The structure and function of the lipopolysaccharide transport system in gram negative bacteria.

Abstract

Gram-negative bacteria contain lipopolysaccharide (LPS), also known as endotoxin, in the outer leaflet of their outer membrane (OM). LPS causes a strong immune response in humans, provides structural support for the bacterium and protects it against hydrophobic molecules, like some antibiotics. LPS is synthesised in separate parts in the cytoplasm and transported across the inner membrane (IM), through the periplasm and across the OM. The synthesis pathway and the transport across the IM have been described. The subsequent removal from the IM and transport to the outside of the OM have not been elucidated entirely. A complex of seven proteins, named LPS transport system (Lpt) A-G, is responsible for LPS transport to the OM. The complex is thought to span the entire envelope of the bacterium, likely at contact points between IM and OM, as seen by Bayer in electron microscopy (EM) studies. This research aims to identify the localisation and interaction partners in the complex for each protein and resolve the structure of four proteins that together span the membrane. The functional interaction domains for either complex partners or LPS will also be determined for these proteins. Neisseria meningitidis will serve as model organism. Its Lpt proteins will be analyzed by EM, Förster resonance energy transfer, Nuclear Magnetic Resonance imaging and by reconstructing an artificial double membrane system with two different kinds of vesicles. This will provide fundamental insight into the process of outer membrane synthesis of gram-negative bacteria and aid in the development of novel antibiotics.