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.
References:
Du D., van Veen HW. and Luisi BF. (2015). Assembly and operation of bacterial tripartite multidrug efflux pumps. Trends Microbiol. 23(5):311-9.
Du, D., A. Neuberger, M.W. Orr, C.E. Newman, P.C. Hsu, F. Samsudin, A. Szewczak-Harris, L.M. Ramos, M. Debela, S. Khalid, G. Storz, and B.F. Luisi. (2020). Interactions of a Bacterial RND Transporter with a Transmembrane Small Protein in a Lipid Environment. Structure. [Epub: Ahead of Print]
Hobbs, E.C., X. Yin, B.J. Paul, J.L. Astarita, and G. Storz. (2012). Conserved small protein associates with the multidrug efflux pump AcrB and differentially affects antibiotic resistance. Proc. Natl. Acad. Sci. USA 109: 16696-16701.
Examples:
TC#	Name	Organismal Type	Example
8.A.50.1.1	AcrAB (TC# 2.a.6.2.2) accessory protein of 49 aas and 1 N-terminal TMS, AcrZ or YbhT. It interacts directly with the transmembrane region of AcrB and stimulates its activity for certain drugs such as tetracycline, chloramphenicol and puromycin (Hobbs et al. 2012; Du et al. 2015).	Proteobacteria	AcrZ of E. coli

8.A.50.1.2	AcrZ homologue of 46 aas	Proteobacteria	AcrZ of Morganella morganii

8.A.50.1.3	AcrZ homologue of 45 aas.	Proteobacteria	AcrZ homologue of Xenorhabdus szentirmaii