Transmembrane topology and histidine protein kinase activity of AgrC, the agr signal receptor in Staphylococcus aureus

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Abstract
The agr P2 operon in Staphylococcus aureus codes for the elements of a density‐sensing cassette made up of a typical two‐component signalling system and its corresponding inducer. It is postulated that the autoinducer, a post‐translationally modified octapeptide generated from the AgrD peptide, interacts with a receptor protein, coded by agrC, to transmit a signal via AgrA regulating expression of staphylococcal virulence genes through expression of agr RNA III. We show by analysis of PhoA fusions that AgrC is a transmembrane protein, and confirm using Western blotting that a 46 kDa protein corresponding to AgrC is present in the bacterial membrane. This protein is autophosphorylated on a histidine residue only in response to supernatants from an agr+ strain, and can also respond to the purified native octapeptide. A recombinant fusion protein where most of the N‐terminal region of AgrC is replaced by the Escherichia coli maltose‐binding protein is also autophosphorylated in response to stimulation by agr+ supernatants or purified octapeptide. We conclude that AgrC is the sensor molecule of a typical two‐component signal system in S. aureus, and that the ligand‐binding site of AgrC is probably located in the third extracellular loop of the protein.