Abstract
Listeria monocytogenes responds to environmental stress using a supra-macromolecular complex, the stressosome, to activate the stress sigma factor SigB. The stressosome structure, inferred from in vitro-assembled complexes, consists of the core proteins RsbR (here renamed RsbR1) and RsbS and, the kinase RsbT. The active complex is proposed to be tethered to the membrane and to support RsbR1/RsbS phosphorylation by RsbT and the subsequent release of RsbT following signal perception. Here, we show in actively-growing cells that L. monocytogenes RsbR1 and RsbS localize mostly in the cytosol in a fully phosphorylated state regardless of osmotic stress. RsbT however distributes between cytosolic and membrane-associated pools. The kinase activity of RsbT on RsbR1/RsbS and its requirement for maximal SigB activation in response to osmotic stress were demonstrated in vivo. Cytosolic RsbR1 interacts with RsbT, while this interaction diminishes at the membrane when RsbR1 paralogues (RsbR2, RsbR3 and RsbL) are present. Altogether, the data support a model in which phosphorylated RsbR1/RsbS may sustain basal SigB activity in unstressed cells, probably assuring a rapid increase in such activity in response to stress. Our findings also suggest that in vivo the active RsbR1-RsbS-RsbT complex forms only transiently and that membrane-associated RsbR1 paralogues could modulate its assembly.
Highlights
Listeria monocytogenes responds to environmental stress using a supra-macromolecular complex, the stressosome, to activate the stress sigma factor SigB
RsbR1 and RsbS are mainly cytosolic in L. monocytogenes live cells regardless of osmotic stress
Recent studies performed in L. monocytogenes show that RsbR and
Summary
Listeria monocytogenes responds to environmental stress using a supra-macromolecular complex, the stressosome, to activate the stress sigma factor SigB. The current model predicts that stress signal perception stimulates the kinase activity of RsbT to phosphorylate the second site in RsbR1 and the single site present in R sbS18–22 These phosphorylation events cause the release of RsbT from the stressosome complex and allow its interaction with RsbU, leading to the dephosphorylation of RsbV ~ P, sequestration of RsbW and thereby SigB activation[10,23,24]. Most of these steps have been inferred from in vitro studies based on recombinant proteins lacking kinase or phosphatase activities. Not discussed in the study of Impens et al in L. monocytogenes[15], the immunoprecipitation of RsbR1 paralogues with the miniprotein Prli[42] opens the possibility of heterogeneous stressosome complexes associating with the plasma membrane
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