Crystal Structure of a Functional Dimer of the PhoQ Sensor Domain

Abstract

The PhoP-PhoQ two-component system is a well studied bacterial signaling system that regulates virulence and stress response. Catalytic activity of the histidine kinase sensor protein PhoQ is activated by low extracellular concentrations of divalent cations such as Mg 2؉ , and subsequently the response regulator PhoP is activated in turn through a classic phosphotransfer pathway that is typical in such systems. The PhoQ sensor domains of enteric bacteria contain an acidic cluster of residues (EDDDDAE) that has been implicated in direct binding to divalent cations. We have determined crystal structures of the wildtype Escherichia coli PhoQ periplasmic sensor domain and of a mutant variant in which the acidic cluster was neutralized to conservative uncharged residues (QNNNNAQ). The PhoQ domain structure is similar to that of DcuS and CitA sensor domains, and this PhoQ-DcuS-CitA (PDC) sensor fold is seen to be distinct from the superficially similar PAS domain fold. Analysis of the wild-type structure reveals a dimer that allows for the formation of a salt bridge across the dimer interface between Arg-50 and Asp-179 and with nickel ions bound to aspartate residues in the acidic cluster. The physiological importance of the salt bridge to in vivo PhoQ function has been confirmed by mutagenesis. The mutant structure has an alternative, nonphysiological dimeric association.

Extracytoplasmic PAS-Like Domains Are Common in Signal Transduction Proteins

“…1d). Differences observed between these domains have recently been pointed out by Cheung et al (1). They suggested that these domains comprise a fold "distinct from the PAS domain structures with which they share a ␤-sheet topology" and named them "PDC" fold based on previously determined structures (PhoQ, DcuS, CitA).…”

Secretion, modification, and regulation of Ax21

“…Low cation concentrations promote activation of mgtA, pbgC, pcgF, pcgG, mgtCB, and psiD genes whereas high concentrations result in the repression of these genes [47,48]. These results indicate that Salmonella PhoQ is a sensor for periplasmic concentrations of divalent cations.…”

“…These results indicate that Salmonella PhoQ is a sensor for periplasmic concentrations of divalent cations. The role of divalent cations as signals for PhoQ is also supported by the crystal structures of the PhoQ periplasmic sensor domains from S. typhimurium and E. coli [48]. Similarly, we have shown that the Xoo PhoPQ system is required for sensing low extracellular Mg 2+ and Ca 2+ concentrations, conditions that the pathogen likely is confronted with upon entry into the xylem of the rice plant [33].…”

Sensor Domain of Histidine Kinase KinB of Pseudomonas

“…The majority of previously characterized SDs adopt two main types of structural scaffolds, the PAS-like and four-helix bundle domains. A PASlike (16) or "PDC" fold (17) is represented by the structures of PhoQ (17,18), DcuS (19), and CitA (16). The double PAS-like domain has also been observed in LuxQ (20), DctB (21,22), and in recently reported KinD (23).…”