Published online 12 January 2011 | Nature | doi:10.1038/news.2011.13

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Near-action shots of vital proteins

Structures of G-protein-coupled receptors visualized in near-active states.

activated receptorLong-sought image of an activated receptor bound to a G-protein surrogate.The Kobilka Lab

Researchers have long sought to know what proteins in a particular group look like when they assemble for action. The quest has now borne fruit, as three articles published in Nature today use pioneering techniques to visualize the structures of proteins that control bodily functions. The findings could help pharmaceutical companies to improve drug design.

Almost 30% of drugs approved by the US Food and Drug Administration work by binding to G-protein-coupled receptors (GPCRs), proteins that are embedded in cell membranes and control the transport of information into and out of the cell. But the many possible structures adopted by these common membrane proteins remain largely unknown. In particular, the shape of the activated receptor, ready to transmit cellular signals, has been nearly impossible to pin down.

Researchers now reveal a glimpse of a vital type of GPCR — the β-adrenergic receptors, which control heart and lung behaviour, among other things — just before they are activated1,2, and at the moment of activation3.

Drugs, hormones and other chemical messengers that activate β-adrenergic receptors have major physiological effects, so pharmaceutical companies hope that reports such as these will help them to refine the design of drugs.

For example, when isoprenaline, a medication for severe asthma attacks, binds to β-adrenergic receptors, it raises an individual's heart rate as well as relaxing their airways. A clearer picture of how binding occurs might help drug developers to design more sophisticated asthma medications without unwanted side-effects on blood pressure.

Setting the stage

Broadly speaking, molecules that activate β-adrenergic receptors are called agonists, and those that inhibit action are called antagonists.

"Pharmaceutical companies would love to know the agonist-bound and antagonist-bound structures of these receptors," says Chris Tate, a membrane-protein biochemist at MRC Laboratory of Molecular Biology in Cambridge, UK, and an author on one of the papers1. "That would be their Christmas present for the next 100 years — but we're just not at that stage yet."

But Tate's group, along with the other teams whose work is published today, is preparing the ground by using clever crystallization techniques to capture the structures of agonist-bound receptors on the atomic level.

The teams found that when agonists bind to a pocket on the outermost portion of the β-adrenergic receptor, certain amino acids in the receptor change their position so that the agonist nestles into the pocket more tightly.

Yet, like a runner poised to start a race, the agonist–receptor complex tends to remain inactive at this stage.

"These subtle changes are the prelude to activation," says Tate. "It's like putting the key in the lock — it's ready for action, but until you turn it you haven't opened the door."

Activation occurs more readily once molecules that communicate chemical messages within cells, such as G-proteins, have bound themselves to the inner side of the receptor, opposite the agonist.

Open sesame

Visualizing this turn of the key has eluded biochemists for a number of reasons. For one, the seven coiled parts that make up GPCRs snake through the fatty membranes of cells, making it difficult for researchers to wash the grease from the extracted receptors without damaging their shapes.

After extraction comes the task of growing crystals of the protein, then shining X-ray beams onto the crystal and mapping how the rays scatter to provide an image of the molecule's structure. Crystallizing agonist-bound β-adrenergic receptors without significantly modifying them first is impossible at the moment, because they change their shapes rapidly.

"We had to reduce this wiggly-ness to stop them from acting like squirming kids," says Brian Kobilka, a biochemist at Stanford University in California and lead author on two of the studies2,3.

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Each team stabilized the agonist-bound receptors in different ways. Tate's group genetically mutated it at six points, to make it less sensitive to heat1. Kobilka's teams modified the agonist so that it could be 'handcuffed' to the binding pocket of the receptor by a chemical bond in one study2 and, in the other, stabilized the complex in an active state by binding it to a type of llama antibody that acts like a G-protein in experiments3.

Computer simulations by Kobilka's team showed that without the G-protein surrogate, the agonist-bound receptor was too unstable to remain in an activated state. That may be why the activated receptor could only be visualized when the surrogate was present, says Kobilka.

"These groups went after one of the biggest questions," says Bob Lefkowitz, a receptor biologist at Duke University Medical Center in Durham, North Carolina. "But the glass is half empty or half full, depending on how you look at it. On the one hand, they are way off from using an un-manipulated, native receptor. But on the other hand, without these tricks we've got nothing." 

  • References

    1. Warne, T. et al. Nature 469, 241-244 (2011). | Article
    2. Rosenbaum, D.M. et al. Nature 469, 236-240 (2011). | Article
    3. Rasmussen, S. et al. Nature 469, 175-180 (2011). | Article

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