Sterol carrier protein-2 selectively alters lipid composition and cholesterol dynamics of caveolae/lipid raft vs nonraft domains in L-cell fibroblast plasma membranes

Although the functional significance of caveolae/lipid rafts in cellular signaling and cholesterol transfer is increasingly recognized, almost nothing is known regarding the lipids, cholesterol dynamics, and factors regulating these properties in caveolae/lipid rafts as opposed to nonlipid raft domains of the plasma membrane. The present findings demonstrate the utility of con-A affinity chromatography for simultaneous isolation of caveolae/lipid raft and nonlipid raft domains from plasma membranes of L-cell fibroblasts. These domains differed markedly in both protein and lipid constituents. Although caveolae/ lipid rafts were enriched in total lipid, cholesterol, and phospholipid as well as other markers for these domains, the cholesterol/phospholipid ratio of caveolae/lipid rafts did not differ from that of nonlipid rafts. Nevertheless, spontaneous sterol transfer was 7-12-fold faster from caveolae/lipid raft than nonlipid raft domains of the plasma membrane. This was largely due to the near absence of exchangeable sterol in the nonlipid rafts. SCP-2 dramatically and selectively enhanced sterol transfer from caveolae/lipid rafts, but not from nonlipid rafts. Finally, overexpression of SCP-2 significantly altered the sterol dynamics of caveolae/lipid rafts to facilitate retention of cholesterol within the cell. These results established for the first time that (i) caveolae/lipid rafts, rather than the nonlipid raft domains, contain significant levels of rapidly transferable sterol, consistent with their role in spontaneous sterol transfer from and through the plasma membrane, and (ii) SCP-2 selectively regulates how caveolae/lipid rafts, but not nonlipid raft domains, mediate cholesterol trafficking through the plasma membrane.