The study is being published in ScienceXpress, an advance online edition of the journal Science, on May 5.
"The development of antibiotics to treat infectious disease is being seriously undermined by the emergence of drug-resistant bacteria," says Geoffrey A. Chang, Ph.D., a Scripps Research associate professor and a member of the Skaggs Institute for Chemical Biology, who led the study. "Multidrug resistance develops in part through the expulsion of drugs by integral membrane transporters like EmrD. Determining the structure of this transporter will add significantly to our general understanding of the mechanism of drug transport through the cell membrane and provide the structural basis for how these proteins go about selecting specific drugs to expel."
Multidrug resistant bacterial infections raise the cost of medical treatment and are far more expensive than treating normal infections. Treating drug-resistant tuberculosis, for example, requires so-called second-line drugs if standard treatment fails. According to the Centers for Disease Control, second-line drugs can cost as much as "$33,000 per patient in industrialized countries compared to $84 for first-line drugs." In addition, the centers noted, second-line drugs need to be taken for longer periods of time-from 18 to 36 months-and may require substantial patient monitoring, making these treatments difficult if not impossible to "be available in many of the resource-poor nations where drug-resistant tuberculosis is emerging."
EmrD belongs to the Major Facilitator Superfamily, a group of transporters among the most prevalent in microbial genomes. These transporters are distinctive in their ability to recognize and expel a highly diverse range of amphipathic compounds. Amphipathic molecules contain both hydrophobic and h ydrophilic groups-molecules that repel or are attracted to water, respectively.
The x-ray structure of the EmrD transporter-determined with data collected at the Stanford University Synchrotron Radiation Laboratory and the Advanced Light Source at the University of California, Berkeley-revealed an interior composed primarily of hydrophobic residues. This finding is consistent with its role of transporting hydrophobic or lipophilic molecules-and similar to the interior of another multidrug transporter, EmrE, which Chang and his colleagues uncovered in a study that was published last year in the journal Science.
This internal cavity is the "most notable difference" between EmrD and most non-Major Facilitator Superfamily multidrug transporters that, the new study noted, typically transport "a relatively narrow range of structurally related" compounds. The hydrophobic residues in the EmrD internal cavity are likely to contribute to the general mechanism transporting various compounds through the cell membrane, and may play "an important role in dictating a level of drug specificity" through a number of molecular interactions.
The study also suggests that EmrD intercepts and binds cyanide m-chlorophenyl hydrazone, a known efflux pump inhibitor, before it reaches the cell cytoplasm. This binding is likely facilitated by hydrophobic interactions within the internal cavity of EmrD. The researchers speculate that cyanide m-chlorophenyl hydrazone is either expelled from the bacterial cell or into the periplasmic space-the space between the outer membrane and the plasma membrane in gram-negative bacteria like E. coli.
"While EmrD and EmrE are completely different proteins from different molecular families," Chang said, "both are multidrug transporters that help bacteria develop multidrug resistance. Together with MsbA, another MDR structure that our laboratory is studying, this new x-ray structure adds another important view of some genera l structural features across multi-drug resistant transporter families."
Other authors of the study include Yong Yin, Xiao He, Paul Szewczyk, and That Nguyen of The Scripps Research Institute.