Scientific Report 2006

Molecular Biology

Structural Biology of Integral Membrane Proteins

G. Chang, S. Aller, A. Chen, Y. Chen, X. He, A. Karyakin, C.R. Reyes, P. Szewczyk, A. Ward, S. Wada, J. Yu, Y. Yin

The structural biology of integral membrane proteins is an exciting frontier. We are interested in 5 areas: (1) the molecular structural basis for lipid and drug transport across the cell membrane by multidrug-resistance (MDR) transporters, (2) the high-resolution structure of yeast and mammalian MDR transporters, (3) signal transduction by receptors, (4) the discovery and design of potent MDR reversal agents, and (5) the development of an in vitro cell-free system capable of overproducing integral membrane proteins suitable for biophysical study. We use several experimental methods, including detergent/lipid protein biochemistry, 3-dimensional crystallization of integral membrane proteins, protein x-ray crystallography, and functional analysis of transporters.

We are addressing the molecular basis of MDR in the treatment of infectious disease and cancer. A major cause of MDR is drug efflux pumps imbedded in the cell membrane. Through our structural studies on MDR transporters, we are gaining insights into the molecular mechanics of translocating amphipathic substrates across the cell membrane and the rational design of powerful inhibitors.

We are combining chemistry and biology with structure for the discovery and design of potent MDR reversal agents for cancer chemotherapy in collaboration with M.G. Finn, Department of Chemistry; I. Urbatsch, Texas Tech University Health Sciences Center, Lubbock, Texas; and S. Reutz, Novartis International AG, Basel, Switzerland. In collaboration with M. Saier, University of California, San Diego, and Q. Zhang, Department of Molecular Biology, we are probing the structures and function of bacterial MDR transporters. In a collaboration with R.A. Milligan, Department of Cell Biology, we are using electron cryomicroscopy to visualize the low-resolution structures of our transporters.

Recently, we determined the x-ray structure of an MDR transporter called EmrD. EmrD is from the Major Facilitator Superfamily, and it expels amphipathic compounds across the inner membrane of E coli. The structure reveals an interior that is composed mostly of hydrophobic residues, a finding consistent with the role of EmrD in transporting amphipathic molecules. Two long loops extend into the inner leaflet side of the cell membrane. This region can recognize and bind substrate directly from the lipid bilayer. We propose that multisubstrate specificity, binding, and transport are facilitated by these loop regions and the internal cavity.

Publications

Yin, Y., He, X., Szewczyk, P., Nguyen, T., Chang, G. Structure of the multidrug transporter EmrD from Escherichia coli. Science 312:741, 2006.