The cell membrane is one of the defining elements of life. It defines the border of each cell and allows the cell to maintain a chemical composition significantly different from its surroundings. The cell membrane does this in two principal ways. First, the double layer of lipid molecules that compose the cell membrane blocks the passage of most chemicals into and out of the cell. Second, membrane proteins embedded in this lipid bilayer facilitate the passage of particular ions and molecules, either by selectively allowing them to diffuse across the cell boundary, or by actively pumping them against a concentration gradient.

Detailed structural studies of membrane proteins have been notoriously difficult. The workings of transporter proteins in particular are a longstanding mystery in structural biology. Perhaps most mysterious of all has been the ATP Binding Cassette (ABC) Transporter family of proteins. Found in the cells of all living things, these membrane proteins use the energy of ATP hydrolysis to transport a wide range of molecules across the cell membrane. The mystery extends beyond understanding the workings of a particular protein. Both the specificity, i.e., how particular ABC transporters allow the passage of only correct chemicals, and the generality, i.e., the mechanism that has enabled the evolution of transporters to traffic an extremely diverse set of molecules, must be explained.

ABC transporters are more than an intriguing biological mystery, however. They play a significant role in human disease, and their role is growing rapidly with time. Mutations of the nearly 50 ABC transporters used by human cells have been connected with human diseases such as cystic fibrosis. Detailed structural and functional analyses of ABC transporters may therefore lead to new methods of treating these diseases. Furthermore, studies of ABC transporters may lead to understanding of multidrug resistance in infectious diseases and cancer. The increasing resistance of microorganisms and cancer cells to commonly prescribed drugs threatens to reverse the gains in health made by modern medicine. Multidrug resistance has been linked to increased production of ABC transporters as disease-causing cells employ ABC transporters to remove drugs from the targeted cell (Higgins and Linton, 2001).

Until recently, the only three dimensional structure of a complete ABC transporter was a 25 Å resolution structure determined by electron diffraction (Rosenberg et al., 1997). Now, using X-ray diffraction data collected on Beam Lines 11-1 and 9-2, Geoffrey Chang and Christopher Roth, The Scripps Research Institute, have determined the first crystal structure of a complete ABC transporter at 4.5 Å resolution (Chang and Roth, 2001).  The structure is of the MsbA transporter from E. coli. This particular ABC transporter is a lipid flippase, a protein that transports lipid molecules from the inner layer of the cell membrane to the outer layer. MsbA is clearly homologous to two human proteins implicated in multidrug resistance, one of which is itself a lipid flippase.

The crystal structure also provides significant insights into the workings of this class of ATP transporters. Two polypeptide chains pack together in the structure to form a homodimer, with contacts between the two subunits restricted to the portions of the transmembrane domains that occupy the external layer of the cell membrane. A large cavity is formed between the portions of the transmembrane domains that occupy the internal layer of the membrane. The distribution of charged residues lining this cavity suggests a mechanism whereby lipids, and other amphipathic molecules such as drugs, may be transported by the flippase from the inner to the outer membrane layer: the substrate to be transported binds within the cavity, and a conformational change of the protein releases the substrate to the outer membrane layer.

Mysteries remain about how ABC transporters work, but this crystal structure is a significant milestone in understanding these critical molecules.
 

This work has been cited in the following article:

Sharff A,  Fanutti C, Shi J, Calladine C,  Luisi B, "The Role of the TolC Family in Protein Transport and Multidrug Efflux: From Stereochemical Certainty to Mechanistic Hypothesis."  Eur. J.  Biochem.  2001 268: 5011-5026

1. Chang G, Roth, CB. “ Structure of MsbA from E. coli: A Homolog of the Multidrug Resistance ATP Binding Cassette (ABC) Transporters.”  Science 2001 293: 1793-1800.
2. Higgins CF, Linton KJ. “The xyz of ABC Transporters.” Science 2001 293: 1782-1784.
3. Rosenberg MF, Callaghan R, Ford RC, Higgins CF. “Structure of the Multidrug Resistance P-glycoprotein to 2.5 nm Resolution Determined by Electron Microscopy and Image Analysis.” J. Biol. Chem. 1997 272(16): 10685-94.