Characterization of lptA and lptB, Two Essential Genes Implicated in Lipopolysaccharide Transport to the Outer Membrane of Escherichia coli

Dipartimento di Scienze Biomolecolari e Biotecnologie, Università degli Studi di Milano, Milan, Italy.
Journal of Bacteriology (Impact Factor: 2.81). 02/2007; 189(1):244-53. DOI: 10.1128/JB.01126-06
Source: PubMed


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.

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    • "The LptC anchors to the IM through an N-terminal transmembrane helix to form a complex with LptBFG and its periplasm domain was found to be the essential function region [8]. However, its function is not well understood within the complex LptBFGC [5] [9] [10] [11]. LptA was proposed to act as a periplasmic chaperone for LPS transport across the periplasm and reported to interact with IM protein LptC [12] [13] and OM protein LptD [13]. "
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    • "Gene MAE 35490 is 99% similar to a gene in two other strains of Microcystis (strain PCC 9701 and T1-4, E-value = 2e−104 each) and has been annotated as having similarity to an ATPase in Trichodesmium erythraeum (strain IMS101). Gene MAE 51260 is most similar to a putative lipopolysaccharide transport protein in other strains of Microcystis (E-value <3e−170) which may be involved in transporting carbohydrates, organic alcohols, and acids [101]. Collectively, the higher transcript levels of these genes in the absence of high levels of inorganic N and/or P suggest they may assist in transporting and degrading organic compounds. "
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    • "Once LPS is extracted from the IM it is proposed that LptC transfers LPS to the periplasmic protein LptA which facilitates transfer of LPS to the OM assembly site LptD/E (Sperandeo et al., 2007; Tran et al., 2008). LptA is an essential soluble periplasmic protein that has been implicated in LPS transport (Sperandeo et al., 2007, 2008). LptA interacts specifically with the lipid A region of LPS and relieves LptC of LPS in vitro (Tran et al., 2008, 2010) supporting the role in LPS transport from LptC in the LptB/C/F/G transporter in the IM to the OM assembly site LptD/E. "
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