Characterization of the Catalytic Activities of the PhoQ Histidine Protein Kinase of Salmonella enterica Serovar Typhimurium

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Abstract
Studies of Escherichia coli membranes that were highly enriched in the Salmonella enterica serovar Typhimurium PhoQ protein showed that the presence of ATP and divalent cations such as Mg 2+ , Mn 2+ , Ca 2+ , or Ba 2+ resulted in PhoQ autophosphorylation. However, when Mg 2+ or Mn 2+ was present at concentrations higher than 0.1 mM, the kinetics of PhoQ autophosphorylation were strongly biphasic, with a rapid autophosphorylation phase followed by a slower dephosphorylation phase. A fusion protein lacking the sensory and transmembrane domains retained the autokinase activity but could not be dephosphosphorylated when Mg 2+ or Mn 2+ was present at high concentrations. The instability of purified [ 32 P]phospho-PhoP in the presence of PhoQ-containing membranes indicated that PhoQ also possesses a phosphatase activity. The PhoQ phosphatase activity was stimulated by increasing the Mg 2+ concentration. These data are consistent with a model in which Mg 2+ binding to the sensory domain of PhoQ coordinately regulates autokinase and phosphatase activities.