A chaperone network controls the heat shock response in E. coli

Abstract

The heat shock response controls levels of chaperones and proteases to ensure a proper cellular environment for protein folding. In Escherichia coli, this response is mediated by the bacterial-specific transcription factor, σ32. The DnaK chaperone machine regulates both the amount and activity of σ32, thereby coupling σ32 function to the cellular protein folding state. In this manuscript, we analyze the ability of other major chaperones in E. coli to regulate σ32, and we demonstrate that the GroEL/S chaperonin is an additional regulator of σ32. We show that increasing the level of GroEL/S leads to a decrease in σ32 activity in vivo and this effect can be eliminated by co-overexpression of a GroEL/S-specific substrate. We also show that depletion of GroEL/S in vivo leads to up-regulation of σ32 by increasing the level of σ32. In addition, we show that changing the levels of GroEL/S during stress conditions leads to measurable changes in the heat shock response. Using purified proteins, we show that that GroEL binds to σ32 and decreases σ32-dependent transcription in vitro, suggesting that this regulation is direct. We discuss why using a chaperone network to regulate σ32 results in a more sensitive and accurate detection of the protein folding environment.