Multidrug resistance (MDR) system. (a) Schematic of the AcrB-AcrA-TolC tripartite efflux system. Arrows indicate the substrate/proton antiport directions. (b) New cartoon representation of the crystal structure of AcrB trimers. Each monomer can be decomposed into trans-membrane domain, porter domain and docking domain. (c) Top view of the porter domains. Each monomer is composed of four subdomains: PC1, PC2, PN1 and PN2. (d) Ball and stick representation of the key residues in TM domain (region indicated by red box in (b)): Asp 407, Asp 408, Lys 940 and Thr978.