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October 6, 2011

New Method Isolates Membrane Proteins

Protein Purification: Technique separates proteins on a lipid bilayer

Leigh Krietsch Boerner

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Anal. Chem. View Enlarged Image
GET IN LINE When researchers apply an electric potential across a lipid bilayer, membrane proteins (shown in black, green, red, and yellow) line up with others that have the same charge and size.

Chemists have a new way using lipid bilayers to separate insoluble membrane proteins (Anal. Chem., DOI: 10.1021/ac201768k). The technique could help scientists study the function of these often difficult-to-handle proteins, including enabling research on pharmaceutically relevant properties such as how small molecules bind to them, the researchers say.

To get membrane proteins into solution for standard purification methods, chemists typically add detergents and sonicate the mixture. But this process can disrupt the proteins' structures and possibly alter their function. Also, because membrane proteins have large hydrophobic regions, they tend to aggregate into insoluble clumps, limiting how well the sonication method works.

In the new technique, called electrophoretic-electroosmotic focusing, the researchers put the proteins in a lipid bilayer. Compared to a detergent solution, the bilayer is more like a cell's natural environment, says the study author Paul Cremer of Texas A&M University. The scientists then apply an electric potential across the bilayer, causing the proteins to move based on their charge: The greater negative charge a protein has, the faster it will move toward the positive electrode. But the proteins also experience a second force based on their size: The greater the amount of protein that sticks out from the membrane, the more it moves in the opposite direction. A protein stops moving in the bilayer when the two forces balance, Cremer says. The result is individual proteins lined up in neat rows on the lipid bilayer.

To test the method, Cremer and his colleagues prepared a mixture of four proteins labeled with fluorescent dyes. After mixing the proteins into the bilayer and applying an electric potential for one hour, the researchers could spot four distinct bands of proteins using fluorescence microscopy.

To identify proteins in unknown mixtures, Cremer's group is now working on coupling the separation technique with matrix-assisted laser desorption/ionization mass spectrometry.

Chemical & Engineering News
ISSN 0009-2347
Copyright © 2011 American Chemical Society
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