title: Characterization of lptA and lptB, two essential genes implicated in lipopolysaccharide transport to the outer membrane of Escherichia coli.
abstract: The outer membrane (OM) of gram-negative bacteria is an asymmetric lipid bilayer that protects the cell from toxic molecules. Lipopolysaccharide (LPS) is an essential component of the OM in most gram-negative bacteria, and its structure and biosynthesis are well known. Nevertheless, the mechanisms of transport and assembly of this molecule in the OM are poorly understood. To date, the only proteins implicated in LPS transport are MsbA, responsible for LPS flipping across the inner membrane, and the Imp/RlpB complex, involved in LPS targeting to the OM. Here, we present evidence that two Escherichia coli essential genes, yhbN and yhbG, now renamed lptA and lptB, respectively, participate in LPS biogenesis. We show that mutants depleted of LptA and/or LptB not only produce an anomalous LPS form, but also are defective in LPS transport to the OM and accumulate de novo-synthesized LPS in a novel membrane fraction of intermediate density between the inner membrane (IM) and the OM. In addition, we show that LptA is located in the periplasm and that expression of the lptA-lptB operon is controlled by the extracytoplasmic sigma factor RpoE. Based on these data, we propose that LptA and LptB are implicated in the transport of LPS from the IM to the OM of E. coli.
authors.affiliation: Dipartimento di Scienze Biomolecolari e Biotecnologie, Università degli Studi di Milano, Milan, Italy.
authors.forename: Paola
authors.initials: P
authors.lastname: Sperandeo
authors.forename: Rachele
authors.initials: R
authors.lastname: Cescutti
authors.forename: Riccardo
authors.initials: R
authors.lastname: Villa
authors.forename: Cristiano
authors.initials: C
authors.lastname: Di Benedetto
authors.forename: Daniela
authors.initials: D
authors.lastname: Candia
authors.forename: Gianni
authors.initials: G
authors.lastname: Dehò
authors.forename: Alessandra
authors.initials: A
authors.lastname: Polissi
mesh_terms: D001425:Bacterial Outer Membrane Proteins / Q000378:metabolism*; D001692:Biological Transport / Q000235:genetics; D004926:Escherichia coli / Q000235:genetics* / Q000378:metabolism*; D029968:Escherichia coli Proteins / Q000378:metabolism*; D005798:Genes, Bacterial / Q000502:physiology*; D008070:Lipopolysaccharides / Q000378:metabolism*; D019897:Periplasm / Q000378:metabolism
chemical_list: D001425:Bacterial Outer Membrane Proteins; D029968:Escherichia coli Proteins; D008070:Lipopolysaccharides