Oxytocin receptors and cholesterol: interaction and regulation

Cholesterol affects the ligand binding function of the oxytocin receptor in a highly specific manner. While the structurally-related cholecystokinin r eceptor shows a strong correlation between the membrane fluidity and its bi nding function, the oxytocin receptor behaves differently. A stringent and unique requirement of the affinity state of the oxytocin receptor for struc tural features of the sterol molecule has been found. The molecular require ments differ both from those postulated for sterol-phospholipid interaction s and from those known to be necessary for the activity of other proteins. Employing a new detergent-free subcellular fractionation protocol, a two-fo ld enrichment of the oxytocin receptors (10-15% of total receptors) has bee n detected in the cholesterol-rich, caveolin-containing membrane domains of the plasma membrane. While most of the properties of the oxytocin receptor s were indistinguishable in cholesterol-poor versus cholesterol-rich membra ne compartments, high-affinity oxytocin receptors localised in caveolin-enr iched low-density membranes showed about a 3-fold higher stability against thermal denaturation at 37 degrees C compared with the oxytocin receptors l ocalised in high-density membranes. Moreover, addition of cholesterol to th e cholesterol-poor high-density membranes fully protected the oxytocin rece ptors against thermal denaturation and partially rescued high-affinity oxyt ocin binding. Although the membrane fluidity of the caveolin-enriched domai ns was lower than that in the high-density membranes, there was no correlat ion between the stability of oxytocin receptors and the fluidity level of t he membrane domains. Finally, in a molecular modelling approach a putative cholesterol binding motif on the extracellular surface of the oxytocin rece ptor was found.