Several groups have hypothesized that speed and specificity of the many different G protein–mediated pathways might be achieved by precoupling the G protein and its receptor, via direct or indirect binding. But the new FRET analyses show that the two are not together until the receptor is activated. Interactions thus depend on random collisions between receptor and G protein. “Lateral diffusion in the membrane is fast enough,” says Bünemann.
Kinetic analyses reveal that each interaction between a receptor and G protein lasts only a small fraction of the total time that the receptor and G protein are active. Thus, one receptor can activate many G proteins. This repetition explains why activation of only a very small fraction of the total receptor pool rapidly elicits full activation of its G protein if enough receptors are expressed. Whereas, says Bünemann, “if they were consistently complexed, you'd need a one-to-one ratio of active receptors to G proteins.”
The authors are not suggesting that other receptors do not precouple. But at least in this case, any precoupling is more likely to occur between the G protein and its specific effector.