Activation of sarcoplasmic reticular Ca2+ transport ATPase by phosphorylation of an associated phosphatidylinositol.

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Abstract
Approximately 1 mol phosphatidylinositol phosphate is formed per mol isolated Ca2+ transport ATPase when the enzyme is incubated with ATP/Mg2+. The phosphorylation of this enzyme‐associated phosphatidylinositol represents the alkylphosphate formation described earlier. The phosphatidylinositol phosphate has been found in the hydrophobic core of the enzyme. A complex of phosphatidylinositol phosphate with protein can be extracted with acidic chloroform/methanol. The protein behaves like proteolipid during chromatography on Sephadex LH 60 and binds the radioactively labelled phosphatidylinositol phosphate. The phosphorylation of approximately 1 mol phosphatidylinositol per 100,000 g protein correlates with an enhancement of the Ca2+ transport ATPase activity which is due to an approximately 7‐fold enhanced affinity for Ca2+ and an approximately 2‐fold enhanced maximal turnover.

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