Modulation of concentration fluctuations in phase-separated lipid membranes by polypeptide insertion.
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.
Fahsel
S
S
University of Dortmund, Department of Chemistry, Physical Chemistry I, D-44221 Dortmund, Germany.
Pospiech
E-M
EM
Zein
M
M
Hazlet
T L
TL
Gratton
E
E
Winter
Roland
R
D000900:Anti-Bacterial Agents / Q000494:pharmacology; D055592:Biophysical Phenomena; D001703:Biophysics; D004134:Dimyristoylphosphatidylcholine / Q000737:chemistry; D006096:Gramicidin / Q000494:pharmacology*; D050356:Lipid Metabolism; D008055:Lipids / Q000737:chemistry*; D008567:Membranes, Artificial; D008856:Microscopy, Fluorescence; D009502:Neutrons; D010455:Peptides / Q000737:chemistry*; D010713:Phosphatidylcholines / Q000737:chemistry; D010743:Phospholipids / Q000737:chemistry; D012542:Scattering, Radiation; D017550:Spectroscopy, Fourier Transform Infrared; D013696:Temperature
D000900:Anti-Bacterial Agents; D008055:Lipids; D008567:Membranes, Artificial; D010455:Peptides; D010713:Phosphatidylcholines; D010743:Phospholipids; D006096:Gramicidin; C010942:1,2-distearoyllecithin; D004134:Dimyristoylphosphatidylcholine