Characterization of the bacterial sensor protein PhoQ. Evidence for distinct binding sites for Mg2+ and Ca2+.
The PhoP/PhoQ two-component regulatory system governs several virulence properties in the Gram-negative bacterium Salmonella typhimurium. The PhoQ protein is a Mg2+ and Ca2+ sensor that modulates transcription of PhoP-regulated genes in response to the extracellular concentrations of these divalent cations. We have purified a 146-amino acid polypeptide corresponding to the periplasmic (i.e. sensing) domain of the PhoQ protein. Mg2+ altered the tryptophan intrinsic fluorescence of this polypeptide whereas Ba2+, which is unable to modulate transcription of PhoP-regulated genes, did not. Mg2+ was more effective than Ca2+ at repressing transcription of PhoP-activated genes in vivo. However, maximal repression was achieved when both cations were present. An avirulent mutant harboring a single amino acid substitution in the sensing domain of PhoQ exhibited lower affinity for Ca2+ but similar affinity for Mg2+. Cumulatively, these experiments demonstrate that Mg2+ can bind to the sensing domain of PhoQ and establish the presence of distinct binding sites for Mg2+ and Ca2+ in the PhoQ protein.
Véscovi
E G
EG
Department of Molecular Microbiology, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
Ayala
Y M
YM
Di Cera
E
E
Groisman
E A
EA
D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000378:metabolism*; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D008274:Magnesium / Q000378:metabolism*; D008969:Molecular Sequence Data; D011994:Recombinant Proteins / Q000378:metabolism; D012486:Salmonella typhimurium / Q000378:metabolism*; D013050:Spectrometry, Fluorescence
D001426:Bacterial Proteins; C079792:PhoQ protein, Bacteria; D011994:Recombinant Proteins; D008274:Magnesium; D002118:Calcium