Odorants Selectively Activate Distinct G Protein Subtypes in Olfactory Cilia*

Chemoelectrical signal transduction in olfactory neurons appears to involve intracellular reaction cascades mediated by heterotrimeric GTP-binding proteins. In this study attempts were made to identify the G protein subtype(s) in olfactory cilia that are activated by the primary (odorant) signal. Antibodies directed against the α subunits of distinct G protein subtypes interfered specifically with second messenger reponses elicited by defined subsets of odorants; odor-induced cAMP-formation was attenuated by Gαs antibodies, whereas Gαoantibodies blocked odor-induced inositol 1,4,5-trisphosphate (IP3) formation. Activation-dependent photolabeling of Gα subunits with [α-32P]GTP azidoanilide followed by immunoprecipitation using subtype-specific antibodies enabled identification of particular individual G protein subtypes that were activated upon stimulation of isolated olfactory cilia by chemically distinct odorants. For example odorants that elicited a cAMP response resulted in labeling of a Gαs-like protein, whereas odorants that elicited an IP3 response led to the labeling of a Gαo-like protein. Since odorant-induced IP3formation was also blocked by Gβ antibodies, activation of olfactory phospholipase C might be mediated by βγ subunits of a Go-like G protein. These results indicate that different subsets of odorants selectively trigger distinct reaction cascades and provide evidence for dual transduction pathways in olfactory signaling.

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