| MECHANISM OF ACTION |
In bacteria and mammals, certain proteins pump all sorts of compounds out of the cell. When the compounds are drugs, the action can lead to bacterial resistance to antibiotics, or therapeutic agents might not work as expected. How these pumps operate has not yet been fully elucidated. ROOMY Huge cavity in the E. coli multidrug transporter AcrB accommodates three ciprofloxacin molecules at once. IMAGE COURTESY OF YU, NIKAIDO, AND KOSHLAND Now, researchers Edward W. Yu, Helen I. Zgurskaya, Hiroshi Nikaido, and Daniel E. Koshland Jr. at the University of California, Berkeley, and Gerry McDermott at Lawrence Berkeley National Laboratory suggest a mechanism. Their proposal is based on high-resolution crystal structures of the Escherichia coli pump called AcrB with four different ligands. The protein/ligand complexes show a huge cavity in AcrB that presents many sites for low-affinity binding interactions. The ligands tested--the dyes rhodamine 6G and ethidium and the drugs dequalinium and ciprofloxacin--are captured by different sets of amino acid residues. Several molecules of ligand are held at once through hydrophobic, aromatic stacking, or van der Waals interactions. Furthermore, ligand binding causes a slight conformational rotation [Science, 300, 976 (2003)]. This is the first time that a pumping protein of this type--called a multidrug transporter--has been seen complexed with drugs, comments Richard G. Brennan, professor of biochemistry at Oregon Health & Science University, Portland. "That's very important because multidrug transporters are major contributors to the failure of treatments for cancer and bacterial infections." -------------------------------------------------------------------------------- "Multidrug transporters are major contributors to the failure of treatments for cancer and bacterial infections." --Richard G. Brennan -------------------------------------------------------------------------------- Although AcrB straddles the cell membrane, the picture presented by the protein/ligand complexes is very similar to that seen with multidrug-binding proteins inside the cell, Brennan notes. Work in his laboratory has shown that cytosolic multidrug-binding proteins have many subsites in the binding pocket and use the same types of nonspecific binding interactions as AcrB. Knowledge of the binding interactions eventually may be used to prevent the expulsion of drugs from cells. Drug designers might take advantage of a possible inverse relationship between binding affinity and ease of being expelled, Brennan suggests. Or they could consider blocking entry to the binding pocket. It's a tough problem, he says. For now, the new results suggest that AcrB acts like an elevator: Several small molecules board, the protein moves and ejects the molecules, and then AcrB reverts to its former shape and gets boarded again. According to Koshland, AcrB is associated with two other proteins outside the cell: AcrA, which is known to be essential for the pumping action, and TolC. The movement of ligand-loaded AcrB, the researchers suggest, brings it closer to AcrA, and the ligands are pushed from AcrB to TolC and out of the cell. "That's how we think the drug goes from the inside to the outside of the cell," he says. The hypothesis is supported by findings that removal of TolC knocks out the pump. Another consequence of the hypothesis, if true, is that overproducing AcrB should lead to cell death because even essential molecules would be expelled. This prediction is being borne out by preliminary studies by Nikaido and coworkers, Koshland says. The mechanism suggested by the bacterial AcrB pump should hold for other similar pumps, Koshland believes. "It's only a guess at the moment," he says. "But it's a good guess." |
|
Want more information ? Interested in the hidden information ? Click here and do your request. |