Ethidium multidrug resistance protein (EmrE) is a member of the small multidrug resistance family of proteins and is responsible for resistance to a diverse group of lipophilic cations. To examine the multimeric state(s), size-exclusion HPLC and sedimentation velocity experiments were performed with EmrE solubilized in N-dodecyl-beta-D-maltopyranoside (DM) detergent. EmrE was purified from Escherichia coli membranes using organic extraction with a 3:1 chloroform:methanol solvent followed by LH-20 chromatography and the recovered pure protein was re-solubilized in a buffer continaing 2% DM. The purified protein was analyzed by SEC-HPLC to estimate the monodispersity and to determine the amount of bound detergent. The results show that EmrE is homogeneous in DM with a Stokes radius of 3.6 nm compatible with that of a monomer. Sedimentation velocity experiments indicated that the EmrE preparation was monodisperse and supports the fact that the organic extracted protein solubilized in DM is monomeric. This monomeric form of the protein analyzed here is also shown to bind substrate in the micromolar range. (C) 2004 Elsevier Inc. All rights reserved.