Analyzing conformational changes in the transport cycle of EmrE

The small multidrug resistance transporters represent a unique model system for studying the mechanism of secondary active transport and membrane protein evolution. However, this seemingly simple protein has been highly controversial. Recent studies have provided experimental evidence that EmrE exists as an asymmetric dimer that exchanges between identical inward-facing and outward-facing states. Re-examination of the published literature in light of these findings fills in many details of the microscopic steps in the transport cycle. Future work will need to examine how the symmetry observed in vitro affects EmrE function in the asymmetric environment of its native Escherichia coli membrane.

► EmrE is oriented both ways in its native E. coli membrane. ► Antiparallel dimer topology confirmed by EPR, smFRET, and cross-linking. ► NMR dynamics provides experimental evidence for AB–BA exchange model. ► Can now fill in structures and rate constants for many steps in the transport cycle.