title: The Lack of the Essential LptC Protein in the Trans-Envelope Lipopolysaccharide Transport Machine Is Circumvented by Suppressor Mutations in LptF, an Inner Membrane Component of the Escherichia coli Transporter.
abstract: The lipopolysaccharide (LPS) transport (Lpt) system is responsible for transferring LPS from the periplasmic surface of the inner membrane (IM) to the outer leaflet of the outer membrane (OM), where it plays a crucial role in OM selective permeability. In E. coli seven essential proteins are assembled in an Lpt trans-envelope complex, which is conserved in γ-Proteobacteria. LptBFG constitute the IM ABC transporter, LptDE form the OM translocon for final LPS delivery, whereas LptC, an IM-anchored protein with a periplasmic domain, interacts with the IM ABC transporter, the periplasmic protein LptA, and LPS. Although essential, LptC can tolerate several mutations and its role in LPS transport is unclear. To get insights into the functional role of LptC in the Lpt machine we searched for viable mutants lacking LptC by applying a strong double selection for lptC deletion mutants. Genome sequencing of viable ΔlptC mutants revealed single amino acid substitutions at a unique position in the predicted large periplasmic domain of the IM component LptF (LptFSupC). In complementation tests, lptFSupC mutants suppress lethality of both ΔlptC and lptC conditional expression mutants. Our data show that mutations in a specific residue of the predicted LptF periplasmic domain can compensate the lack of the essential protein LptC, implicate such LptF domain in the formation of the periplasmic bridge between the IM and OM complexes, and suggest that LptC may have evolved to improve the performance of an ancestral six-component Lpt machine.
authors.affiliation: Dipartimento di Bioscienze, Università degli Studi di Milano, Milan, Italy.
authors.forename: Mattia
authors.initials: M
authors.lastname: Benedet
authors.affiliation: Dipartimento di Bioscienze, Università degli Studi di Milano, Milan, Italy.
authors.forename: Federica A
authors.initials: FA
authors.lastname: Falchi
authors.affiliation: Scuola di Dottorato in Medicina Molecolare e Traslazionale, Università degli Studi di Milano, Segrate, Italy.|Istituto di Tecnologie Biomediche, Consiglio Nazionale delle Ricerche, Milan, Italy.
authors.forename: Simone
authors.initials: S
authors.lastname: Puccio
authors.affiliation: Dipartimento di Bioscienze, Università degli Studi di Milano, Milan, Italy.
authors.forename: Cristiano
authors.initials: C
authors.lastname: Di Benedetto
authors.affiliation: Istituto di Tecnologie Biomediche, Consiglio Nazionale delle Ricerche, Milan, Italy.
authors.forename: Clelia
authors.initials: C
authors.lastname: Peano
authors.affiliation: Dipartimento di Biotecnologie e Bioscienze, Università degli Studi di Milano-Bicocca, Milan, Italy.
authors.forename: Alessandra
authors.initials: A
authors.lastname: Polissi
authors.affiliation: Dipartimento di Bioscienze, Università degli Studi di Milano, Milan, Italy.
authors.forename: Gianni
authors.identifier: http://orcid.org/0000-0001-5492-7766
authors.initials: G
authors.lastname: Dehò
mesh_terms: D018528:ATP-Binding Cassette Transporters / Q000235:genetics* / Q000378:metabolism*; D019943:Amino Acid Substitution; D001692:Biological Transport; D002462:Cell Membrane / Q000378:metabolism*; D004926:Escherichia coli / Q000166:cytology* / Q000235:genetics / Q000378:metabolism; D029968:Escherichia coli Proteins / Q000235:genetics* / Q000378:metabolism*; D008070:Lipopolysaccharides / Q000378:metabolism*; D008565:Membrane Proteins / Q000172:deficiency*; D010641:Phenotype; D013489:Suppression, Genetic
chemical_list: D018528:ATP-Binding Cassette Transporters; D029968:Escherichia coli Proteins; D008070:Lipopolysaccharides; C543318:LptC protein, E coli; C529275:LptF protein, E coli; D008565:Membrane Proteins