Atomic force microscopy on domains in biological model membranes

Abstract

This thesis describes the preparation and imaging of supported lipid bilayers, which can be regarded as biological modelmembranes, in the light of the formation of domains. The bilayers were prepared with either the Langmuir-Blodgett method, or with vesicle fusion. They were imaged with Atomic Force Microscopy, which has proven to be a powerful tool to image biological samples under aqueous conditions.Many biological membranes are asymmetric, and several membrane proteins associate with anionic lipids. Hence asymmetric bilayers were prepared, with a first leaflet of zwitterionic, and a second leaflet of anionic phospholipids. Chapter 2 reports on the morphology of such bilayers. The defects present in these bilayers were surrounded by elevations. The formation of these elevations was found to arise from electrostatic interactions, and was called bilayer blistering. Chapters 3 and 4 deal with domain formation due to peptide-lipid interactions. In order to study the lateral organization of lipid bilayers mixed with transmembrane peptides, DPPC bilayers with transmembrane model peptides incorporated were imaged. Chapter 3 describes how WALP peptides induced the formation of domains with an unusual, highly ordered pattern and it describes the effect of the length of WALP peptides on these striated domains. Chapter 4 deals with the influence of flanking residues other than tryptophans on the formation of striated domains. In Chapter 5 domain formation due to lipid-lipid interaction is studied. Bilayers containing sphingolipids and cholesterol were chosen because these lipids are known to preferentially associate, and in certain cell membranes they are suspected to form functional domains which are assumed to correspond to certain parts of cell membranes that are resistant to non-ionic detergents. The formation of such domains as well as their resistance to a non-ionic detergent was imaged. In Chapter 6 all the results presented in this thesis are summarized and discussed