Domain formation in models of the renal brush border membrane outer leaflet

The plasma membrane outer leaflet plays a key role in determining the exist ence of rafts and detergent-resistant membrane domains. Monolayers with lip id composition mimicking that of the outer leaflet of renal brush border me mbranes (BBM) have been deposited on mica and studied by atomic force micro scopy. Sphingomyelin (SM) and palmitoyloleoyl phosphatidylcholine (POPC) mi xtures, at molar ratios varying from 2:1 to 4:1, were phase-separated into liquid condensed (LC) SM-enriched phase and liquid expanded (LE) POPC-enric hed phase. The LC phase accounted for 33 and 58% of the monolayers surface for 2:1 and 4:1 mixtures, respectively. Addition of 20-50 mol % cholesterol (Chl) to the SM/POPC (3:1) mixtures induced marked changes in the topology of monolayers. Whereas Chi promoted the connection between SM domains at 2 0 mol %, increasing Chi concentration progressively reduced the size of dom ains and the height differences between the phases. Lateral heterogeneity w as, however, still present at 33 mol % Chi. The results indicate that the l ipid composition of the outer leaflet is most likely responsible for the BB M thermotropic transition properties. They also strongly suggest that the c ommon maneuver that consists of depleting membrane cholesterol to suppress rafts does not abolish the lateral heterogeneity of BBM membranes.