Impact of the glycostructure of amphiphilic membrane components on the function of the outer membrane of Gram-negative bacteria as a matrix for incorporated channels and a target for antimicrobial peptides or proteins

Abstract

The architecture of the lipid matrix of Gram-negative bacteria is extremely asymmetric with respect to its lipid distribution: Whereas the inner leaflet is composed of a phospholipid mixture, the outer leaflet is built up by glycolipids. For most Gram-negative species, these glycolipids are lipopolysaccharides, for a few species, however, glycosphingolipids. We describe here our experimental approach and results thereof to get answers on the function of the asymmetric architecture and, in particular of the glycostructure of the outer leaflet (i) for the incorporation of porin channels into the bilayer and their function inside the membrane, (ii) the role of the glycolipid surface in the activation of the complement system, and (iii) the formation of transient lesions or stable pores by the interaction of antimicrobial peptides, e.g. polymyxin B, the bactericidal/permeability-increasing protein BPI, and cathelicidins. Furthermore, we investigated the influence of the glycocomponent on basic biophysical characteristics of glycolipid-containing membranes such as their electric properties and the lateral organization of the glycolipids in the membrane. To this end we established and applied a number of various reconstitution systems of the outer membrane reaching from monolayers at the air–water interface, via solid supported symmetric and asymmetric bilayers, free-standing symmetric and asymmetric bilayers prepared according to Montal-Mueller technique to three-dimensional aggregates such as liposomes. To obtain answers on the questions outlined above, we investigated the influence of the various applied molecules on physical parameters of these model membranes.