Presence of a Na+-activated ATPase in the Plasma Membrane of the Marine Raphidophycean Heterosigma akashiwo

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Abstract
Highly purified plasma membranes were isolated from Heterosigma akashiwo cells, a marine raphidophycean unicellular biflagellate, by the silica microbead method, and the ATPase activity of the membranes was characterized. The ionic requirements and spectrum of effective inhibitors enable us to identify a novel Na+-activated ATPase in the plasma membrane of this organism. Furthermore, we detected two phosphorylated intermediate forms of ATPases, with molecular weights of 150 kDa and 95 kDa as judged by acid SDS-polyacrylamide gel electrophoresis of extracts of isolated plasma membrane. The 150 kDa intermediate was phosphorylated in the presence of both Mg2+ and Na+, while the 95 kDa intermediate was phosphorylated in the presence of Mg2+ alone. Both were dephosphorylated in the presence of monovalent cations. These results indicate that the former intermediate was a Na+-activated ATPase, similar to Na+,K+-ATPases from animals, and the latter was similar to H+,K+-ATPases from higher plants. The physiological significance of the two kinds of ATPase in the plasma membrane of marine algae.