Bacterial Innate Immunity: Mechanisms of PhoQ Sensing that Promote Salmonella Virulence

Abstract

Salmonella PhoQ is a histidine kinase with a periplasmic sensor domain that promotes virulence by detecting the hostile environment of the host macrophage phagosome. PhoQ kinase activity is repressed by divalent cations and induced in environments of acidic pH, limited divalent cations, and cationic antimicrobial peptides (CAMP). Upon activation, PhoQ phosphorylates the response regulator PhoP, coordinating regulation of greater than three hundred genes involved in virulence and intracellular survival. Previously, it was unclear which host signals are sensed by salmonellae to promote PhoPQ-mediated virulence. Using NMR and random activating mutagenesis, we defined conformational changes produced in the PhoQ periplasmic domain on exposure to acidic pH that indicate structural flexibility is induced in α-helices 4 and 5, suggesting this region contributes to sensing fluctuations in periplasmic pH. Therefore, we engineered a disulfide bond between the residues W104C and A128C in the PhoQ periplasmic domain that restrains conformational flexibility between the α-helices 4 and 5 and the α/β-core of the periplasmic domain. PhoQW104C-A128C is responsive to CAMP, but is inhibited for activation by acidic pH and divalent cation limitation. Salmonella enterica Typhimurium with the phoQW104C-A128C allele are virulent in mice and macrophages, indicating that acidic pH and divalent cation sensing by PhoQ are dispensable for virulence.