Loss of activation of Gs but not Gi following expression of an alpha2A-adrenoceptor-Gi1alpha fusion protein.

Both the porcine alpha2A-adrenoceptor and a fusion protein between this receptor and a pertussis toxin-resistant form of Gila were stably expressed in Rat-1 fibroblasts. The agonist UK14304 mediated a biphasic regulation of adenylyl cyclase activity via the isolated receptor with inhibition of the enzyme activity at low concentrations of the compound which was subsequently reversed at higher concentrations. By contrast, stimulation of the fusion protein with this agonist could only produce inhibition of enzyme activity. This inhibition was produced by activation of endogenous Gi rather than the fused alpha subunit of Gi1, as pertussis toxin treatment obliterated inhibitory regulation of adenylyl cyclase via the fusion construct. Pertussis toxin treatment potentiated stimulation of adenylyl cyclase via the isolated receptor but such treatment was unable to uncover capacity of the fusion protein to produce such an effect.