Scientists first identified olfactory receptors in mammals 10 years ago a discovery that won the 2004 Nobel Prize in Physiology or Medicine. Each olfactory neuron bears only one type of olfactory protein receptor on its surface, forming the cellular basis for discrimination among smells, Matsunami explained. Mice have as many as 1,000 different odorant receptors, while humans with a relatively poorer sense of smell have 350 of the protein receptors for smell. However, the selectivity of those different receptors for chemical stimuli remains unclear, he added.
The researchers' extensive search for proteins involved in delivering odorant receptor proteins to the surface of olfactory nerves revealed two proteins, which they named receptor transporting proteins one and two (RTP1 and RTP2). The researchers found that genes encoding the proteins were specifically active in olfactory neurons. Furthermore, RTP1 and RTP2 interact with olfactory receptor proteins and enhance their function, they found.
The researchers reported similar, although much weaker effects, for a third protein, which they called receptor expression enhancing protein one (REEP1).
To demonstrate the utility of the accessory proteins for elucidating the sense of smell, the team created cells with constantly high levels of RTP1, RTP2, REEP1, and another protein known to bind olfactory receptors. The enhancement of these protein levels made it possible for the researchers to induce the newly created cells to express odorant receptors at their surfaces. The team then tested the receptors' response to various chemicals, including the fishy-smelling aliphatic acids and sweet-smelling coumarine and piperonal.