The 10-state free-exchange model of EmrE transport. All states and transitions observed for EmrE are incorporated into one unified model. Multiple pathways exist for proton-coupled antiport (2:1 or 1:1 proton/drug stoichiometry), proton-coupled symport, uncoupled drug transport, and uncoupled proton transport (leak). Colors are as in Fig. 1, and states are labeled (apo, E; proton-bound, EH; double-proton-bound, EHH; drug-bound, ED; and proton/drug-bound, EHD). The membrane is not shown for clarity due to the complexity of this model, but the states of the transporter open to either side of the membrane are separated by the orange dotted line and labeled ext (open-out) or int (open-in).