Calcium‐Independent Activation of Adenylate Cyclase by Calmodulin

Adenylate cyclase of Bordetella pertussis is stimulated by calmodulin by two distinct interactions. At low activator concentrations (∼ 1 nM) the process is Ca2+‐dependent (i.e. inhibited by EGTA added before calmodulin). High activator concentrations (∼ 0.1–10 μM) stimulate adenylate cyclase also in the presence of EGTA, an effect not accounted for by residual Ca2+ or low concentrations of Ca · calmodulin, which thus appears to be due to calcium‐free calmodulin. Some calmodulin dose‐response curves show both phases of stimulation, separated by a plateau of activity, and half‐maximal activating concentrations differ by 100–300‐fold. Both effects are on the V and not the Km for ATP and are not mimicked by 105‐fold greater concentrations of parvalbumin or by various polyanions.In addition, adenylate cyclase stimulation at high calmodulin concentrations is greater in the presence of EGTA than in its absence. This enhancement is also produced by 1,10‐phenanthroline and 8‐hydroxyquinoline but not by non‐chelating isomers. These compounds are poor Ca2+ chelators, stimulate at any calmodulin concentration (unlike EGTA), and suggest regulation of this adenylate cyclase by a second metal ion.