Until recently, scientists had the wrong impression about G protein-coupled receptors, even though the 1,000-or-so receptors seem to do just about everything. The receptors let us see, tell us when to stop eating, and are targeted by drugs that help treat heart disease and schizophrenia. About half of all drugs on the market target a member of the G protein-coupled receptor family.
But researchers mistakenly thought the receptors were solitary proteins in the cell membrane, only making connections with ligands that bind the receptor and the intracellular G proteins they activate.
In the past few years, though, researchers have begun to find evidence that most receptors pair up with each other to form dimers. But whether the subunits of the dimers acted independently or needed each other to work remained unclear. Recent research, however, has shown that in some dimers, the two subunits communicate with each other before sending a signal into the cell, even though no one knew where that communication took place.
Now P&S researchers have located where the subunits of a G protein-coupled receptorthe D2 dopamine receptorjoin and communicate. The research by Dr. Jonathan Javitch, associate professor of psychiatry and pharmacology in the Center for Molecular Recognition, and his postdoctoral researchers, Drs. Wen Guo and Lei Shi, was reported in the Feb. 14 Journal of Biological Chemistry. The D2 dopamine receptor is targeted by drugs that treat schizophrenia; the new finding should help researchers more fully understand how the receptor and other G protein-coupled receptors work.
The D2 dopamine receptor, like all other G protein-coupled receptors, is composed of seven helices that span the membrane. The transmembrane segments of the receptor are thought to form a bundle around a central binding pocket for dopamine.
Dr. Javitch had been studying the pocket to find out how different ligands affect the receptor's activity. But when the only crystal structure of a G protein-coupled receptor, bovine rhodopsin, was solved in 2000, he realized some parts of the receptor that affect binding weren't actually in the binding pocket. Instead, they were on the edge of the receptor in the fourth transmembrane segment (TM4). Dr. Javitch then speculated the TM4 region joins two D2 receptors together in a dimer.
To find out, he used a technique called cysteine cross-linking. A chemical, copper phenanthroline, promotes cross-linking between nearby cysteines.
When Dr. Javitch added this chemical to cells expressing the human D2 dopamine receptor, the extracellular tip of TM4 in one receptor cross-linked with the same TM4 tip in a second receptor. Additional unpublished cross-linking experiments extend the dimer interface at least one helical turn down the length of the TM4 segment.
"Now the question is whether important information moves through the dimer interface," Dr. Javitch says. Dimerized receptors could just be a byproduct of the way receptors must be made in the endoplasmic reticulum to be shipped to the cell membrane, but dimerization may also be necessary for the receptors to work. It's already clear that at least one G protein-coupled receptorGABAbcannot work as a single protein.
Dr. Javitch's lab is currently investigating if the D2 receptor also functions as a dimer. The lab has previously shown that the binding of drugs changes the shape of TM4, suggesting that the shape changes may be transmitting information across the dimer interface.
To find out if the D2 dopamine receptor really works as a dimer, Dr. Javitch will probably have to pry the dimers apart with a drug to see if the monomers are functional. If dimerization is essential, the interface is a novel target for drug design.
The discovery of dimers also has raised the possibility that cells combine different types of receptors to make heterodimers or even oligomers. Such combinations could vastly increase the number of different possible receptors, each with different pharmacological and signaling properties. "It's complicating our whole understanding of how these receptors work," Dr. Javitch says. "We have a lot to learn."
The research was supported by the National Institutes of Health, the Lebovitz Fund, and the Lieber Center.