Defining key roles for auxiliary proteins in an ABC transporter that maintains bacterial outer membrane lipid asymmetry

Elife. 2016 Aug 16;5:e19042. doi: 10.7554/eLife.19042.


In Gram-negative bacteria, lipid asymmetry is critical for the function of the outer membrane (OM) as a selective permeability barrier, but how it is established and maintained is poorly understood. Here, we characterize a non-canonical ATP-binding cassette (ABC) transporter in Escherichia coli that provides energy for maintaining OM lipid asymmetry via the transport of aberrantly localized phospholipids (PLs) from the OM to the inner membrane (IM). We establish that the transporter comprises canonical components, MlaF and MlaE, and auxiliary proteins, MlaD and MlaB, of previously unknown functions. We further demonstrate that MlaD forms extremely stable hexamers within the complex, functions in substrate binding with strong affinity for PLs, and modulates ATP hydrolytic activity. In addition, MlaB plays critical roles in both the assembly and activity of the transporter. Our work provides mechanistic insights into how the MlaFEDB complex participates in ensuring active retrograde PL transport to maintain OM lipid asymmetry.

Keywords: E. coli; biochemistry; infectious disease; intermembrane lipid transport; mammalian cell entry (MCE) domain; microbiology; phospholipid binding; protein complex assembly; sulfate transport and anti-sigma factor antagonist (STAS) domain.

MeSH terms

  • ATP-Binding Cassette Transporters / metabolism*
  • Escherichia coli / enzymology*
  • Escherichia coli / metabolism*
  • Escherichia coli Proteins / metabolism*
  • Membrane Lipids / metabolism*
  • Membrane Proteins / metabolism*
  • Phospholipids / metabolism*


  • ATP-Binding Cassette Transporters
  • Escherichia coli Proteins
  • Membrane Lipids
  • Membrane Proteins
  • MlaB protein, E coli
  • MlaD protein, E coli
  • Phospholipids

Grants and funding

The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.