FUNCTIONAL COLOCALIZATION OF TRANSFECTED CA2-STIMULABLE ADENYLYL CYCLASES WITH CAPACITATIVE CA2+ ENTRY SITES()

Three adenylyl cyclases (ACI, ACIII, and ACVIII) have been described, which are putatively Ca2+-stimulable, based on in vitro assays, Howeve r, it is not clear that these enzymes can be regulated by physiologica l rises in [Ca2+](i) when expressed in intact cells. Furthermore, it i s not known whether transfected adenylyl cyclases might display the st rict requirement for capacitative Ca2+ entry that is shown by the Ca2-inhibitable ACVI, which is indigenous to C6-2B glioma cells (Chiono, M., Mahey, R., Tate, G., and Cooper, D. M. F. (1995) J. Biol. Chem. 27 0, 1149-1155). In the present study, ACI, ACIII, and ACVIII were heter ologously expressed in HEK 293 cells, and conditions were devised that distinguished capacitative Ca2+ entry from both internal release and nonspecific elevation in [Ca2+](i) around the plasma membrane. Remarka bly, not only were ACI and ACVIII largely insensitive to Ca2+ release from stores, but they were robustly stimulated only by capacitative Ca 2+ entry and not at all by a substantial increase in [Ca2+](i) at the plasma membrane elicited by ionophore. (ACIII, reflecting its feeble i n vitro sensitivity to Ca2+, was unaffected by any [Ca2+](i) rise.) Th ese results suggest a quite unsuspected, essential association of Ca2-sensitive adenylyl cyclases with capacitative Ca2+ entry sites, even when expressed heterologously.