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 responses elicited by defined subsets of odorants; odor-induced cAMP-formation was attenuated by Gα(s) antibodies, whereas Gα(o) antibodies 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 Gα(s)-like protein, whereas odorants that elicited an IP₃ response led to the labeling of a Gα(s)-like protein. Since odorant-induced IP₃ formation was also blocked by G(β) antibodies, activation of olfactory phospholipase C might be mediated by βγ subunits of a G(o)-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.