8.A.50 The AcrZ RND auxiliary subunit (AcrZ) Family

The AcrAB-TolC multidrug efflux pump confers resistance to a wide variety of antibiotics and other compounds in Escherichia coli. Hobbs et al. 2012 showed that AcrZ (formerly named YbhT), a 49-amino-acid inner membrane protein with a single TMS, associates with the AcrAB-TolC complex. Co-purification of AcrZ with AcrB in the absence of both AcrA and TolC, two-hybrid assays and suppressor mutations all indicated that this interaction occurs through the inner membrane protein AcrB. The highly conserved acrZ gene is coregulated with acrAB through induction by the MarA, Rob, and SoxS transcriptional regulators. Mutants lacking AcrZ are sensitive to many, but not all, of the antibiotics transported by AcrAB-TolC. This differential antibiotic sensitivity suggests that AcrZ may enhance the ability of the AcrAB-TolC pump to export certain classes of substrates including tetracycline, puromycin and chloramphenicol (Hobbs et al. 2012).

The AcrBZ complex in a lipid environment has been solved by cryoEM, and comparisons suggest that conformational changes occur in the drug-binding pocket of AcrB as a result of AcrZ binding (Du et al. 2020). Simulations indicate that cardiolipin preferentially interacts with the AcrBZ complex, due to increased contact surface, and the chloramphenicol sensitivity of bacteria lacking AcrZ is exacerbated when combined with cardiolipin deficiency. Thus, AcrZ and lipid cooperate to allosterically modulate AcrB activity.