Separate effects of long-chain phosphatidylcholines on dephosphorylation of the Ca(2+)-ATPase and on Ca2+ binding.

The steady-state activity of the Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum (SR) is low when reconstituted into bilayers of the long-chain phosphatidylcholines dierucyl phosphatidylcholine [di(C22:1)PC] or dinervonyl phosphatidylcholine [di(C24:1)PC]. In di(C24:1)PC the ATPase binds a single Ca2+ ion, whereas in di(C22:1)PC it binds two, as in the native SR [Starling, East and Lee (1993) Biochemistry 32, 1593-1600]. In di(C22:1)PC, rates of phosphorylation of the ATPase by ATP and the rate of ATP-induced Ca2+ dissociation are slightly lower than in the native ATPase. However, a much more marked decrease is observed in di(C22:1)PC in the rate of dephosphorylation of the phosphorylated ATPase, which explains the low steady-state ATPase activity. The level of phosphorylation of the ATPase by Pi was little affected by reconstitution in di(C22:1)PC, suggesting that the rate of phosphorylation by Pi is also decreased. The very similar effects of di(C22:1)PC and di(C24:1)PC (Starling, East and Lee (1995) Biochem. J. 310, 875-879) on phosphorylation and dephosphorylation suggest that changes in these steps and the change in Ca2+ binding stoichiometry observed in di(C24:1)PC represent independent changes on the ATPase.