G protein amplification occurs automatically in an activated rod cell, but not so simply in olfactory cells, according to Vikas Bhandawat, Johannes Reisert, and King-Wai Yau (Johns Hopkins University, Baltimore, MD).
Photon activation of a single rhodopsin molecule activates many G proteins, thereby amplifying the signal until rhodopsin is inactivated by phosphorylation. “Based on this one well-studied system, it has generally been assumed that other G protein pathways behave similarly,” says Yau. Now, his group's analyses of single olfactory receptor neurons reveal a low amplification system.
An individual odorant-bound receptor exhibited a very low probability of activating even one downstream G protein molecule, as odorant receptor binding was transient—lasting 1 ms or less. “We expect many other ligand-triggered G protein pathways to behave similarly,” says Yau.
Olfaction amplification therefore requires increasing the probability of G protein activation. This could be achieved either via many odorant molecules that continuously bind to receptors, or via a large number of receptors, so that odorants at low concentrations will still be able to find a receptor. “When these events are summated across all receptor molecules on the cell, and all cells express the same receptor protein,” says Bhandawat, “this produces substantial signal amplification and therefore high sensitivity in the brain.”