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HHMI investigator Tamir Gonen is leading a multidisciplinary team of scientists in a quest to find a better way to determine the structure of membrane proteins.
Tamir Gonen of the University of Washington, an HHMI early career scientist, also studies stretch-activated channels, specifically the water channel aquaporin-0 (AQP-0), found in the lens of the eye. “These channels are much more than just a hole,” Gonen says. “They let water through very efficiently, billions of molecules per second.” When they're not working due to certain mutations, the eye's lens clouds over with cataracts.
In 2004, while working in the lab of HHMI investigator Thomas Walz at Harvard Medical School, Gonen determined the structure of AQP-0. While x-ray crystallography usually uses three-dimensional crystals of membrane proteins that are enveloped by detergents, Gonen analyzed the protein while it was embedded in a membrane, one of only a few successful attempts to do this. He's convinced that observing a protein structure while it's in a membrane gives the best picture of how the protein really looks.
“The way we need to look at membrane proteins is not in isolation,” Gonen says. “We need to look at them with the membrane, because in the cell they're not operating in a vacuum.”
HHMI investigator John Kuriyan has a story that illustrates Gonen's point. Kuriyan, at the University of California, Berkeley, studies SOS, a signaling protein that, when mutated, causes Noonan syndrome, a developmental disorder that causes heart malformation, mental retardation, and other problems. SOS sends signals by activating Ras, a membrane-associated protein. Researchers assumed that the Noonan syndrome-causing mutation changed the interaction between SOS and Ras.
But when Kuriyan placed mutated SOS and its normal version in solution and compared how they interacted with Ras, he saw no differences. When he repeated the experiment with Ras bound to an artificial membrane built by Groves, however, the mutated SOS kept Ras activated. The interaction, Kuriyan realized, took place at the membrane and both SOS and Ras needed the membrane environment to behave as they do in the cell. The results appeared in Nature Structural & Molecular Biology in May 2008.
Certain that membrane proteins are inexorably linked in function to the lipids around them, Jay Groves at UC Berkeley studies how groups of proteins are arranged in membranes. The first theories on membrane organization arose in the 1980s when scientists dissolved membranes and observed that they separated into distinct layers—one with phospholipids and one with other lipid-like membrane molecules, including cholesterol. Proteins separated into both layers, and the scientists hypothesized that the cholesterol wasn't just scattered around the membrane but was in distinct “rafts” where certain proteins localized.
So scientists began experimenting with whatever membrane proteins they studied, testing whether removing cholesterol from the membrane—a simple experiment—altered the function of the protein.
“Lo and behold, everyone who did that experiment, no matter what they looked at, messed up the function of the protein,” says Groves. “And so everything became ‘raft-associated.’ You can find hundreds of papers with this in the title.”
Photo: Paul Fetters