(A) The components of a tripartite efflux system (adapted from Protein Data Bank ID 5O66) visualized in side view with the inner membrane pump (IMP, blue), membrane fusion protein (MFP, green) and outer membrane protein (OMP, purple). Pictured is the IMP:MFP:OMP subunit ratio of 3:6:3, which is the most common assembly pattern. The outer membrane (OM) and inner membrane (IM) are designated by dashed lines. (B) A magnified view of one subunit of an RND-type inner membrane pump. Periplasmic subunits are designated DC (yellow), DN (red), PN1 (light blue), PN2 (purple), PC1 (orange), and PC2 (green). The transmembrane (TM) and membrane-associated helices are designated blue. RND, resistance–nodulation–cell division.