Abstract
The perturbations induced by second messenger diacylglycerols (DAGs) into bovine brain phosphatidylcholine (BBPC) bilayers in the presence or absence of bovine brain sphingomyelin (SM) and/or cholesterol were studied by 2 H NMR. Addition of 15 mol% DAG to BBPC bilayers did not induce non-bilayer lipid phases in the temperature range 30–60 °C. Similar measurements performed in the presence of cholesterol revealed that cholesterol progressively destabilizes PC bilayers with respect to DAG-induced perturbations. Thus, at 40 mol% cholesterol, addition of 15 mol% DAG induced the formation of non-bilayer (isotropic and inverted hexagonal) phases at 60 °C. Whereas some lateral separation of the bilayers into domains of different cholesterol contents was observed in BBPC/cholesterol membranes, such a lateral heterogeneity was greatly facilitated by the addition of SM. Since both a tendency to form non-bilayer lipid phases and lateral heterogeneity of the membranes are associated with increased activation of a number of membrane-associated enzymes, our results suggest that SM- and cholesterol-enriched regions of biological membranes (rafts) provide an environment with increased sensitivity to the generation of lipid second messengers and modified transmembrane signal transduction properties.
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