure the unidirectional movement of CTA1 from the ER to the cytosol. Hsp90 bound to the N-terminus of CTA1 and did not release CTA1 after refolding the toxin. The continued association of Hsp90 with CTA1 allowed the toxin to maintain an active conformation at 37 °C. Hsp90 thus plays two key roles CT intoxication: it couples toxin refolding with toxin extraction from the ER, and it maintains the cytosolic toxin in a functional conformation.