Distinct Mechanisms of Regulation by Ca2+/Calmodulin of Type 1 and 8 Adenylyl Cyclases Support Their Different Physiological Roles

Nine membrane-bound mammalian adenylyl cyclases (ACs) have been identified. Type 1 and 8 ACs (AC1 and AC8), which are both expressed in the brain and are stimulated by Ca2+/calmodulin (CaM), have discrete neuronal functions. Although the Ca2+ sensitivity of AC1 is higher than that of AC8, precisely how these two ACs are regulated by Ca2+/CaM remains elusive, and the basis for their diverse physiological roles is quite unknown. Distinct localization of the CaM binding domains within the two enzymes may be essential to differential regulation of the ACs by Ca2+/CaM. In this study we compare in detail the regulation of AC1 and AC8 by Ca2+/CaM both in vivo and in vitro and explore the different role of each Ca2+-binding lobe of CaM in regulating the two enzymes. We also assess the relative dependence of AC1 and AC8 on capacitative Ca2+ entry. Finally, in real-time fluorescence resonance energy transfer-based imaging experiments, we examine the effects of dynamic Ca2+ events on the production of cAMP in cells expressing AC1 and AC8. Our data demonstrate distinct patterns of regulation and Ca2+ dependence of AC1 and AC8, which seems to emanate from their mode of regulation by CaM. Such distinctive properties may contribute significantly to the divergent physiological roles in which these ACs have been implicated.