Molecular Simulations of Bacterial Lipoprotein Biogenesis

Abstract

Lipoproteins perform critical roles in bacterial physiology, pathogenicity, and antibiotic resistance. Their roles include modulation of the cell envelope structure, signal transduction and transport. Lipoproteins are processed by a pathway of membrane proteins - Sec, Lgt, LspA, Lnt and Lol - which insert, cleave and transport the protein substrate, while affixing lipid moieties to an absolutely conserved cysteine and therefore permitting their tethering to the cell envelope.The recent determination of the three-dimensional protein structures of Lgt and LspA have enlivened lipoprotein research. Furthermore, the latest structures of the Sec translocon with associated signal peptide provide a means to study the initiation point of this pathway and mechanism by which the lipoproteins are inserted into the cell membrane. Structural studies of mature lipoproteins have also recently come to the fore, including the BAM complex, of which four of the five proteins are lipoproteins, and LptDE, an outer membrane barrel with a lipoprotein plug. Both complexes are essential to gram-negative bacteria; respectively, folding the protein-conduits that enable nutrient transport through the outer membrane and assembling the outermost bacterial fortifications.Here we have used a range of molecular simulation, modelling and bioinformatics methods to study this pathway. With initial focus on the first two enzymes of the pathway, Lgt and LspA, our studies have elucidated the mode of binding of signal peptides to both enzymes and interpreted from this the molecular mechanisms involved in the enzymatic reactions. These studies highlight key roles for the most highly conserved residues, whilst also providing a means to inhibit the enzymes, as illustrated by the antibiotic, globomycin, bound to LspA.We have also developed molecular parameters for the cysteine lipid-moeities, and applied these post-translational modifications to simulate the dynamics of both individual lipoproteins and the lipoprotein complexes of BAM and LptDE within the bacterial outer membrane.