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

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.