Protecting Bacterial Cells from Rupture: A Multiscale Simulation Study of Braun's Lipoprotein

Abstract

The cell envelope of Gram-negative bacteria is composed of two membranes. The inner membrane has a relatively simple phospholipid composition, whilst the asymmetric outer membrane is a complex mixture of lipopolysaccharide (LPS) and phospholipids. The region between the two membranes, know as the periplasm, is largely composed of peptidoglycan that maintains the cell shape and protects its from rupture. Braun's lipoprotein (BLP), a coiled-coiled trimeric protein located within the periplasm, has long been known to covalently bind to the peptidoglycan and to anchor the peptidoglycan to the inner leaflet of the outer membrane. Recently, however, it has been shown that in addition to this structural role, BLP is able to adopt a trans-membrane orientation across the outer membrane.In this present study we have performed a series of molecular dynamics simulations of BLP using both atomistic and coarse-grained approaches. An atomistic model of the peptidoglycan has been developed and incorporated with our complex model of the E. coli outer membrane that contains LPS in the outer leaflet and mixtures of phospholipids in the inner leaflet. Multiple simulations of BLP, including the peptidoglycan bound and free forms, plus BLP in a trans-membrane orientation have been performed. These simulations have enabled us to explore the conformational dynamics, oligomerisation and environmental interactions of BLP in these different orientations.