Outer membrane component of bacteria drug efflux pump, TolC
Antibiotics are used in the treatment of infection by pathogenic bacteria. However, chronic administration of antibiotics can induce drug resistance of bacteria and result in spread of epidemics. One mechanism of drug resistance acquisition is derived from the activity of efflux transporters that recognize broad range of chemical compounds. In Gram-negative bacteria, the transporter takes the form of three-component efflux pumps that span the inner and outer membranes and the periplasm. The pump comprises an outer membrane channel, inner membrane protein that transduces electrochemical energy, and a bridging protein that links the transmembrane components, respectively. A drug efflux transporter in E.coli corresponds to an outer membrane protein TolC, an inner membrane protein AcrB, and a periplasmic protein AcrA.
TolC is homotrimeric channel assembly with a β-barreled outer membrane end and a periplasmic α helical bundle that has two distinct sections: an equatorial domain and an AcrB engaging end.
The intersubunit interaction that regulates channel opening is maintained by a network of hydrogen bonds and salt bridges mediated by Y362 and R367. A double mutant (Y362F and R367E) disrupts the interaction and enhances the sensitivity to antibiotics.
The inner membrane faced sides of wild type (PDB ID: 1EK9) and double mutant TolC (PDB ID: 2VDD) are compared. The pore size is different : the pore diameter of “closed” wild type and “open” double mutant are 8.5 Å and 20 Å respectively. Mobile H7/H8 helix swinging makes the pore size larger in double mutant.
Although the structure of TolC-AcrB-AcrA complex is unknown, AcrB- and AcrA binding to TolC is supposed to regulate the channel opening. Interactions of AcrB-AcrB engaging end and AcrA-H7/H8 helices would induce tilting of H7/H8 helices, that result in the opening of the channel.
Protein Data Bank (PDB)
Bavro, V.N. Pietras, Z. Furnham, N. Perez-Cano, L. Fernandez-Recio, J. Pei, X.Y. Truer, R. Misra, R. Luisi, B.; "Assembly and Channel Opening in a Bacterial Drug Efflux Machine."; Molecular Cell; (2008) 30:114- PubMed:18406332.
author: Daisuke Ino