Peptides in Membranes                               



Research Highlights:



Precise Structures









Peptides in Membranes






Water Permeability Through Membranes



Questions: What is the location of an antimicrobial peptide, alamethicin, in a membrane? How is the barrel stave formed by this transmembrane peptide affected by the thickness of the lipid bilayer?  How does the lipid bilayer thickness affect the barrel stave?  How does alamethicin affect the elastic properties of the lipid membrane? 


Alamethicin Aggregation in Lipid Membranes

J Membrane Biol (2009) 231:11–27

Jianjun Pan, Stephanie Tristram-Nagle and                         John F. Nagle

Physics Department, Carnegie Mellon University,            Pittsburgh, Pennsylvania 15213, USA


a b s t r a c t                                                                                                                                                        X-ray scattering features induced by aggregates of alamethicin (Alm) were obtained in oriented stacks of model membranes of DOPC(diC18:1PC) and diC22:1PC.  The first feature obtained near full hydration was Bragg rod in-plane scattering near 0.11 Å^-1 in DOPC and near 0.08 Å^-1 in diC22:1PC at a 1:10 Alm:lipid ratio. This feature is interpreted as bundles consisting of n Alm monomers in a barrel-stave configuration surrounding a water pore. Fitting the scattering data to previously published molecular dynamics simulations indicates that the number of peptides per bundle is n = 6 in DOPC and n C 9 in diC22:1PC. The larger bundle size in diC22:1PC is explained by hydrophobic mismatch of Alm with the thicker bilayer. A second diffuse scattering peak located at qr & 0.7 Å^-1 is obtained for both DOPC and diC22:1PC at several peptide concentrations.  Theoretical calculations indicate that this peak cannot be caused by the Alm bundle structure. Instead, we interpret it as being due to two-dimensional hexagonally packed clusters in equilibrium with Alm bundles. As the relative humidity was reduced, interactions between Alm in neighboring bilayers produced more peaks with three dimensional crystallographic character that do not index with the conventional hexagonal space groups.

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kAlamethicin in lipid bilayers: Combined use of X-ray scattering and MD Simulations

Biochimica et Biophysica Acta 1788 (2009), 1387–1397

Jianjun Pan1, D. Peter Tieleman2, John F. Nagle1,3, Norbert Kučerka1,4 and Stephanie Tristram-Nagle1

1Biological Physics Group, Physics Department, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213, USA  2Department of Biological Sciences, University of Calgary, Calgary, AB, Canada  3Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA 15213, USA  4Canadian Neutron Beam Centre, National Research Council, Chalk River, Ontario K0J1J0, Canada

a b s t r a c t                                                                                                                                    We study fully hydrated bilayers of two di-monounsaturated phospholipids diC18:1PC (DOPC) and diC22:1PC with varying amounts of alamethicin (Alm). We combine the use of X-ray diffuse scattering and molecular dynamics simulations to determine the orientation of alamethicin in model lipids.  Comparison of the experimental and simulated form factors shows that Alm helices are inserted trans- membrane at high humidity and high concentrations, in agreement with earlier results. The X-ray scattering data and the MD simulations agree that membrane thickness changes very little up to 1/10 Alm/DOPC. In contrast, the X-ray data indicate that the thicker diC22:1PC membrane thins with added Alm, a total decrease in thickness of 4 Å at 1/10 Alm/diC22:1PC. The different effect of Alm on the thickness changes of the two bilayers is consistent with Alm having a hydrophobic thickness close to the hydrophobic thickness of 27 Å for DOPC; Alm is then mismatched with the 7 Å thicker diC22:1PC bilayer. The X-ray data indicate that Alm decreases the bending modulus (Kc) by a factor of ~2 in DOPC and a factor of ~10 in diC22:1PC membranes (P/L~1/10). The van der Waals and fluctuational interactions between bilayers are also evaluated through determination of the anisotropic B compressibility modulus.

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