INDIVIDUAL LEAFLETS OF A MEMBRANE BILAYER CAN INDEPENDENTLY REGULATE PERMEABILITY

Water rapidly crosses most membranes, but only slowly crosses apical m embranes of barrier epithelia such as bladder and kidney collecting du ct, a feature essential to barrier function. How apical membrane struc ture reduces permeabilities remains unclear, Cell plasma membranes con tain two leaflets of distinct lipid composition; the role of this bila yer asymmetry in membrane permeability is unclear, To determine how as ymmetry of leaflet composition affects membrane permeability, effects on bilayer permeation of reducing single leaflet permeability were det ermined using two approaches: formation of asymmetric bilayers in an U ssing chamber, with only one of two leaflets containing cholesterol su lfate, and stabilization of the external leaflet of unilamellar vesicl es with praeseodymium (Pr3+). In both systems, permeability measuremen ts showed that each leaflet acts as an independent resistor of water p ermeation, These results show that a single bilayer leaflet can act as the barrier to permeation and provide direct evidence that segregatio n of lipids to create a low permeability exofacial leaflet may act to reduce the permeability of barrier epithelial apical membranes.