title: Differential stiffness and lipid mobility in the leaflets of purple membranes.
abstract: Purple membranes (PM) are two-dimensional crystals formed by bacteriorhodopsin and a variety of lipids. The lipid composition and density in the cytoplasmic (CP) leaflet differ from those of the extracellular (EC) leaflet. A new way of differentiating the two sides of such asymmetric membranes using the phase signal in alternate contact atomic force microscopy is presented. This method does not require molecular resolution and is applied to study the stiffness and intertrimer lipid mobility in both leaflets of the PM independently over a broad range of pH and salt concentrations. PM stiffens with increasing salt concentration according to two different regimes. At low salt concentration, the membrane Young's normal modulus grows quickly but differentially for the EC and CP leaflets. At higher salt concentration, both leaflets behave similarly and their stiffness converges toward the native environment value. Changes in pH do not affect PM stiffness; however, the crystal assembly is less pronounced at pH > or = 10. Lipid mobility is high in the CP leaflet, especially at low salt concentration, but negligible in the EC leaflet regardless of pH or salt concentration. An independent lipid mobility study by solid-state NMR confirms and quantifies the atomic force microscopy qualitative observations.
authors.affiliation: Interdisciplinary Research Collaboration in Bionanotechnology, Department of Physics, Clarendon Laboratory, University of Oxford, Oxford OX1 3PU, United Kingdom. voitchovsky1@physics.ox.ac.uk
authors.forename: Kislon
authors.initials: K
authors.lastname: Voïtchovsky
authors.forename: Sonia
authors.initials: S
authors.lastname: Antoranz Contera
authors.forename: Miya
authors.initials: M
authors.lastname: Kamihira
authors.forename: Anthony
authors.initials: A
authors.lastname: Watts
authors.forename: J F
authors.initials: JF
authors.lastname: Ryan
mesh_terms: D022081:Cell Surface Extensions / Q000737:chemistry*; D004548:Elasticity; D008560:Membrane Fluidity; D008563:Membrane Lipids / Q000737:chemistry*; D018625:Microscopy, Atomic Force / Q000379:methods*; D008968:Molecular Conformation; D018717:Purple Membrane / Q000737:chemistry*; D012492:Salts / Q000737:chemistry*; D013314:Stress, Mechanical
chemical_list: D008563:Membrane Lipids; D012492:Salts