EFFECTS OF PHOSPHATIDYLETHANOLAMINES ON THE ACTIVITY OF THE CA2-ATPASE OF SARCOPLASMIC-RETICULUM()

ATPase activities for the Ca2+-ATPase of skeletal muscle sarcoplasmic reticulum reconstituted into dioleoylphosphatidylethanolamine [di(C-18 :1)PE] are, at temperatures higher than 20 degrees C, lower than in di oleoylphosphatidylcholine [di(C-18:1)PC], whereas in egg yolk phosphat idylethanolamine the activities are the same as in di(C-18:1)PC up to 25 degrees C, suggesting that low ATPase activities occur when the pho sphatidylethanolamine species is in the hexagonal H-II phase. ATPase a ctivities measured in mixtures of di(C-18:1)PC and di(C-18:1)PE do not change with changing di(C-18:1)PE content up to 80%. It is concluded that curvature frustration in bilayers containing di(C-18:1)PE has no effect on ATPase activity. The rates of phosphorylation and of Ca2+ tr ansport are identical for the native ATPase and for the ATPase in di(C -18:1)PE. Dephosphorylation of the phosphorylated ATPase in di(C-18:1) PE at 25 degrees C is, however, slower than for the native ATPase, exp laining the lower steady-state rate of ATP hydrolysis; in egg yolk pho sphatidylethanolamine at 25 degrees C the rate of dephosphorylation is equal to that for the unreconstituted ATPase. Phosphorylation of the ATPase by P-i in the absence of Ca2+ is unaffected by reconstitution i n di(C-18:1)PE. The stoichiometry of Ca2+ binding to the ATPase is als o unaltered. Studies of the effect of di(C-18:1)PE on the fluorescence intensity of the ATPase labelled with 7-chloro-4-nitro-2,1,3-benzoxad iazole are consistent with an increase in the E1/E2 equilibrium consta nt, where E1 is the conformation of the ATPase with two high-affinity binding sites for Ca2+ exposed to the cytoplasm, and E2 is a conformat ion unable to bind cytoplasmic Ca2+. A slight increase in affinity for Ca2+ can be attributed to the observed increase in the E1/E2 equilibr ium constant.