MCE domain proteins: conserved inner membrane lipid-binding proteins required for outer membrane homeostasis

Abstract

Bacterial proteins with MCE domains were first described as being important for Mammalian Cell Entry. More recent evidence suggests they are components of lipid ABC transporters. In Escherichia coli, the singledomain protein MlaD is known to be part of an inner membrane transporter that is important for maintenance of outer membrane lipid asymmetry. Here we describe two multi MCE domain-containing proteins in Escherichia coli, PqiB and YebT, the latter of which is an orthologue of MAM-7 that was previously reported to be an outer membrane protein. We show that all three MCE domain-containing proteins localise to the inner membrane. Bioinformatic analyses revealed that MCE domains are widely distributed across bacterial phyla but multi MCE domain-containing proteins evolved in Proteobacteria from single-domain proteins. Mutants peer-reviewed) is the author/funder. All rights reserved. No reuse allowed without permission.The copyright holder for this preprint (which was not . http://dx.doi.org/10.1101/159053 doi: bioRxiv preprint first posted online Jul. 3, 2017; 2 defective in mlaD, pqiAB and yebST were shown to have distinct but partially overlapping phenotypes, but the primary functions of PqiB and fig. S1). Other multi-MCE domain-containing proteins were detected in Proteobacteria, but at much lower frequencies than type III and IV proteins (Supplementary table S1, Supplementary fig. S1).Some MCE domains were detected in proteins from eukaryotic genomes (Supplementary fig. S2). Type I proteins were found in plant phyla Chlorophyta and Streptophyta. In Arabidopsis they are involved in the trafficking of phosphatidic acid from the outer to the inner membrane of chloroplasts 18 . A small number of MCE proteins were identified in animal genomes. Manual inspection of the DNA sequences encoding these proteins revealed that all but one could be attributed to contamination with bacterial DNA, the exception being in Trichoplax adhaerens, an animal known to have an unusually large mitochondrial genome 22,23 .Protein clustering and evolution of multi-domain proteins. To understand the evolutionary relationships between MCE proteins, protein-protein similarity networks were constructed and coloured by architecture type and phylum (Fig. 2). MCE proteins generally cluster within phyla, suggesting that little or no horizontal transmission of these genes has occurred and variants have arisen through speciation.peer-reviewed) is the author/funder. All rights reserved. No reuse allowed without permission.The (Fig. 3B). In many of these peer-reviewed) is the author/funder. All rights reserved. No reuse allowed without permission.copyright holder for this preprint (which was not . http://dx.doi.org/10.1101/159053 doi: bioRxiv preprint first posted online Jul. 3, 2017The copyright holder for this preprint (which was not . http://dx.doi.org/10.1101/159053 doi: bioRxiv preprint first posted online Jul. 3, 2017; 7 neighbourhoods, for example in Neisseria meningitidis 27 , the outer membrane component MlaA (VacJ) is found in ...

Structure of LetB reveals a tunnel for lipid transport across the bacterial envelope

“…These seven MCE rings that make up LetB stack to form an elongated macromolecular barrel with a single protomer extending the entire length of the barrel. The N-terminal transmembrane helix of each protomer is expected to be anchored in the IM, as previously reported 9 , with the MCE1 ring abutting the IM and the MCE7 ring closest to the OM.…”

“…Disruption of pqiAB results in a moderate growth defect in the presence of the detergent lauryl sulfobetaine (LSB), while the growth of a letAB mutant is unaffected 9 . However, disruption of letAB in combination with pqiAB (pqiAB letAB double knockout) significantly exacerbates LSB sensitivity, and complementation with wild-type letAB from a plasmid relieves this exacerbation, as previously described 9 (Extended Data Fig. 1c).…”

“…1a). E. coli and Salmonella strains lacking letAB exhibit phenotypes consistent with outer membrane defects 9,15 (Extended Data Fig. 1).…”

“…MCE proteins (which contain Mammalian Cell Entry domains) have been shown to interact with IM ABC transporters, or are present in operons with integral membrane proteins that have been proposed to encode membrane transporters 7,9,10 . These multi-subunit MCE systems are broadly conserved in double-membraned bacteria and chloroplasts 11 , and have been implicated in the import and/or export of lipids, cholesterol or other hydrophobic molecules 6,8,12 .…”

“…These multi-subunit MCE systems are broadly conserved in double-membraned bacteria and chloroplasts 11 , and have been implicated in the import and/or export of lipids, cholesterol or other hydrophobic molecules 6,8,12 . Members of this transporter family play an important role in the outer membranes of both Gram-negative bacteria 6,9 and Mycobacteria 13 , and are critical during host infection of various bacterial pathogens 14,15 .…”