Adenylate cyclase in cilia from Paramecium: Localization and partial characterization

A particulate adenylate cyclase was identified in the excitable ciliary membrane from Paramecium tetraurelia. MnATP was preferentially used as substrate, the K_m was 67 μM, V_max was 1 nmol cAMP.min^?1.mg^?1, a marked temperature optimum of 37°C was observed. Adenylate cyclase was not inhibited by 100 μM EGTA or 100 μM La³⁺, whereas under these conditions guanylate cyclase activity was abolished. Fractionation of ciliary membrane vesicles by a Percoll density gradient yielded two vesicle populations with adenylate cyclase activity. In contrast, calmodulin/Ca-dependent guanylate cyclase was associated with vesicles of high buoyant density only.