NMR INVESTIGATIONS OF CELL MEMBRANE STRUCTURE

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The objectives of this project are to: (1) investigate the interaction of alcohol with proteins and lipids in biological membranes; (2) study structure and dynamics of membranes composed of lipids with polyunsaturated fatty acids such as docosahexaenoic acid (DHA) 22:6n-3; and (3) study lipid-protein interactions related to alcoholism and lipid polyunsaturation. (1) The interface location of ethanol lowers interfacial energy of bilayers, increasing the area per lipid molecule. This provides more space for movement of the lipid hydrocarbon chains which become progressively more disordered with increasing ethanol concentration. Ethanol-induced chain disordering is smaller in polyunsaturated bilayers, most likely because polyunsaturated hydrocarbon chains already occupy a larger area per molecule and are therefore less sensitive to ethanol-induced disordering. The ethanol molecules at the lipid-water interface block pathways for water diffusion through lipid bilayers as seen in decreased rates of water permeation. (2) The membranes of brain synaptosomes and retinal rod outer segments contain 30-50 mol% of the six-fold unsaturated docosahexaenoic acid (DHA) as lipid hydrocarbon chains. One possible role of DHA is to alter membrane mechanical properties important for activity of receptor proteins. There is controversy as to the nature of the perturbation which DHA chains induce on membrane hydrocarbon order. The six methylene- interrupted cis double bonds within DHA's 22 carbon unit reduce the number of degrees of freedom for structural transitions, which led to the suggestion that these chains have a specific rigid conformation such as angle-iron or helical. However, direct measurements of DHA chain order parameters reveal a different picture. Using a magic angle spinning NMR experiment which re-couples 13C-1H dipolar interactions, assigned DHA order parameters were obtained, and dimensions of the DHA chain unit cell were determined by x-ray diffraction. The results suggest that DHA chains in membranes prefer looped conformations and undergo rapid structural transitions, providing increased flexibility to receptor-rich neural membranes. (3) The structure of the cytolytic peptide fragment 828-848 (P828s) from the carboxy-terminus of the envelope glycoprotein gp41 of HIV-1 was investigated by high resolution NMR in water and bound to negatively charged SDS micelles. The depth of peptide incorporation into phosphatidylserine membranes and the resulting perturbation of the lipid matrix were studied by solid state 2H NMR. The peptide is unstructured in water and converts to a partially helical conformation upon binding to either negatively charged liposomes or micelles. NMR crosspeak patterns, and induced variation in chemical shift, suggest formation of a very flexible 3(10)-helix that covers the first 14 residues of the peptide. The C-terminus of the peptide that holds three of the six positively charged arginines of P828s appears to be unstructured. The location of the peptide in the bilayer has been investigated by selective deuteration of the four isoleucines I3, I13, I16, and I20. The 2H NMR sidechain order parameters suggested a penetration of the I3 sidechain into the membrane's hydrophobic core and an interface location of the other sidechains. The membrane perturbation that resulted from peptide binding was studied on sn-1 chain deuterated SOPS- d35 membranes. Peptide binding at a lipid/peptide ratio of 20/1 resulted in a membrane thinning of about 1 angstrom. The decrease of order parameters was larger in the center of the bilayer than near the glycerol region confirming the location of the peptide's backbone in the lipid/water interface. We propose that P828's incorporation into the lipid/water interface results in membrane curvature stress that becomes the driving force for pore formation and cell lysis.