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J Biol Chem, Vol. 274, Issue 6, 3541-3548, February 5, 1999
From the School of Biological Sciences, University of Sydney,
Sydney, New South Wales 2006, Australia
The multidrug efflux pump QacA from
Staphylococcus aureus confers resistance to an extensive
range of structurally dissimilar compounds. Fluorimetric analyses
demonstrated that QacA confers resistance to the divalent cation
4',6-diamidino-2-phenylindole, utilizing a proton motive
force-dependent efflux mechanism previously demonstrated
for QacA-mediated resistance to the monovalent cation ethidium. Both
the ionophores nigericin and valinomycin inhibited QacA-mediated export
of ethidium, indicating an electrogenic
drug/nH+ (n 
2) antiport
mechanism. The kinetic parameters, Km and
Vmax, were determined for QacA-mediated export
of four fluorescent substrates, 4',6-diamidino-2-phenylindole,
3',3'-dipropyloxacarbocyanine, ethidium, and pyronin Y. Competition
studies showed that QacA-mediated ethidium export is competitively
inhibited by monovalent cations, e.g. benzalkonium, and
non-competitively inhibited by divalent cations, e.g.
propamidine, which suggests that monovalent and divalent cations bind
at distinct sites on the QacA protein. The quaternary ammonium salt,
1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene, was used as a
membrane-specific fluorescence probe and demonstrated that the amount
of substrate entering the inner leaflet was significantly reduced in
QacA-containing strains, supporting the notion that the substrate is
extruded directly from the membrane.
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