|Scientists have obtained their first three-dimensional look
at a member of a large family of proteins that play a central role in the development of
cystic fibrosis, can block the therapeutic effects of medications, and are also involved
in a wide range of vital biological processes. The structure of this representative
protein was solved by a team at the U.S. Department of Energy's Lawrence Berkeley National
Laboratory (Berkeley Lab) and the University of California at Berkeley (UCB).
Sung-Hou Kim, a chemist with Berkeley Lab's Physical Biosciences Division and a professor with UCB's Chemistry Department, and Giovanna Ferro-Luzzi Ames, a professor in UCB's Molecular and Cell Biology Department, led the research team which used intense beams of x-rays produced at Berkeley Lab's Advanced Light Source (ALS) to solve the structure of a protein called HisP.
HisP is a "conserved subunit" of a family of proteins known as ATP-binding cassette (ABC) transporters. ABC transporters are responsible for carrying substances back and forth across the inner membranes of cells.
Among the many medically significant proteins in the ABC transporter family are the cystic fibrosis transmembrane regulator (CFTR) and a multidrug resistance protein (MDR) called P-glycoprotein. Cystic fibrosis is the most common fatal genetic disease in the United States today, occurring in approximately one of every 3,300 live births. It is caused by mutations in the CTFR gene that result in defective CFTR proteins. MDR proteins are the bane of the medical community because they counteract the effects of pharmaceutical drugs, forcing doctors to increase prescribed dosages in order to obtain desired results.
"Cystic fibrosis occurs when the ABC transporters are not working well enough, and multidrug resistance occurs when ABC transporters are working too well," says Kim. "With our 3-D crystal structure, we have provided a structural basis for understanding the properties of ABC transporters."
The HisP protein that Kim and his colleagues imaged comes from the recently completed Escherichia coli genome. The scientists resolved the protein's crystal structure to 1.5 angstroms, a level of detail made possible by the quality of the x-ray beams and instrumentation available at the ALS' Macromolecular Crystallography Facility. Once they had a 3-D image of their HisP protein, the research team was able to correlate structural details with the biochemical, genetic, and biophysical properties of wild-type and mutant HisP proteins, as well as with some of the known mutants of CFTR and MDR proteins.
"Our findings indicate that HisP is a good model for the ATP-binding domain of ABC transporters in general," says Kim. "It could be used to better understand and perhaps treat cystic fibrosis or to design ways to inhibit multidrug resistance."
ABC transporters contain two ATP-binding domains which serve as molecular engines powering the molecular machinery of two membrane-spanning domains (MSDs). Because ABC transporters play such a critical role in so many different biological processes, scientists would really like to know what MSDs look like and how the ATP-binding domains power them. Kim says that solving the HisP crystal structure represents an important step towards this goal. Kim, Ames, and their colleagues reported their research results in the December 17 issue of the journal Nature at http://www.nature.com.
Co-authoring the Nature paper with Kim and Ames were Li-Wei Hung, Iris Xiaoyan Wang, Kishiko Nikaido, and Pei-Qi Liu.
Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.