E. coli Protein Structure in Major Science Article Uses Data from DOE Synchrotrons

The Science

The structure of a key membrane-spanning transporter protein in the bacterium Escherichia coli is reported in Science for May 10, 2002. The protein, vitamin B-12 importer BtuCD, is a member of a class of more than a thousand known proteins, called ABC transporters, that import or export specific chemical species into or out of cells. These proteins are implicated in a number of diseases, including cystic fibrosis, and are involved in development of multidrug resistance in cancer cells. Gaining knowledge of the exact structure of these transporters is thus a high priority for structural molecular biology research, as such structures would enable explaining how a given transporter selects the specific chemical compound(s) it transports. The new article reports the first structure with sufficient spatial resolution, 3.2 Å, to locate all of the components of an ABC transporter.

The two ATP-binding cassettes (BtuD) are in close contact with each other, as are the two membrane-spanning subunits (BtuC); this arrangement is distinct from that observed for the E. coli lipid flippase MsbA. The BtuC subunits provide 20 transmembrane helices grouped around a translocation pathway that is closed to the cytoplasm by a gate region whereas the dimer arrangement of the BtuD subunits resembles the ATP-bound form of the Rad50 DNA repair enzyme. A prominent cytoplasmic loop of BtuC forms the contact region with the ATP-binding cassette and appears to represent a conserved motif among the ABC transporters. The three-dimensional structure shows the locations of the four subunits of the protein in relationship to each other and also precisely positions the many helical and strand-like portions of the subunits. The structure is used by the authors to develop a mechanism for how BtuCD imports vitamin B-12 into E. coli.

BER Program Manager

U.S. Department of Energy, Biological and Environmental Research (SC-33)
Biological Systems Science Division
[email protected]

Funding

The authors acknowledge use of the Stanford Synchrotron Radiation Laboratory (SSRL), the Advanced Photon Source (APS), the Advanced Light Source, and the National Synchrotron Light Source in crystal screening and data collection. The actual high resolution data were obtained at the BER-funded stations at SSRL and the APS.

References

Locher, K. P., A. T. Lee, and D. C. Rees. 2002. “The E. coli BtuCD Structure: A Framework for ABC Transporter Architecture and Mechanism,” Science 296, 1091–98. DOI:10.1126/science.1071142.