Human adenylyl cyclase 9 is auto-stimulated by its isoform-specific C-terminal domain and equipped with paradigm-switching mechanisms

Human transmembrane adenylyl cyclase 9 (AC9) is not regulated by heterotrimeric G proteins. Key to the resistance to stimulation by Gs-coupled receptors (GsR) is auto-inhibition by the COOH-terminal domain (C2b). The present study investigated the role of the C2b domain in the regulation of cyclic AMP production by AC9 in HEK293FT cells expressing the Glosensor22F cyclic AMP-reporter protein. Surprisingly, we found C2b to be essential for sustaining the basal output of cyclic AMP by AC9. A human mutation (E326D) in the parallel coiled-coil formed by the signalling helices of AC9 dramatically increased basal activity, which was also dependent on the C2b domain. Intriguingly, the same mutation enabled stimulation of AC9 by GsRs. In summary, auto-regulation by the C2b domain of AC9 sustains its basal activity and quenches activation by GsR. Thus AC9 appears to be tailored to support constitutive activation of cyclic AMP effector systems. A switch from this paradigm to stimulation by GsRs may be occasioned by conformation changes at the coiled-coil or removal of the C2b domain. ### Competing Interest Statement The authors have declared no competing interest.