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

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Abstract

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.

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Shuhua Thong

Department of Chemistry, National University of Singapore, Singapore, Singapore

Competing interests
The authors declare that no competing interests exist.
Bilge Ercan

Department of Chemistry, National University of Singapore, Singapore, Singapore

Competing interests
The authors declare that no competing interests exist.
Federico Torta

Department of Biochemistry, National University of Singapore, Singapore, Singapore

Competing interests
The authors declare that no competing interests exist.
Zhen Yang Fong

Department of Chemistry, National University of Singapore, Singapore, Singapore

Competing interests
The authors declare that no competing interests exist.
Hui Yi Alvina Wong

Department of Chemistry, National University of Singapore, Singapore, Singapore

Competing interests
The authors declare that no competing interests exist.
Markus R Wenk

Department of Biochemistry, National University of Singapore, Singapore, Singapore

Competing interests
The authors declare that no competing interests exist.
Shu-Sin Chng

Department of Chemistry, National University of Singapore, Singapore, Singapore

For correspondence
chmchngs@nus.edu.sg

Competing interests
The authors declare that no competing interests exist.

"This ORCID iD identifies the author of this article:" 0000-0001-5466-7183

Funding

National University of Singapore (Start-up funding)

Shu-Sin Chng

Ministry of Education - Singapore (Academic Research fund Tier 2 (MOE2013-T2-1-148))

Shu-Sin Chng

National Research Foundation Singapore (NRFI2015-05)

Markus R Wenk

Agency for Science, Technology and Research (BMRC-SERC 112 148 0006)

Markus R Wenk

National University of Singapore (Life Science Institute)

Markus R Wenk

Ministry of Education - Singapore (Academic Research Fund Tier 1)

Shu-Sin Chng

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

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