Stoichiometry of lipid–protein interaction assessed by hydrophobic photolabeling

Here we undertook a comparative study of the composition of the lipid annulus of three ATPases pertaining to the P-type family: plasma membrane calcium pump (PMCA), sarcoplasmic reticulum calcium pump (SERCA) and Na,K-ATPase. The photoactivatable phosphatidylcholine analogue [125I]TID-PC/16 was incorporated into mixtures of dimyristoyl phosphatidylcholine (DMPC) and each enzyme with the aid of the nonionic detergent C12E10. After photolysis, the extent of the labeling reaction was assessed to determine the lipid:protein stoichiometry: 17 for PMCA, 18 for SERCA, 24 for the Na,K-ATPase (α-subunit) and 5.6 mol PC/mol protein for the Na,K-ATPase (β-subunit).