Schematic view of the organization of transport systems. Schematic view of the organization of transport systems. In gram-negative bacteria, substrates can cross the outer membrane by facilitated diffusion through porins, which are trimeric channels. Red circles represent transported substrates, small green circles represent cotransported ions, and small blue circles represent phosphates. (a) Group translocators and secondary transporters. (A) PTSs. PTSs consist of a set of cytoplasmic energy-coupling proteins and various integral membrane permeases/sugar phosphotransferases, each specific for a different sugar. The E. coli mannitol permease consists of two cytoplasmic domains (EIIA and EIIB) involved in mannitol phosphorylation and an integral membrane domain (EIIC) which is sufficient to bind mannitol but which transports mannitol at a rate that is dependent on phosphorylation of the EIIA and EIIB domains. The two other components are common to all PTS systems. The soluble enzyme I (EI) autophosphorylates in the presence of Mg2+. The histidine protein (HPr) is the energy-coupling protein and delivers phosphoryl groups from EI to the sugar-specific transporters (EIIs). (B) TRAP transporters. A periplasmic BP, which is unrelated to an ABC BP at the sequence level but similar in secondary structure, functions in association with two membrane components, namely, a large TM subunit involved in the translocation process and a smaller membrane component of unknown function. The driving force for solute accumulation is an electrochemical ion gradient, not ATP hydrolysis. (C) Ion-driven MFS transporters. These transporters typically consist of a single cytoplasmic membrane protein with 12 TM segments that couples transport of small solutes to existing gradients of ions, such as protons or sodium ions. Symporters pump two or more types of solutes in the same direction simultaneously, using the electrochemical gradient of one of the solutes as the driving force. Antiporters (not shown) are driven in a similar way, except that the solutes are transported in opposite directions across the membrane. (D) Uniporters transport one type of solute and are driven directly by the substrate gradient. (b) ABC import systems. (E) Vitamin B12 importer. The vitamin B12 uptake system of E. coli includes a high-affinity OMR, BtuB, that translocates the substrate through the outer membrane in an energy-dependent step that requires an active TonB-ExbB-ExbD complex. Substrates are captured by the periplasmic BP BtuF in the periplasmic space and presented to a cytoplasmic complex made of two copies each of BtuC and BtuD. This complex mediates the ATP hydrolysis-dependent translocation of vitamin B12 into the cytoplasm. (F) Maltose-maltodextrin importer. The transport of maltodextrins larger than maltotriose through the outer membrane requires the trimeric maltoporin LamB. Substrates are captured by the maltose-BP MalE in the periplasmic space and presented to a cytoplasmic complex made of MalF, MalG, and two copies of MalK. (c) Comparison between secondary RND and primary ABC export systems. (G) AcrAB-TolC exporter. This hypothetical model of the RND family AcrA-AcrB-TolC drug efflux pump is based on the trimeric structures determined for TolC and AcrB. TolC is predicted to contact the apex of the AcrB trimer. Two molecules of the MFP AcrA are shown, but it is probable that this protein exists as higher-order oligomers in the complex. Hydrophobic drugs are probably pumped out of the membrane lipid bilayer coupled to the downhill movement of protons across the cytoplasmic membrane. (H) Hemolysin HlyBD-TolC exporter. This hypothetical assembled model consists of a TolC trimer, a dimer of the IM-ABC protein HlyB, and the MFP HlyD. The exact oligomeric state of HlyD is not known accurately, though it may be trimeric. The TM and ABC domains of HlyB are represented by red rectangles and green circles, respectively. Hemolysin is translocated through the envelope by an ATP hydrolysis-dependent process. Amy L. Davidson et al. Microbiol. Mol. Biol. Rev. 2008; doi:10.1128/MMBR.00031-07