Unfolding of metastable linker region is at the core of Hsp33 activation as a redox-regulated chaperone.

Hsp33, a molecular chaperone specifically activated by oxidative stress conditions that lead to protein unfolding, protects cells against oxidative protein aggregation. Stress sensing in Hsp33 occurs via its C-terminal redox switch domain, which consists of a zinc center that responds to the presence of oxidants and an adjacent metastable linker region, which responds to unfolding conditions. Here we show that single mutations in the N terminus of Hsp33 are sufficient to either partially (Hsp33-M172S) or completely (Hsp33-Y12E) abolish this post-translational regulation of Hsp33 chaperone function. Both mutations appear to work predominantly via the destabilization of the Hsp33 linker region without affecting zinc coordination, redox sensitivity, or substrate binding of Hsp33. We found that the M172S substitution causes moderate destabilization of the Hsp33 linker region, which seems sufficient to convert the redox-regulated Hsp33 into a temperature-controlled chaperone. The Y12E mutation leads to the constitutive unfolding of the Hsp33 linker region thereby turning Hsp33 into a constitutively active chaperone. These results demonstrate that the redox-controlled unfolding of the Hsp33 linker region plays the central role in the activation process of Hsp33. The zinc center of Hsp33 appears to act as the redox-sensitive toggle that adjusts the thermostability of the linker region to the cell redox status. In vivo studies confirmed that even mild overexpression of the Hsp33-Y12E mutant protein inhibits bacterial growth, providing important evidence that the tight functional regulation of Hsp33 chaperone activity plays a vital role in bacterial survival.