Gsα stimulation of mammalian adenylate cyclases regulated by their hexahelical membrane anchors

Nine mammalian adenylate cyclase (mAC) isoforms are pseudoheterodimers with two dissimilar hexahelical membrane-anchors, isoform-specifically conserved for more than half a billion years. In the past, we have postulated a receptor function for these membrane anchors. Here, we exchanged both membrane anchors of the human AC isoform 2 (hAC2) with the hexahelical membrane domain of the quorum-sensingreceptor from Vibrio, CqsS, which has a known ligand, Cholera- Autoinducer-1 (CAI-1). In the chimera, cyclase activity was stimulated by Gsα, whereas CAI-1 by itself had no effect. Surprisingly, CAI-1 inhibited Gsα stimulation, shifting the concentration-response curve and attenuating the maximal response. Extending these data we report that Gsα stimulation of hAC2 expressed in Sf9insect cells is inhibited concentration-dependently by human serum whereas human serum albumin does not. The data establish the existence of an allosteric linkage in mACs, in which the membrane anchors, as receptors, can transduce extracellular signals to the catalytic dimer, regulating the extent of mAC stimulation by G-protein-coupled receptor-signaling. A general mechanistic model of AC regulation is presented which is compatible with all known regulatory inputs into mammalian ACs, i.e. stimulation by Gsα as well as modulations by secondary modifications. The data allow to unequivocally designate the membrane anchors of mammalian ACs as orphan receptors, and thus establish a new level of AC regulation in mammals. This opens the possibility to start a targeted ligand search.