3D target: Membrane protein structure

G force

 

The three-dimensional structure of a complete, unmodified G-protein-coupled receptor in its native, membrane environment under physiological conditions has been obtained NMR spectroscopy.

In October, chemists the world over were stunned by the announcement from the Nobel Committee that protein biochemistry was to be the winner of this year's prize for Chemistry. The subject of the prize, as SpectroscopyNOW reported, was G-protein-coupled receptors. These membrane proteins are involved in the functioning of almost half of all prescription drugs and so are an incredibly important area of research in medicinal chemistry especially as many new pharmaceuticals are yet to be discovered that target these proteins in a wide range of diseases.

As we discussed last month, the GPCRs work as a modular system allowing cells to transmit different chemical signals across their membranes, to other cells and over long distances in the body. The proteins are the mediators of the flow of information and while X-ray crystallographic structures have provided useful structural information about these species, NMR spectroscopy provides the opportunity to observe them in their native, physiological state without recourse to crystallization, which is often impossible for membrane proteins.

Stanley Opella of the University of California, San Diego and colleagues have now mapped the protein CXCR1, which is involved in the detection of the inflammatory signal carried by interleukin 8. This biomolecules acts on a G-protein located within the cell that triggers a cascade of events to ultimately mobilize cells of the immune system in response to disease or injury.

"This finding will have a major impact on structure-based drug development since for the first time the principal class of drug receptors can be studied in their biologically active forms where they interact with other proteins and potential drugs," explains Opella. Structural studies often provide the insights needed to find small molecules to inhibit or control the activation of proteins that have gone awry in disease or fail to carry out their function for other reasons. Opella and colleagues describe details of their findings in the journal Nature. The team points out that by using NMR spectroscopy not only do they preclude the need for a complex crystallisation process they also avoid having to remove parts of the protein chain to simplify the studies. "Our approach was to not touch the protein," Opella explains. "We are working with molecules in their active form."

The research has ultimately given science a new perspective on these receptors. Earlier work had suggested that the protein contains seven helices that are woven into the cell membrane. The team has now demonstrated that there is actually an eighth helix that simply lies on the surface of the cell membrane. They suggest that this structural characteristic is likely to be present in other G protein-coupled receptors too. Opella points out that, "Others have seen helix 8 in some, but not all crystal structures. In particular, it is not present or very highly distorted in the crystal structure of the only chemokine receptor that has been reported, CXCR4. This either due to its C-terminus being truncated or the absence of a membrane bilayer for interaction with the amphipathic helix." He adds, however, that this new structural work has shown that a piece of received wisdom concerning another structural feature has been overturned. Previous studied had led to the claim that the protein loops inside and outside of the cell were mobile when in fact it turns out that they are instead structured. "The signals we get from the loops aren't any weaker than the other parts of the protein as they would be if they were waving about," Opella adds.

The researchers point out the CXCR1 protein itself has been implicated specifically in the progression of various forms of cancer. Indeed, pre-clinical studies have already demonstrated that by blocking this receptor it is possible to inhibit the growth of undifferentiated stem cells within breast cancer tumours, which causes cell death in all tumour cell types and prevents them from seeding new malignancies within the breast tissue.

Such findings are encouraging and coupled with the work by Opella and colleagues regarding changes in the receptor's configuration as it binds to interleukin 8 and drug candidates might one day lead to more effective and precise cancer treatments that reduce or avoid some of the deleterious side effects of older, conventional chemical therapies.