Residence of adenylyl cyclase type 8 in caveolae is necessary but not sufficient for regulation by capacitative Ca2+ entry

Abstract

Ca2+-sensitive adenylyl cyclases (ACs) depend on capacitative Ca2+ entry (CCE) for their regulation. Residence of the endogenous Ca2+-inhibitable adenylyl cyclase of C6-2B glioma cells in cholesterol-enriched caveolae is essential for its regulation by CCE (Fagan, K. A., Smith, K. E., and Cooper, D. M. F. (2000) J. Biol. Chem. 275, 26530-26537). In the present study, we established that depletion of cellular cholesterol ablated the regulation by CCE of a Ca2+-stimulable adenylyl cyclase, AC8, heterologously expressed in HEK293 cells. We considered the possibility that a calmodulin-binding domain in the N terminus of AC8, which is not required for in vitro regulation by Ca2+, might play a targeting role. Deletion and mutation of the N terminus did attenuate the enzyme's sensitivity to CCE without altering its in vitro responsiveness to Ca2+/calmodulin. Both N terminus-deleted AC8 and wild type AC8 were expressed at the plasma membrane, as shown by imaging analysis of green fluorescence protein-tagged constructs. However, not only wild type AC8 but also the CCE-insensitive mutants occurred in caveolar fractions of the plasma membranes, even though a Ca2+-insensitive adenylyl cyclase, AC7, was excluded from caveolae. Finally, the AC8 mutants were no more responsive to nonphysiological elevation of Ca2+ than the wild type. We conclude that (i) not all adenylyl cyclases reside in caveolae, (ii) the calmodulin-binding domain in the N terminus of AC8 does not play a role in caveolar targeting, (iii) the N terminus does play a role in associating AC8 with factors that confer sensitivity to CCE, and (iv) residence of Ca2+-sensitive adenylyl cyclases in caveolae is essential but not sufficient for regulation by CCE.