| Abstract: | When incubated for 14 h at 37°C in the absence of energy supply, brush-border membrane vesicles from rabbit kidney cortex maintain, as judged by the use of sphingomyelinase and trinitrobenzene sulfonate as membrane probes, their highly asymmetrical phospholipid distribution. In particular, sphingomyelin still accounts for 75% of the phospholipids present on the outer membrane leaflet. Pretreatment of the vesicles with 5 mM diamide resulted in extensive crosslinking of membranous and cytoskeletal proteins. Although it had no immediate effect on the topology of phospholipids, this crosslinking resulted in a limited but significant increase in the amount of aminophospholipids present on the outer membrane leaflet after 14-h incubations. Degradation of aminophospholipids, upon incubation with hog pancreas and bee venom phospholipases A2, was also enhanced by diamide. However, this enhanced hydrolysis was observed immediately after the diamide treatment. A similar increase in degradation of aminophospholipids was obtained when vesicles were incubated with dihydrocytochalasin B. Our results strongly suggest that cytoskeletal proteins, via interactions with aminophospholipids, stabilize the lipid bilayer of the brush-border membrane. It is also suggested that, due to a low transbilayer migration rate, sphingomyelin may play an important role in the maintenance of the lipid asymmetry in these membranes. |
