Lipid Transport Across Bacterial Membranes

Abstract

The movement of lipids within and between membranes in bacteria is essential for building and maintaining the bacterial cell envelope. Moving lipids to their final destination is often energetically unfavorable and does not readily occur spontaneously. Bacteria have evolved several protein-mediated transport systems that bind specific lipid substrates and catalyze the transport of lipids across membranes and from one membrane to another. Specific protein flippases act in translocating lipids across the plasma membrane, overcoming the obstacle of moving relatively large and chemically diverse lipids between leaflets of the bilayer. Active transporters found in double-membraned bacteria have evolved sophisticated mechanisms to traffic lipids between the two membranes, including assembling to form large, multiprotein complexes that resemble bridges, shuttles, and tunnels, shielding lipids from the hydrophilic environment of the periplasm during transport. In this review, we explore our current understanding of the mechanisms thought to drive bacterial lipid transport.

Keywords: bacteria; cell envelope; lipids; lipopolysaccharide; membranes; transport.

Figure 1

Architectures of the bacterial cell envelope. (a) Gram-positive and (b) gram-negative cell envelopes are depicted, highlighting the lipid composition of each. (c) Cartoon representation of lipids, corresponding to components of panels a and b, with examples of chemical structures of each lipid shown.

Figure 2

Common mechanisms of active transport. Transporters are depicted in gray, and red balls indicate a generic substrate in all parts of the figure. (a) Ion-driven alternating access mechanism. After substrate binding to the inward-facing state of the protein, conformational changes result in a transition to an outward-facing state. In the outward-facing state, the substrate is released and a counterion, indicated here by a cation, binds to the transporter and results in a conformational change back to an inward-facing state, resetting the transporter. (b) ATP-driven alternating access mechanism. After substrate binding to the inward-facing state of the protein, ATP binds and is hydrolyzed. ATP binding drives conformational change to the outward-facing state, transferring the substrate to the outer leaflet of the membrane or to the extracellular or periplasmic space (Holland & Blight 1999). (c) ATP-driven extrusion model. After substrate binding to the outward-open state of the transporter, ATP binding triggers a conformational change in the system that results in a collapsed state, in which the substrate-binding pocket closes and the substrate is extruded out of the transporter. (d) Putative flipping and pumping mechanism. After substrate binding to the closed-channel state of the protein, ATP binding and hydrolysis trigger conformational changes that allow the processive movement of a longer substrate (red dashed line) through an open channel in the transporter. Multiple cycles of ATP hydrolysis are thought to be necessary for the complete translocation of the substrate. (e) Gated-channel mechanism. In the resting state, the protein contains cytoplasm-facing and extracellular-facing cavities, separated by a gate. After substrate binding to the cytoplasm-facing cavity, the gate opens, allowing passage of the substrate to the extracellular-facing cavity and translocation into the outer leaflet.

Figure 3

Transporters responsible for the translocation of substrates across the plasma membrane. (a) Flippase MurJ in an inward-facing state (PDB 6NC7), responsible for flipping Lipid II from the inner to the outer leaflet of the plasma membrane. (b) Flippase TarGH in an inward-facing state (PDB 6JBH;), responsible for flipping the nascent Und-PP-WTA moiety from the inner to the outer leaflet of the plasma membrane. (c) Flippase LtaA in an outward-facing state (PDB 6S7V), responsible for flipping the LTA precursor, gentiobiosyl-diacylglycerol, from the inner to the outer leaflet of the plasma membrane. (d) Flippase MprF (PDB 7DUW), responsible for flipping aminoacyl-PG from the inner to the outer leaflet of the plasma membrane. Abbreviations: aminoacyl-PG, aminoacyl-phosphatidylglycerol; LTA, lipoteichoic acid; Und-PP, undecaprenyl diphosphate; WTA, wall teichoic acid.

Figure 4

Transporters responsible for lipopolysaccharide (LPS) transport across the cell envelope. (a) Flippase MsbA in an LPS-bound, inward state [PDB (Protein Data Bank) 5TV4], responsible for flipping the LPS core moiety from the inner leaflet to the outer leaflet of the plasma membrane. (b) ATP-binding cassette transporter WzmWzt in an ATP-bound, closed resting state (PDB 7K2T), responsible for exporting lipid-anchored O-antigen polysaccharide from the inner to the outer leaflet of the plasma membrane. The O-antigen can then be coupled to the LPS core by the enzyme WaaL. (c) The Lpt system, which transports LPS from the plasma membrane to the outer leaflet of the outer membrane. LptBFGC (PDB 6MJP), LptA (PDB 2R19), and LptDE (PDB 5IV9) proteins are shown; the question mark indicates the unknown number of LptA subunits.

Figure 5

Transporters involved in phospholipid and other lipid transport. (a) The Mla system, which is responsible for phospholipid transport across the gram-negative cell envelope. The MlaFEDB complex (PDB 6XBD), MlaC (PDB 5UWA), and MlaA-OmpF (PDB 5NUO) are shown. Bidirectional arrows between the plasma membrane and OM components and MlaC indicate uncertainty in the direction of transport. (b) The Let system, which is proposed to transport lipids or other substrates across the cell envelope. A single monomer of the homohexameric LetB (PDB 6V0C) is highlighted in the darker purple. LetA is shown as a gray circle, as the structure is currently unknown. Question marks indicate uncertainty in substrate and directionality. (c) The Pqi system, which is proposed to transport lipids or other substrates across the cell envelope. A single monomer of the homohexameric PqiB (PDB 5UVN) [data from Ekiert et. al (2017)] is shown in the darker red. PqiA is shown as a gray circle, as the structure is currently unknown. Question marks indicate uncertainty in substrate and directionality. (d) YhdP is proposed to mediate passive diffusion of phospholipids in bulk across the cell envelope. Although no structures are available for the protein, it is thought to be anchored in the plasma membrane and span across the periplasm. Question marks indicate uncertainty in substrate and directionality. (e) HpnN (PDB 5KHN), which is responsible for trafficking hopanoids from the outer leaflet of the plasma membrane to the periplasm. (f) FadL (PDB 1T16) is responsible for importing LCFAs from the extracellular space across the OM to the periplasm. FadD (PDB 3G7S) is responsible for activating the fatty acid to prevent its diffusion out of the cell. Abbreviations: Hpn, hopanoid; LCFA, long-chain fatty acid; Let, lipophilic envelope-spanning tunnel; Mla, maintenance of lipid asymmetry; OM, outer membrane; Omp, outer membrane protein; PDB, Protein Data Bank; Pqi, paraquat inducible.