Repeated structure of a voltage-dependent potassium ion channel. Courtesy of Science .
Thanks to a special group of proteins embedded in the membrane of cells, our nerves send signals, our muscles flex, and our hormones get secreted. Yet these medically important molecules remain relatively mysterious, largely because scientists can’t easily visualize their three-dimensional shapes—key to understanding how they work and to designing drugs that alter their functions.
A major challenge in visualizing a membrane protein structure has been obtaining sufficient quantities of the protein in its purest form. To do this, researchers can either extract the protein from a source in which it is naturally abundant or use a simple organism, such as yeast or bacteria, to express it in large quantities. But when it comes to producing quantities sufficient for solving the structure of a mammalian membrane protein, only the first approach has worked.
Now, researchers led by Nobel laureate Roderick MacKinnon, M.D., of Rockefeller University in New York City have used a yeast expression system to express and obtain the structure of a mammalian voltage-dependent potassium ion channel—a type of membrane protein that helps transmit electrical signals into and out of heart and nerve cells. The work was supported by the National Institute of General Medical Sciences (NIGMS), a component of the National Institutes of Health (NIH), and is described in two papers published in the August 5 issue of Science.
Previously, the MacKinnon group determined the structure of a similar bacterial ion channel by using antibodies to stabilize the protein and to aid the formation of the crystal needed to capture structural information. At the time, the results signaled a significant advance for structural biology.
Taking the work to the next level, MacKinnon determined the structure of the related eukaryotic potassium ion channel. The protein was expressed in a yeast system of sufficient quantity and quality that it could be purified for structural studies. The researchers then used a novel combination of lipids, detergents, and reducing agents to stabilize the structure and to entice the proteins to aggregate into a crystal. They used X-ray crystallography and a technique called molecular replacement to obtain a complete model of the channel in the open position.
The recent findings showcase an ion channel structure that confirms conclusions from the earlier study and reveals many new features. MacKinnon plans to determine the structure of the ion channel in the closed position. The ultimate result could be new drugs that control channel activity in a precise way.
In the process, the research team may have opened the door to understanding the structures and functions of other eukaryotic membrane proteins, according to Jeremy M. Berg, Ph.D., NIGMS director.
“The use of expression systems has completely revolutionized structural studies of non-membrane proteins,” Berg said. “This new work offers a proof of principle that such an approach could apply to membrane proteins as well, and it likely foreshadows many future advances.”
Berg and others anticipate that additional methods for producing abundant quantities of eukaryotic membrane proteins and for solving their structures will be developed as researchers involved in related NIH efforts advance their studies. These include:
- Centers for Innovation in Membrane Protein Production, part of the structural biology initiative of the NIH Roadmap for Medical Research, funded in September 2004 at Scripps Research Institute in La Jolla, CA, and the University of California, San Francisco. More information on the Roadmap: http://nihroadmap.nih.gov/
- Membrane Protein Production and Structure Determination research grants, also part of the NIH Roadmap, funded in September 2005. More information on these funded grants: http://nihroadmap.nih.gov/grants/fundedresearch.asp
- The Protein Structure Initiative (PSI), an NIGMS initiative that supports 10 research centers, six of which focus on developing new technology and methods for determining the structures of proteins, including membrane proteins, that are difficult to study. More information on the PSI: http://www.nigms.nih.gov/Research/SpecificAreas/PSI