Site-Directed Disulfide Cross-Linking Shows that Cleft Flexibility in the Periplasmic Domain Is Needed for the Multidrug Efflux Pump AcrB of Escherichia coli

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Abstract
Escherichia coli AcrB is a multidrug efflux transporter that recognizes multiple toxic chemicals having diverse structures. Recent crystallographic studies of the asymmetric trimer of AcrB suggest that each protomer in the trimeric assembly goes through a cycle of conformational changes during drug export. However, biochemical evidence for these conformational changes has not been provided previously. In this study, we took advantage of the observation that the external large cleft in the periplasmic domain of AcrB appears to become closed in the crystal structure of one of the three protomers, and we carried out in vivo cross-linking between cysteine residues introduced by site-directed mutagenesis on both sides of the cleft, as well as at the interface between the periplasmic domains of the AcrB trimer. Double-cysteine mutants with mutations in the cleft or the interface were inactive. The possibility that this was due to the formation of disulfide bonds was suggested by the restoration of transport activity of the cleft mutants in a dsbA strain, which had diminished activity to form disulfide bonds in the periplasm. Furthermore, rapidly reacting, sulfhydryl-specific chemical cross-linkers, methanethiosulfonates, inactivated the AcrB transporter with double-cysteine residues in the cleft expressed in dsbA cells, and this inactivation could be observed within a few seconds after the addition of a cross-linker in real time by increased ethidium influx into the cells. These observations indicate that conformational changes, including the closure of the external cleft in the periplasmic domain, are required for drug transport by AcrB.