EmrE dimerization depends on membrane environment

The small multi-drug resistant (SMR) transporter EmrE functions as a homodimer. Although the small size of EmrE would seem to make it an ideal model system, it can also make it challenging to work with. As a result, a great deal of controversy has surrounded even such basic questions as the oligomeric state. Here we show that the purified protein is a homodimer in isotropic bicelles with a monomer–dimer equilibrium constant (K_MD^2D) of 0.002–0.009 mol% for both the substrate-free and substrate-bound states. Thus, the dimer is stabilized in bicelles relative to detergent micelles where the K_MD^2D is only 0.8–0.95 mol% (Butler et al. 2004). In dilauroylphosphatidylcholine (DLPC) liposomes K_MD^2D is 0.0005–0.0008 mol% based on Förster resonance energy transfer (FRET) measurements, slightly tighter than bicelles. These results emphasize the importance of the lipid membrane in influencing dimer affinity.