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Superposition of central helices of EmrE and AcrB. Although there is no significant sequence similarity between the two proteins, the proton-binding site as described by our model and found in the x-ray structure of AcrB is remarkably similar. In both cases, the central, symmetry-related helix constitutes the proton-binding site, although with different residues: EmrE uses two Glu, whereas Acrb uses a Lys and two Asp (only one shown).
