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4.6: The G Switch

  • Page ID
    211483
  • G proteins act like relay batons to pass messages from circulating hormones into cells.
    (a) A hormone (red) encounters a receptor (blue) in the membrane of a cell.
    (b) A G protein (green) becomes activated and makes contact with the receptor to which the hormone is attached.
    (c) The G protein passes the hormone's message to the cell by switching on a cell enzyme (purple) that triggers a response.

    Imagine yourself sitting on a cell, looking outward to the bloodstream rushing by. Suddenly, a huge glob of something hurls toward you, slowing down just as it settles into a perfect dock on the surface of your cell perch. You don't realize it, but your own body sent this substance—a hormone called epinephrine—to protect you, telling you to get out of the way of a car that just about sideswiped yours while drifting out of its lane. Your body reacts, whipping up the familiar, spine-tingling, "fight-or-flight" response that gears you to respond quickly to potentially threatening situations such as this one.

    How does it all happen so fast?

    Getting into a cell is a challenge, a strictly guarded process kept in control by a protective gate called the plasma membrane. Figuring out how molecular triggers like epinephrine communicate important messages to the inner parts of cells earned two scientists the Nobel Prize in physiology or medicine in 1994. Getting a cellular message across the membrane is called signal transduction, and it occurs in three steps. First, a message (such as epinephrine) encounters the outside of a cell and makes contact with a molecule on the surface called a receptor. Next, a connecting transducer, or switch molecule, passes the message inward, sort of like a relay baton. Finally, in the third step, the signal gets amplified, prompting the cell to do something: move, produce new proteins, even send out more signals.

    One of the Nobel Prize winners, pharmacologist Alfred G. Gilman of the University of Texas Southwestern Medical Center at Dallas, uncovered the identity of the switch molecule, called a G protein. Gilman named the switch, which is actually a huge family of switch molecules, not after himself but after the type of cellular fuel it uses: and energy currency called GTP. As with any switch, G proteins must be turned on only when needed, then shut off. Some illnesses, including fatal diseases like cholera, occur when a G protein is errantly left on. In the case of cholera, the poisonous weaponry of the cholera bacterium "freezes" in place one particular type of G protein that controls water balance. The effect is constant fluid leakage, causing life-threatening diarrhea.

    In the few decades since Gilman and the other Nobel Prize winner, the late National Institutes of Health scientist Martin Rodbell, made their fundamental discovery about G protein switches, pharmacologists all over the world have focused on these signaling molecules. Research on G proteins and on all aspects of cell signaling has prospered, and as a result scientists now have an avalanche of data. In the fall of 2000, Gilman embarked on a groundbreaking effort to begin to untangle and reconstruct some of this information to guide the way toward creating a "virtual cell." Gilman leads the Alliance for Cellular Signaling, a large, interactive research network. The group has a big dream: to understand everything there is to know about signaling inside cells. According to Gilman, Alliance researchers focus lots of attention on G proteins and also on other signaling systems in selected cell types. Ultimately, the scientists hope to test drugs and learn about disease through computer modeling experiments with the virtual cell system.

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    Describe how G proteins work.

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