The ATPase activity of the chloroplast coupling factor 1 (CF1) isolated from the green alga Dunaliella is completely latent. A brief heat treatment irreversibly induces a Ca2+ -dependent activity. The Ca2+ dependent ATPase activity can be reversibly inhibited by ethanol, which changes the divalent cation dependency from Ca2+ to Mg2+. Both the Ca2+ -dependent and Mg2+ -dependent ATPase activities of heat-treated Dunaliella CF1 are inhibited by monospecific antisera directed against Chlamydomonas reinhardi CF1. However, when assayed under identical conditions, the Ca2+ -dependent ATPase activity is significantly more sensitive to inhibition by the antisera than is the Mg2+ -dependent activity. These data are interpreted as indicating that soluble Dunaliella CF1 can exist in a variety of conformations, at least one of which catalyzes a Ca2+ -dependent ATPase and two or more of which catalyze an Mg2+ -dependent ATPase.
