It's just a matter of probability, Sommer says: In brighter light, arrestin interacts with two activated receptors simply because there are more of them around.
"Although there were two fairly clear-cut theories regarding how arrestin binds rhodopsin, what was totally unexpected is that both can occur," she says.
But, what does this mean for the other senses and physiological functions controlled by other rhodopsin-like proteins? Rhodopsin is the most-studied member of the large family of G-protein coupled receptors, or GPCRs, and many well-known drugs target GPCRs. For example, when morphine binds to a GPCR, it affects the release of neurotransmitters in the brain and thus reduces pain signals. Meanwhile, beta-blockers, which are used to treat cardiac conditions and hypertension, block the activation of GPCRs by standing in the way of natural activating molecules.
"Nearly all GPCRs are normally bound by arrestin, and arrestin can greatly influence what happens to the GPCRs when they are acted on by drugs," says Sommer. "For example, many GPCR-targeted drugs become less effective with continued use. Part of this is because of arrestin. Arrestin binds to the activated GPCR and tells the cell to remove it from the cell surface. In other words, arrestin causes the cell to become less sensitive to the drug because it loses the receptors that normally catch the drug molecules."
By understanding how arrestin interacts with receptors like rhodopsin under healthy conditions, she says, researchers will be able to design better drugs that avoid such problems as desensitization.
|Contact: Angela Hopp|
American Society for Biochemistry and Molecular Biology