Stressosome and model of RsbRA-RsbS-RsbT activation.

<p>(A) Cytoplasmic stressosome complex (RsbR-RsbS-RsbT) controls activation of the environmental phosphatase (RsbU) in response to diverse physical and chemical signals. STAS domains of RsbR and RsbS form the stressosome core (red); N-terminal domains of RsbR are hypothetical sensors (red crosshatch); dissociable RsbT kinase (green) is positive activator of RsbU (yellow). RsbU removes the phosphoryl group (orange) from S56 on the RsbV anti-anti-σ, ultimately activating the σ^B stress factor. The energy phosphatase (RsbP) and the phosphatase-independent cold stress pathways are shown in dotted outline. Arrowheads indicate activation of protein targets or enzymatic reactions; T-headed lines indicate inhibition. (B) Model of stressosome control of RsbU activity. Stressosome core comprises partially redundant RsbRA, RB, RC, and RD co-antagonists (represented here as RsbRA) and the RsbS antagonist, which binds the RsbT kinase. In unstressed cells RsbT phosphorylates RsbRA on T171, facilitating subsequent activation of RsbT kinase activity toward RsbS (+ arrow). During the stress response RsbT phosphorylates RsbS on S59; RsbT is released to bind and activate RsbU. The RsbX feedback phosphatase (dotted outline) dephosphorylates RsbS-P. (C) RsbS comprises a single STAS domain (red rectangle), whereas the larger RsbRA has an N-terminal non-heme globin domain (red crosshatching) and C-terminal STAS domain (red). Phosphorylated S59 and T171 residues lie in the STAS domains; T205 (light grey) is phosphorylated only under extreme stress (see text). RsbRB, RC and RD are similar to RsbRA, with corresponding phosphorylated residues (not shown); these three paralogs were removed in some strains. YtvA is an RsbR family member that increases stressosome output in response to blue light, sensed by its N-terminal LOV (Light-Oxygen-Voltage) domain (blue). Figure modified from ref <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0090741#pone.0090741-Gaidenko3" target="_blank">[20]</a>.