Interaction of Lipopolysaccharide and Phospholipid in Mixed Membranes: Solid-State ³¹P-NMR Spectroscopic and Microscopic Investigations

¹ Suntory Institute for Bioorganic Research
² National Institute of Advanced Industrial Science and Technology
³ Research Center Borstel

^* To whom correspondence should be addressed. E-mail: nomura{at}sunbor.or.jp.

Submitted on February 20, 2008
Revised on March 19, 2008
Accepted on 16 April 2008

	Abstract

Lipopolysaccharide (LPS), which constitutes the outermost layer of Gram-negative bacterial cells as a typical component essential for their life, induces the first line defense system of innate immunity of higher animals. To understand the basic mode of interaction between bacterial LPS and phospholipid cell membranes, distribution patterns were studied by various physical methods of deep rough mutant LPS (ReLPS) of Escherichia coli incorporated in phospholipid bilayers as simple models of cell membranes. Solid-state ³¹P-NMR spectroscopic analysis suggested that a substantial part of ReLPS is incorporated into 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) lipid bilayers when multilamellar vesicles (MLVs) were prepared from mixtures of these. In egg L--phosphatidylcholine (egg-PC)-rich membranes, ReLPS undergoes micellization. In phosphatidylethanolamine (PE)-rich membranes, however, micellization was not observed. We studied by microscopic techniques the location of ReLPS in membranes of ReLPS / egg-PC (1:10 M/M) and ReLPS / egg-PC / 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG) (1:9:1 M/M/M). The influence of ReLPS on the physicochemical properties of the membranes was also studied as well. Microscopic images of both giant unilamellar vesicles (GUVs) and supported planar lipid bilayers (SPBs) showed that LPS was uniformly incorporated in the egg-PC lipid bilayers. In the egg-PC / POPG (9:1 M/M) lipid bilayers, however, ReLPS is only partially incorporated and becomes a part of the membrane in a form of aggregates (or as mixed aggregates with the lipids) on the bilayer surface. The lipid lateral diffusion coefficient measurements at various molar ratios of ReLPS / egg-PC / POPG indicated that the incorporated ReLPS reduces the diffusion coefficients of the phospholipids in the membrane. The retardation of diffusion became more significant with increasing POPG concentrations in the membrane at high ReLPS / phospholipid ratios. The present work demonstrated that the phospholipid composition has critical influence on the distribution of added ReLPS in the respective lipid membranes and also on the morphology and physicochemical property of the resulting membranes. A putative major factor causing these phenomena is reasoned to be the miscibility between ReLPS and individual phospholipid compositions.

Key Words: Lipopolysaccharide, Microscopy, Phospholipid membrane, Solid-State NMR Spectroscopy, diffusion coefficient