Molecular biology of olfactory receptors.

OR proteins bind odorant ligands and transmit a G-protein-mediated intracellular signal, resulting in generation of an action potential. The accumulation of DNA sequences of hundreds of OR genes provides an opportunity to predict features related to their structure, function and evolutionary diversification. The OR repertoire has evolved a variable ligand-binding site that ascertains recognition of multiple odorants, coupled to constant regions that mediate the cAMP-mediated signal transduction. The cellular second messenger underlies the responses to diverse odorants through the direct gating of olfactory-specific cation channels. This situation necessitates a mechanism of cellular exclusion, whereby each sensory neuron expresses only one receptor type, which in turn influences axonal projections. A 'synaptic image' of the OR repertoire thus encodes the detected odorant in the central nervous system.