Compartmentalization of phosphatidylinositol 4,5-bisphosphate metabolism into plasma membrane liquid-ordered/raft domains

Abstract

Possible segregation of plasma membrane (PM) phosphoinositide metabolism in membrane lipid domains is not fully understood. We exploited two differently lipidated peptide sequences, L10 and S15, to mark liquid-ordered, cholesterol-rich (Lo) and liquid-disordered, cholesterol-poor (Ld) domains of the PM, often called raft and nonraft domains, respectively. Imaging of the fluorescent labels verified that L10 segregated into cholesterol-rich Lo phases of cooled giant plasma-membrane vesicles (GPMVs), whereas S15 and the dye FAST DiI cosegregated into cholesterol-poor Ld phases. The fluorescent protein markers were used as Förster resonance energy transfer (FRET) pairs in intact cells. An increase of homologous FRET between L10 probes showed that depleting membrane cholesterol shrank Lo domains and enlarged Ld domains, whereas a decrease of L10 FRET showed that adding more cholesterol enlarged Lo and shrank Ld. Heterologous FRET signals between the lipid domain probes and phosphoinositide marker proteins suggested that phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] and phosphatidylinositol 4-phosphate (PtdIns4P) are present in both Lo and Ld domains. In kinetic analysis, muscarinic-receptor-activated phospholipase C (PLC) depleted PtdIns(4,5)P2 and PtdIns4P more rapidly and produced diacylglycerol (DAG) more rapidly in Lo than in Ld. Further, PtdIns(4,5)P2 was restored more rapidly in Lo than in Ld. Thus destruction and restoration of PtdIns(4,5)P2 are faster in Lo than in Ld. This suggests that Lo is enriched with both the receptor G protein/PLC pathway and the PtdIns/PI4-kinase/PtdIns4P pathway. The significant kinetic differences of lipid depletion and restoration also mean that exchange of lipids between these domains is much slower than free diffusion predicts.