Cyclic AMP‐Dependent Protein Phosphorylation in Chemosensory Neurons: Identification of Cyclic Nucleotide‐Regulated Phosphoproteins in Olfactory Cilia

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Abstract
Chemosensory dendritic membranes (olfactory cilia) contain protein kinase activity that is stimulated by cyclic AMP and more efficiently by the nonhydrolyzable GTP analog guanosine‐5′‐O‐(3‐thio)triphosphate (GTPγS). In control nonsensory (respiratory) cilia, the cyclic AMP‐dependent protein kinase is practically GTPγS‐insensitive. GTPγS activation of the olfactory enzyme appears to be mediated by a stimulatory GTP‐binding protein (G‐pro‐tein) and adenylate cyclase previously shown to be enriched in the sensory membranes. Protein kinase C activity cannot be detected in the chemosensory cilia preparation under the conditions tested. Incubation of olfactory cilia with [γ‐32P]ATP leads to the incorporation of [32P]phosphate into many polypeptides, four of which undergo covalent modification in a cyclic nucleotide‐dependent manner. The phosphorylation of one polypeptide, pp24, is strongly and specifically enhanced by cyclic AMP at concentrations lower than 1 μM. This phosphoprotein is not present in respiratory cilia, but is seen also in membranes prepared from olfactory neuroepithelium after cilia removal. Cyclic AMP‐dependent protein kinase and phosphoprotein pp24 may be candidate components of the molecular machinery that transduces odor signals.