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V‐ AND P‐TYPE Ca2+‐STIMULATED ATPases IN A CALCIFYING STRAIN OF PLEUROCHRYSIS SP. (HAPTOPHYCEAE)

Coccolithophorids are marine unicellular algae characterized by their ability to carry out controlled, subcellular calcification. The biochemical and kinetic features of membrane‐bound Ca2+‐stimulated ATPases have been examined. Membranes and organelles from axenic cultures of Pleurochrysis sp. (CCMP299) were isolated by means of sucrose density centrifugation. High levels of Ca2+‐stimulated ATPase were detected in chloroplasts, Golgi apparatus, plasma membrane, and coccolith vesicles. The sensitivity of the enzyme activity in the organelles and membranes was assessed with pharmacologic agents that are known to be specific for the several isoforms of Ca2+‐stimulated ATPase. The Ca2+‐stimulated ATPase activity in the Golgi and coccolith vesicle preparations was sensitive to nitrate, thiocyanate, and sodium azide and insensitive to vanadate, cyclopiazonic acid, and thapsigargin. ATP‐dependent H+ movement, but not 45Ca2+ transport, across the coccolith vesicle was demonstrated. The Ca2+‐stimulated ATPase in the plasma membrane preparation was sensitive to vanadate. ATP‐dependent, vanadate‐sensitive efflux of 45Ca2+ was demonstrated for microsomal material derived from gradient‐isolated plasma membrane. Polypeptides from isolated Golgi and coccolith vesicle preparations cross‐reacted to an antibody raised against a subunit of the oat root proton pump, whereas polypeptides from the chloroplast preparations did not cross‐react. These findings show that a V‐type Ca2+‐stimulated ATPase is located on the coccolith vesicle membrane and a P‐type Ca2+‐stimulated ATPase is located on the plasma membrane.