1. Overview

AcrB is a major multidrug efflux transporter in Escherichia coli cooperating with an outer membrane channel, TolC and a membrane fusion protein, AcrA. Here, we describe crystal structures of AcrB with and without substrates.

The AcrB-drug complex consists of three protomers, each of which has different conformation corresponding to one of the three functional states of the transport cycle. Bound substrate was found in the periplasmic domain of one of the three protomers. The voluminous binding pocket is aromatic and allows multi-site binding. The structures show that drugs are presumably exported by a three-step functionally rotating mechanism in which drugs undergo ordered binding change.