The characterization of plasma membrane Ca2+-ATPase in rich sphingomyelin–cholesterol domains

According to the raft hypothesis, sphingolipid–cholesterol (CHOL) microdomains are involved in numerous cellular functions. Here, we have prepared liposomes to simulate the lipid composition of rafts/caveolae using phosphatidylchone, sphingomyelin (SPM)–CHOL in vitro. Experiments of both 1,6-diphenyl-1,3,5-hexatriene and merocyanine-540 fluorescence showed that a phase transition from ld to lo can be observed clearly. In particular, we investigated the behavior of a membrane protein, plasma membrane Ca2+-ATPase (PMCA), in lipid rafts (lo phase). Three complementary approaches to characterize the physical appearance of PMCA were employed in the present study. Tryptophan intrinsic fluorescence increase, fluorescence quenching by both acrylamid and hypocrellin B decrease, and MIANS fluorescence decrease, indicate that the conformation of PMCA embedded in lipid lo phase is more compact than in lipid ld phase. Also, our results showed that PMCA activity decreased with the increase of SPM–CHOL content, in other words, with the increase of lo phase. This suggests that the specific domains containing high SPM–CHOL concentration are not a favorable place for PMCA activity. Finally, a possible explanation about PMCA molecules concentrated in caveolae/rafts was discussed.