Structure of Phosphorylated-like ClpXP Adaptor RssB Reveals an Interface Switch for Activation

SUMMARY In γ-proteobacteria such as Escherichia coli, the general stress response is mediated by σs, the stationary phase dissociable promoter specificity subunit of RNA polymerase. σs is degraded by ClpXP during active growth in a process dependent on the RssB adaptor, which acts catalytically and is thought to be stimulated by phosphorylation of a conserved aspartate in its N-terminal receiver domain. Here we present the crystal structure of full-length RssB bound to a beryllofluoride phosphomimic. Compared to the inhibited IraD anti-adaptor-bound RssB structure, our study reveals movements and coil-to-helix transitions in the C-terminal region of the RssB receiver domain and in the inter-domain segmented helical linker, accompanied by packing of the C-terminal effector domain onto the α4-β5-α5 (4-5-5) “signaling” face of the RssB receiver domain. This face is often the locus of protein-protein interactions in unphosphorylated receiver domains, but its masking is unusual in their phosphorylated forms. Our structure emphasizes the remarkable plasticity that underpins regulatory strategies within the large family of response regulators.