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...
