The projection structure of EmrE, a proton-linked multidrug transporter from Escherichia coli, at 7 angstrom resolution

EmrE belongs to a family of eubacterial multidrug transporters that confer resistance to a wide variety of toxins by coupling the influx of protons to toxin extrusion. EmrE was purified and crystallized in two dimensions by r econstitution with dimyristoylphosphatidylcholine into lipid bilayers, Imag es of frozen hydrated crystals were collected by cryo-electron microscopy a nd a projection structure of EmrE was calculated to 7 Angstrom resolution. The projection map shows an asymmetric EmrE dimer with overall dimensions s imilar to 31 x 40 Angstrom, comprising an are of highly tilted helices sepa rating two helices nearly perpendicular to the membrane from another two he lices, one tilted and the other nearly perpendicular. There is no obvious 2 -fold symmetry axis perpendicular to the membrane within the dimer, suggest ing that the monomers may have different structures in the functional unit.