Joyce M Baldwin | PubMed Health

PubMed2003

Three-dimensional structure of the bacterial multidrug transporter EmrE shows it is an asymmetric homodimer.

The small multidrug resistance family of transporters is widespread in bacteria and is responsible for bacterial resistance to toxic aromatic cations by proton-linked efflux. We have determined the three-dimensional (3D) structure of the Escherichia coli multidrug transporter EmrE by electron cryomicroscopy of 2D crystals, including data to 7.0 A resolution. The structure of EmrE consists of a bundle of eight transmembrane alpha-helices with one substrate molecule bound near the centre. The substrate binding chamber is formed from six helices and is accessible both from the aqueous phase and laterally from the lipid bilayer. The most remarkable feature of the structure of EmrE is that it is an asymmetric homodimer. The possible arrangement of the two polypeptides in the EmrE dimer is discussed based on the 3D density map.

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PubMed2003

Conformational changes in the multidrug transporter EmrE associated with substrate binding.

EmrE is a bacterial multidrug transporter of the small multidrug resistance family, which extrudes large hydrophobic cations such as tetraphenylphosphonium (TPP(+)) out of the cell by a proton antiport mechanism. Binding measurements were performed on purified EmrE solubilized in dodecylmaltoside to determine the stoichiometry of TPP(+) binding; the data showed that one TPP(+) molecule bound per EmrE dimer. Reconstitution of purified EmrE at low lipid:protein ratios in either the presence or the absence of TPP(+) produced well ordered two-dimensional crystals. Electron cryo-microscopy was used to collect images of frozen hydrated EmrE crystals and projection maps were determined by image processing to 7A resolution. An average native EmrE projection structure was calculated from the c222 and p222(1) crystals, which was subsequently subtracted from the average of two independent p2 projection maps of EmrE with TPP(+) bound. The interpretation of the difference density image most consistent with biochemical data suggested that TPP(+) bound at the monomer-monomer interface in the centre of the EmrE dimer, and resulted in the movement of at least one transmembrane alpha-helix.

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