October 13, 2006

VUMC researchers visualize crucial molecular ‘switch’

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Research by Heidi Hamm, Ph.D., and William Oldham, Ph.D., is shedding new light on a crucial cell signaling pathway.
Photo by Susan Urmy

VUMC researchers visualize crucial molecular ‘switch’

Vanderbilt University Medical Center researchers have solved an elusive puzzle in the activation of G protein-coupled receptor (GPCR) signaling pathways — one of the most physiologically and medically important cell signaling pathways.

In the September issue of Nature Structural and Molecular Biology, Heidi Hamm, Ph.D., and colleagues provide some of the first evidence of the protein shape change that initiates the cellular response to GPCR activation.

This new model could inform the development of more specific and safer drugs.

“G protein-coupled receptors are incredibly important both physiologically and therapeutically,” said Hamm, the Earl W. Sutherland Jr. Professor of Pharmacology and chair of the department. Indeed, GPCRs are the targets of nearly half of all drugs currently prescribed.

“There are more than 300 different GPCRs, and they mediate every aspect of physiology, behavior, and even mood, but exactly how they work is not known.”

GPCRs are a large class of proteins that weave through the cell membrane. They receive external chemical signals and relay them inside the cell with the help of molecular “switches” called G proteins. Binding of an external signal (like a hormone) to its cell surface receptor turns the G protein “on.” The G protein then switches “off” after passing the signal on to the next protein in the chain.

G proteins got their name from the small molecules that bind to them — guanosine diphosphate (GDP) and guanosine triphosphate (GTP). When “activated” or turned on by the membrane-bound GPCR, G proteins drop their usual cargo, GDP, and exchange it for a molecule of GTP.

But the region of the G protein that touches the receptor is quite distant — on a molecular scale — from the region that holds onto GDP, begging the question, “How does the receptor act at a distance to cause GDP release from the G protein?”

Hamm and William Oldham, Ph.D., a student in the Medical Scientist Training Program (MSTP), proposed that the receptor must cause a shape change in the G protein to drop the GDP and prepare to be switched on. They used a technique called site-directed spin-labeling to “watch” the movements of each region of the G protein as it became activated.

They found a “hotspot” region where the G protein appeared to be changing shape in response to receptor activation. The receptor uses this piece of the protein as a lever to open up the GDP binding site.

When they replaced the rigid protein lever with a floppy protein sequence, the receptor was no longer able to cause GDP release, suggesting that this movement is the basis for G protein activation.

“This gave us a really clear picture of a concerted mechanism, a conformational change induced by the receptor that could account for GDP release by the G protein,” Hamm said.

Understanding this switching mechanism has important implications for drug development. All currently used drugs that target this pathway act at the extracellular side of the receptor. But figuring out this downstream mechanism could provide alternative therapeutic targets.

“If we can disrupt the interaction between a receptor and a G protein — or if we can figure out how to interrupt this conformational change — then that would be a good therapeutic target,” Oldham said. “Because, once you get past that point, you've already set things in motion.”

The models of the receptor-G protein interaction developed through their research could also hint at the molecules that could be used to interfere with the pathway.

“Understanding what the G protein looks like when it's bound to the receptor…may help us to design therapeutics to prevent that interaction or block the conformational change,” said Oldham.

Intervening at this step could have distinct advantages over targeting the cell surface receptor.

“These receptors actually activate multiple G protein pathways,” Hamm said. “We would be able to distinguish them and to turn off just one.”

“That would conceivably give a drug with fewer side effects.”

Other authors on the paper include Ned Van Eps and Wayne L. Hubbell at the University of California, Los Angeles and Anita M. Preininger, Ph.D., of VUMC.