A Swiss-French team has developed a way to grow crystals of proteins that normally reside in the membranes of cells or organelles, opening the way for researchers to get sharper pictures of their structures. In today's issue of Science*, the researchers describe their first success: The most detailed three-dimensional (3D) structure yet of bacteriorhodopsin, a key protein enabling a salt-loving bacterium to turn sunlight into chemical energy. The structure may provide a model for the workings of similar membrane proteins, including several receptors for hormones and neurotransmitters.
In order to probe a protein's structure with x-rays, researchers must first crystallize the protein. Proteins located in cell membranes, however, often can't withstand being removed from their normal environment: They tend to unfold and become disorganized. In the past, this limited investigators to making two-dimensional bacteriorhodopsin crystals, which can be analyzed by a lower resolution technique called electron crystallography. But x-ray crystallography requires 3D crystals.
Physical chemist Ehud Landau of the University of Basel, Switzerland, and colleagues at the Institute of Biological Structure in Grenoble, France, reasoned that the best way to grow 3D crystals of bacteriorhodopsin would be to keep the protein in a congenial environment. They custom-built a lattice of lipids, fats similar to those in membranes, to house the bacteriorhodopsin during crystallization. "We have faked the natural environment," says Jurg Rosenbusch of Basel.
As a result, extracted membrane proteins that, previously, fell apart in a matter of hours were "happy and functioning for months," yielding very small, 3D crystals. With the help of physicist Christian Riekel at the European Synchrotron Radiation Facility (ESRF) in Grenoble, the team probed the lattice with a very narrow and bright x-ray beam. The result was a picture of bacteriorhodopsin broadly similar to the electron crystallographic analysis, yet at higher resolution.
The technique "offers a very creative approach to growing crystals of membrane-bound proteins that, in the past, have proved difficult to prepare," says Eric Gouaux, a Columbia University protein crystallographer. Eventually, researchers may be able to design and build artificial membranes with different lattice sizes to snare and crystallize membrane proteins of varying sizes and shapes for structural studies.