Biophys J, July 2002, p. 334-344, Vol. 83, No. 1

Modulation of Concentration Fluctuations in Phase-Separated Lipid Membranes by Polypeptide Insertion

S. Fahsel, E.-M. Pospiech, M. Zein, T. L. Hazlet, E. Gratton, and Roland Winter

University of Dortmund, Department of Chemistry, Physical Chemistry I, D-44221 Dortmund, Germany; and  Laboratory for Fluorescence Dynamics, University of Illinois, Urbana, Illinois 61801 USA

The lateral membrane organization and phase behavior of the binary lipid mixture DMPC (1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine) - DSPC (1,2-distearoyl-sn-glycero-3-phosphatidylcholine) without and with incorporated gramicidin D (GD) as a model biomembrane polypeptide was studied by small-angle neutron scattering, Fourier-transform infrared spectroscopy, and by two-photon excitation fluorescence microscopy on giant unilamellar vesicles. The small-angle neutron scattering method allows the detection of concentration fluctuations in the range from 1 to 200 nm. Fluorescence microscopy was used for direct visualization of the lateral lipid organization and domain shapes on a micrometer length scale including information of the lipid phase state. In the fluid-gel coexistence region of the pure binary lipid system, large-scale concentration fluctuations appear. Infrared spectral parameters were used to determine the peptide conformation adopted in the different lipid phases. The data show that the structure of the temperature-dependent lipid phases is significantly altered by the insertion of 2 to 5 mol% GD. At temperatures corresponding to the gel-fluid phase coexistence region the concentration fluctuations drastically decrease, and we observe domains in the giant unilamellar vesicles, which mainly disappear by the incorporation of 2 to 5 mol% GD. Further, the lipid matrix has the ability to modulate the conformation of the inserted polypeptide. The balance between double-helical and helical dimer structures of GD depends on the phospholipid chain length and phase state. A large hydrophobic mismatch, such as in gel phase one-component DSPC bilayers, leads to an increase in population of double-helical structures. Using an effective molecular sorting mechanism, a large hydrophobic mismatch can be avoided in the DMPC-DSPC lipid mixture, which leads to significant changes in the heterogeneous lipid structure and in polypeptide conformation.

Biophys J, July 2002, p. 334-344, Vol. 83, No. 1
© 2002 by the Biophysical Society   0006-3495/02/07/334/11  $2.00

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