Multidrug resistance proteins QacA and QacB from Staphylococcus aureus: membrane topology and identification of residues involved in substrate specificity

Proc Natl Acad Sci U S A. 1996 Apr 16;93(8):3630-5. doi: 10.1073/pnas.93.8.3630.


The closely related multidrug efflux pumps QacA and QacB, from the bacterial pathogen Staphylococcus aureus, both confer resistance to various toxic organic cations but differ in that QacB mediates lower levels of resistance to divalent cations. Cloning and nucleotide sequencing of the qacB gene revealed that qacB differs from qacA by only seven nucleotide substitutions. Random hydroxylamine mutagenesis of qacB was undertaken, selecting for variants that conferred increased resistance to divalent cations. Both QacA and the QacB mutants capable of conferring resistance to divalent cations contain an acidic residue at either amino acid 322 or 323, whereas QacB contains uncharged residues in these positions. Site-directed mutagenesis of qacA confirmed the importance of an acidic residue within this region of QacA in conferring resistance to divalent cations. Membrane topological analysis using alkaline phosphatase and beta-galactosidase fusions indicated that the QacA protein contains 14 transmembrane segments. Thus, QacA represents the first membrane transport protein shown to contain 14 transmembrane segments, and confirms that the major facilitator superfamily contains a family of proteins with 14 transmembrane segments.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Sequence
  • Bacterial Proteins / chemistry*
  • Bacterial Proteins / genetics
  • Bacterial Proteins / metabolism
  • Base Sequence
  • Binding Sites
  • Carrier Proteins / chemistry*
  • Carrier Proteins / genetics
  • Carrier Proteins / metabolism
  • Cloning, Molecular
  • DNA Primers / genetics
  • DNA, Bacterial / genetics
  • Drug Resistance, Multiple / genetics*
  • Membrane Transport Proteins*
  • Molecular Sequence Data
  • Molecular Structure
  • Mutagenesis, Site-Directed
  • Staphylococcus aureus / drug effects*
  • Staphylococcus aureus / genetics*
  • Substrate Specificity


  • Bacterial Proteins
  • Carrier Proteins
  • DNA Primers
  • DNA, Bacterial
  • Membrane Transport Proteins
  • QacB protein, Staphylococcus aureus
  • qacA protein, Staphylococcus aureus

Associated data

  • GENBANK/AF053772