A Novel SRP Recognition Sequence in the Homeostatic Control Region of Heat Shock Transcription Factor σ32

Abstract

Heat shock response (HSR) generally plays a major role in sustaining protein homeostasis. In Escherichia coli, the activity and amount of the dedicated transcription factor σ 32 transiently increase upon heat shock. The initial induction is followed by chaperone-mediated negative feedback to inactivate and degrade σ 32. Previous work reported that signal recognition particle (SRP)-dependent targeting of σ 32 to the membrane is essential for feedback control, though how SRP recognizes σ 32 remained unknown. Extensive photo-and disulfide cross-linking studies in vivo now reveal that the highly conserved regulatory region of σ 32 that lacks a consecutive hydrophobic stretch interacts with the signal peptide-binding site of Ffh (the protein subunit of SRP). Importantly, the σ 32-Ffh interaction observed was significantly affected by mutations in this region that compromise the feedback regulation, but not by deleting the DnaK/DnaJ chaperones. Homeostatic regulation of HSR thus requires a novel type of SRP recognition mechanism.