AcrA is a highly asymmetric protein capable of spanning the periplasm

AcrA protein is a component of the multi-drug efflux complex AcrAB-TolC of Escherichia coli. Judged by the hypersusceptibility phenotype of acrA mutants, the AcrAB-TolC system pumps out an extraordinarily wide variety of antibiotics, chemotherapeutic agents, detergents and dyes. This complex traverses both the inner and outer membranes of E. coli and catalyzes efflux of the drugs directly into the medium. The coordinated operation of the inner membrane transporter AcrB and outer membrane channel TolC is thought to be mediated by AcrA. The latter is a lipoprotein located in the periplasmic space. We show here that a lipid-deficient derivative of AcrA is functionally active as demonstrated by the complementation of the hypersusceptibility phenotype of the acrA mutant. Purified non-lipidated and intact forms of AcrA were able to restore, with similar efficiency, the activity of AcrA-dependent efflux of erythromycin in Ca2+-sucrose-treated E. coli cells. Using analytical ultracentrifugation and dynamic light scattering techniques we determined hydrodynamic properties of the non-lipidated AcrA and found that AcrA exists in solution as a highly asymmetric monomeric molecule with an axial ratio of 8. This elongated shape of AcrA is compatible with the hypothesis that this protein spans the periplasmic space coordinating the concerted operation of inner and outer membrane components of the complex.