Lipopolysaccharide Induced Dynamic Lipid Organizations: Lipid Tubules, Membrane Perforations and Multi-Lamellar Stacking

Supported lipid bilayer assemblies (sLBAs) are generally thought of as relatively stable, predictable model membranes, relevant to a biological system. Lipopolysaccahride (LPS) is a unique lipoglycan, with two major functions: (i) as a major component of the outer membrane of Gram-negative bacteria and (ii) as a highly potent human toxin when released from cells into solution ("endotoxin"). Divalent cations have long been known to neutralize and stabilize LPS in the outer membrane, whereas LPS in the presence of monovalent cations forms highly mobile negatively-charged aggregates. We report fluorescence microscopy and atomic force microscopy analysis of the interaction between soluble LPS and a single component fluid-phase sLBA. Three remarkably different deformations are induced by LPS on the simple lipid membrane, dependent on cation availability. LPS is an amphiphile that spontaneously inserts into the outer leaflet of lipid bilayers to bury its hydrophobic lipidic domain and expose the hydrophilic polysaccharide chain to the exterior polar solvent. Net negative (LPS-Na+) induces membrane curvature due to electrostatic repulsive effects between clustered LPS. This leads to (1) the growth of 100μm-long flexible lipid tubules from surface associated lipid vesicles and (2) destabilization of the sLBA leading to micron-sized hole formation. In contrast, Ca2+ promotes self-association and bridging of LPS, and (LPS-Ca2+) induces (3) growth of 100μm-wide planar single- or multi-lamellar sheets of lipid and LPS from surface associated lipid vesicles that exhibit 2-D membrane fluidity and represent a potential means of organizing layer-by-layer membrane construction. Our findings have important implications about the physical interaction of LPS and lipids and the potential of using LPS and other amphiphilic materials as membrane soft-lithography tools.