Adenylyl Cyclase from the Green Alga Volvox carteri f . nagariensis: Partial Purification and Characterisation

Abstract

In order to understand changes in cyclic adenylate levels of Volvox carteri during the process of sexual induction, we investigated the biochemical properties of its membrane-bound adenylyl cyclase. Membrane preparations possess low levels of Mg²⁺ -dependent or Mn²⁺ -dependent adenylyl cyclase activity. This activity was solubilised and then purified 7800-fold. The enzyme detergent complex has an apparent molecular mass of 100 kDa. Purified preparations contain a major ATP-binding protein of 33 kDa as shown by affinity labelling. The Mg²⁺ -dependent basal enzyme activity is regulated by Ca²⁺, and is highest in the presence of 10-7 M Ca²⁺, but is inhibited by Ca²⁺ above 10^-5 M. La³⁺ at 10^-4M also blocks activity. Neither calmodulin nor its antagonists affect the enzyme activity, nor do the purified preparations interact with immobilised calmodulin. Further mediators of G-protein action (NaF, or GTP and its derivatives) and forskolins have no influence on the basal activity of this plant enzyme. The function of adenylyl cyclase in sexual induction of Volvox is discussed.