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Biophys. J. BioFAST: First Published December 30, 2005. doi:10.1529/biophysj.105.072405
© 2005 by the Biophysical Society.

A more recent version of this article appeared on March 15, 2006.
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MEMBRANES

Differential stiffness and lipid mobility in the leaflets of purple membranes

Kislon Voitchovsky 1*, Sonia Antoranz Contera 1, Miya Kamihira 1, Anthony Watts 1 and John Ryan 1

1 University of Oxford

* To whom correspondence should be addressed. E-mail: k.voitchovsky{at}physics.ox.ac.uk.

Submitted on August 9, 2005
Revised on October 29, 2005
Accepted on 6 December 2005


  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 differs 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 (AC-AFM) is presented. This method does not require molecular resolution, and is applied to study the stiffness and inter-trimer 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≥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 nuclear magnetic resonance confirms and quantifies the AFM qualitative observations.

Key Words: Young's modulus, atomic force microsopy, bacteriorhodopsin, phospholipid, purple membrane, solid-state nuclear magnetic resonance




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Copyright © 2005 by the Biophysical Society.