title: Defining key roles for auxiliary proteins in an ABC transporter that maintains bacterial outer membrane lipid asymmetry.
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.
authors.affiliation: Department of Chemistry, National University of Singapore, Singapore, Singapore.
authors.forename: Shuhua
authors.initials: S
authors.lastname: Thong
authors.affiliation: Department of Chemistry, National University of Singapore, Singapore, Singapore.
authors.forename: Bilge
authors.initials: B
authors.lastname: Ercan
authors.affiliation: Department of Biochemistry, National University of Singapore, Singapore, Singapore.
authors.forename: Federico
authors.initials: F
authors.lastname: Torta
authors.affiliation: Department of Chemistry, National University of Singapore, Singapore, Singapore.
authors.forename: Zhen Yang
authors.initials: ZY
authors.lastname: Fong
authors.affiliation: Department of Chemistry, National University of Singapore, Singapore, Singapore.
authors.forename: Hui Yi Alvina
authors.initials: HY
authors.lastname: Wong
authors.affiliation: Department of Biochemistry, National University of Singapore, Singapore, Singapore.
authors.forename: Markus R
authors.initials: MR
authors.lastname: Wenk
authors.affiliation: Department of Chemistry, National University of Singapore, Singapore, Singapore.|Singapore Center on Environmental Life Sciences Engineering, National University of Singapore, Singapore, Singapore.
authors.forename: Shu-Sin
authors.identifier: 0000-0001-5466-7183
authors.initials: SS
authors.lastname: Chng
mesh_terms: D018528:ATP-Binding Cassette Transporters / Q000378:metabolism*; D004926:Escherichia coli / Q000201:enzymology* / Q000378:metabolism*; D029968:Escherichia coli Proteins / Q000378:metabolism*; D008563:Membrane Lipids / Q000378:metabolism*; D008565:Membrane Proteins / Q000378:metabolism*; D010743:Phospholipids / Q000378:metabolism*
chemical_list: D018528:ATP-Binding Cassette Transporters; D029968:Escherichia coli Proteins; D008563:Membrane Lipids; D008565:Membrane Proteins; C000624279:MlaB protein, E coli; C000620815:MlaD protein, E coli; D010743:Phospholipids
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