Process of Dissociation

Proteins Offer Insight into Drug Action

How drugs such as adrenalin do primarily one thing—in this case increase the heart rate—now makes more sense to scientists.

“Any time you get a sudden jolt, adrenaline is why your heart rate goes up,” said Dr. Nevin A. Lambert, an MCG biophysicist.

Research published in the Nov. 21, 2006 issue of Proceedings of the National Academy of Sciences may help explain how cells respond correctly to adrenaline, or epinephrine.

Most drugs never penetrate cells; they interact with external receptors that activate G proteins inside cells. “It’s like a relay,” said Dr. Lambert. “G proteins collide with receptors. The receptor itself does not do anything other than turn on these G proteins.”

There are only four classes of G proteins, but cells contain thousands of copies of them which interact with hundreds of surface receptors. Each G protein is actually three protein subunits stuck together: alpha, beta and gamma.

Textbooks teach that once G proteins are activated, the alpha protein splits from the beta and gamma subunits, which are irrevocably stuck together as a betagamma pair. Each half of the now dissociated G protein can cause the cell to do something different. “Sometimes they help each other out; sometimes they work at cross purposes,” said Dr. Lambert.

With epinephrine, that should mean the alpha subunit enables production of cyclic AMP, which increases the heart rate, while the betagamma pair should activate ion channels, making cells less electrically excitable and decreasing the heart rate.

However, it has been known for some time that while epinephrine does increase cyclic AMP in heart cells, it does not activate ion channels. It has been unclear how the cell allows one response and supresses the other.

The answer likely is because the G proteins activated by epinephrine receptors don’t readily dissociate, contrary to the textbook picture. MCG researchers have also shown that at least one other class of G proteins does dissociate, suggesting the textbook picture is at least partly correct.

Why the difference? Previous work on G proteins, including their discovery and studies of their role in signal transduction, was mostly done in test tubes using purified proteins. MCG researchers used a technique they developed to actually look at G protein function inside living human cells.

Their findings suggest that epinephrine interacts with a G protein that doesn’t release the betagamma subunit.

“There was a constant question about how drugs sometimes avoid doing unwanted things,” said Dr. Lambert. “This helps us understand how drugs can be specific. The flipside of the coin is some drugs acting on some receptors will have multiple actions because the G proteins do dissociate.”

The newfound information is no doubt only one step toward better understanding how hundreds of receptors can act through just four classes of G proteins and produce so many physiologic results. “It’s like 100 cars driving down four roads and ending up in 100 different places,” Dr. Lambert said.

But it’s a timely finding as science moves toward designer drugs, including some that might target G proteins directly, bypassing intermediary receptors, with the hope of getting a more robust response.

In Dr. Lambert’s lab, MCG graduate student Gregory J. Digby, first author on the PNAS paper, is now looking at G protein subunits that do and don’t fall apart with the longrange goal of designing ones that do what the researchers want.

“Right now, it’s all engineering for the sake of understanding how they work,” said Dr. Lambert.

Researchers suspect the stickiness between the subunits determines whether they split, and that the bottom line will be two classes of G proteins dissociating and two not.

Other coauthors include Robert M. Lober, M.D./Ph.D. student, and Pooja R. Sethi, laboratory technician. Dr. Alfred G. Gilman, longtime chair of pharmacology and now dean of the University of Texas Southwestern Medical School who won the 1994 Nobel Prize in Medicine for discovering G proteins, edited the paper.

The research was funded by the National Science Foundation and the National Institutes of Health.

Toni Baker