Microbiology news and views
J. Mol. Micro. Biotechnol. 3: 155-162
Precious Things Come in Little Packages
Shimon Schuldiner, Dorit Granot, Sonia Steiner Mordoch, Shira Ninio, Dvir Rotem, Michael Soskin, and Hagit Yerushalmi
The 110-amino acid multidrug transporter from
E. coli, EmrE, is a member of the family of MiniTexan or
Smr drug transporters. EmrE can transport acriflavine, ethidium bromide, tetraphenylphosphonium
(TPP+), benzalkonium and several other drugs with relatively high affinities. EmrE is an
H+/drug antiporter, utilizing the proton electrochemical gradient generated across the bacterial cytoplasmic membrane by exchanging
two protons with one substrate molecule. The EmrE multidrug transporter is unique in its small size and
hydrophobic nature. Hydropathic analysis of the EmrE sequence predicts four
a-helical transmembrane segments. This model is experimentally supported by FTIR studies that confirm the high
a-helicity of the protein and by high-resolution heteronuclear NMR analysis of the protein structure. The TMS of EmrE are tightly packed in
the membrane without any continuous aqueous domain, as was shown by Cysteine scanning experiments.
These results suggest the existence of a hydrophobic pathway through which the substrates are translocated.
EmrE is functional as a homo-oligomer as suggested by several lines of evidence, including
co-reconstitution experiments of wild-type protein with inactive mutants in which negative dominance has been observed.
EmrE has only one membrane embedded charged residue, Glu-14, that is conserved in more than fifty
homologous proteins and it is a simple model system to study the role of carboxylic residues in ion-coupled
transporters. We have used mutagenesis and chemical modification to show that Glu-14 is part of the substrate-binding
site. Its role in proton binding and translocation was shown by a study of the effect of pH on ligand binding,
uptake, efflux and exchange reactions. We conclude that Glu-14 is an essential part of a binding site, common
to substrates and protons. The occupancy of this site is mutually exclusive and provides the basis of the
simplest coupling of two fluxes. Because of some of its properties and its size, EmrE provides a unique system
to understand mechanisms of substrate recognition and translocation.
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