Identification of a calmodulin‐stimulated (Ca2++ Mg2+)‐ATPase in a plasma membrane fraction isolated from maize (Zea mays) leaves

Plasma membranes were isolated from light‐grown, 14‐day‐old maize leaves ( Zea mays L . cv. Golden Cross Bantam) using aqueous two‐phase partitioning. The plasma membrane (PM) fraction contained < 0.3% of the total chlorophyll, < 0.2% of the mitochondrial marker enzyme activity, minimal contamination by endomembranes and 34% of the total PM. A calmodulin‐stimulated (Ca2++ Mg2+)‐ATPase was identified in the PM‐enriched fraction. The Ca2++ calmodulin stimulation was dependent on Mg2+, saturated at ca 25 μM total Ca2+, had a pH maximum at 7.2 and was maximally stimulated by 600 n M bovine brain calmodulin. The stimulation was not greatly affected by the anion present and showed a divalent cation specificity of Ca2+ > Sr+2 ± Mn+2 > Co2+± Cu2+ > Ba2+. The napthalenesulfonamide W7, an antagonist of calmodulin action, completely inhibited the calmodulin stimulation at 175 μM , while the less active analogue W5 was ineffective at this concentration. La3+, an inhibitor of PM Ca2+ transport, showed a 50% inhibition of calmodulin‐stimulated ATPase activity at ca 200 μM . Taken as a whole, these data demonstrate the presence of a calmodulinstimulated, (Ca2++ Mg2+)‐ATPase on the cytoplasmic surface of the plasma membrane of maize leaf cells.