Purpose: In recent years, detergent-resistant membranes (DRMs) have been isolated in in vitro models of lipid rafts, from photoreceptor outer segments (ROS), and the localization of a specific complement of photoreceptor proteins has been demonstrated. However, surprisingly little is known about the lipid composition of these important membrane domains. The present study provides the first characterization of phospholipids and fatty acids from ROS-derived DRMs.
Methods: Bovine ROS membranes were incubated with 1% Triton X-100 at 4 degrees C and subjected to density gradient centrifugation to isolate DRMs from the parent membranes. Lipids of ROS and DRMs were separated by two-dimensional, thin-layer chromatography and converted to methyl esters, and fatty acids were analyzed by gas chromatography. Proteins of ROS and DRMs were analyzed by SDS-PAGE and Western blot analysis.
Results: The DRMs represented 8% and 3%, respectively, of total ROS lipid and protein. In general, DRMs were enriched in saturated fatty acids when compared with ROS membranes. Relative to ROS, DRMs were enriched in free fatty acids (FFAs) and a specific phosphatidylcholine (PC) fraction that was almost devoid of polyunsaturated fatty acids (PUFAs). DRMs contained less phosphatidylethanolamine (PE) and phosphatidylserine (PS). Ceramide (CM) from ROS contained PUFAs but no saturated fatty acids; the converse was true of CM from DRMs. Docosahexaenoic acid was diminished in DRM PS and was not detected in the FFAs, but was equally abundant in ROS and DRM PE. ROS-derived DRMs were dramatically enriched in caveolin-1, contained significant amounts of transducin-alpha and c-Src, and were relatively devoid of arrestin.
Conclusions: The relatively saturated lipid environment observed in DRMs is likely to promote the localization of signaling proteins modified with saturated fatty acyl chains. Based on the lipid composition of DRMs, the authors conclude that they would not efficiently support phototransduction.