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LPS-export porin (organic solvent tolerance protein, OstA)
OstA of Neisseria meningitidis (NP_273336)

LPS export porin complex, LptBCFG-A-DE, consists of LptD (Omp; OmpA; 784 aas)-LptE (RlpB; 193 aas; O.M. lipoprotein)-LptA (KdsD; YhbN; OstA small; 185 aas periplasmic chaparone protein)-LptB (KdsC; YhbG; 241 aas cytoplasmic ABC-type ATPase)-LptC (YrbK, 199aas;1 N-terminal TMS)- LptFG, part of the ABC transporter. LptDE (1:1 stoichiometry) comprise a two-protein β-barrel-lipoprotein complex in the outer membrane that assembles and exports LPS (Chng et al., 2010).  After LPS (or a precursor) is transported across the inner membrane by MsbA (3.A.1.106.1), this seven component system translocates LPS from the outer surface of the inner membrane to the outer surface of the outer membrane using ATP hydrolysis to sequentially energize transfer from one binding site to another in several steps (Freinkman et al. 2012; Okuda et al. 2012; Sherman et al. 2014).  LPS interacts with LptC and LptA sequentially before being passed to the LptD outer membrane porin, anchored by the LptE lipoprotein on the inner surface of the outer membrane.  LptF and LptG are the transmembrane consituents of the ABC pump, and LptB is the ATPase of an ABC-like system that energizes the transport using several ATP molecules (Okuda et al. 2012; Sherman et al. 2014).  LptC interconnects the LptBFG ABC system with the periplasmic LptA protein via its large periplasmic domain (Villa et al. 2013).  LptDE form a complex in the outer membrane which inserts LPS into this membrane.  The 3-D strcture of the complex shows that the LptE lipoprotein inserts into the 26 stranded barrel of LptD as a plug.  The first two strands of LptD contain prolines and are therefore distorted, possibly creating a portal for lateral diffusion of LPS into the outer leaflet of the outer membrane (Qiao et al. 2014).  The 3-d structure of the Pseudomonas aeruginosa LptA, LptH, has been solved at 2:75 Å resolution revealing a β-jellyroll fold similar to that in LptD (Bollati et al. 2015). Direct interaction of LptB and LptC has been demonstrated (Martorana et al. 2016). A specific binding site in the LptB ATPase for the coupling helices of the transmembrane LptFG complex is responsible for coupling ATP hydrolysis by LptB with LptFG function to achieve LPS extraction (Simpson et al. 2016). After biosynthesis, bacterial lipopolysaccharides (LPS) are transiently anchored to the outer leaflet of the inner membrane (IM). The ABC transporter LptB2FG extracts LPSs from the IM and transports them to the outer membrane. Luo et al. 2017 reported the crystal structure of nucleotide-free LptB2FG from P. aeruginosa. It shows that LPS transport proteins LptF and LptG each contain a TM domain (TMD), a periplasmic beta-jellyroll-like domain and a coupling helix that interacts with LptB on the cytoplasmic side. The LptF and LptG TMDs form a large outward-facing V-shaped cavity in the IM. Mutational analyses suggested that LPS may enter the central cavity laterally, via the interface of the TMD domains of LptF and LptG, and is expelled into the beta-jellyroll-like domains upon ATP binding and hydrolysis by LptB. These studies suggest a mechanism for LPS extraction by LptB2FG that is distinct from those of classical ABC transporters that transport substrates across the IM (Luo et al. 2017). Transport involves a stable association between the inner (LptBFG) and outer  (LptDE) membrane components, supporting a mechanism in which lipopolysaccharide molecules are pushed one after the other across a protein bridge (LptCA) that connects the inner and outer membranes (Sherman et al. 2018).  The ABC transporter, LptB2FG, which tightly associates with LptC, extracts lipopolysaccharide out of the inner membrane.Li et al. 2019 characterized the structures of LptB2FG and LptB2FGC in nucleotide-free and vanadate-trapped states, using single-particle cryo-electron microscopy. These structures resolve the bound lipopolysaccharide, reveal transporter-lipopolysaccharide interactions with side-chain details and uncover how the capture and extrusion of lipopolysaccharide are coupled to conformational rearrangements of LptB2FGC. LptC inserts its TMS between the two transmembrane domains of LptB2FG, which represents a previously unknown regulatory mechanism for ABC transporters. These results suggest a role for LptC in achieving efficient lipopolysaccharide transport, by coordinating the action of LptB2FG in the inner membrane and Lpt protein interactions in the periplasm (Li et al. 2019). cryo-EM structures of LptB2FG alone and complexed with LptC are known, revealing conformational changes between these states. Two functional transmembrane arginine-containing loops interact with bound AMP-PNP which induces an inward rotation and shift of the transmembrane helices of LptFG and LptC to tighten the cavity, with the closure of two lateral gates, to eventually expel LPS into the bridge (Tang et al. 2019). The ABC transporter, LptB2FGC extracts LPS from the inner membrane and places it onto a periplasmic protein bridge. Lundstedt et al. 2020 showed that residue E86 of LptB is essential for coupling the function of this ATPase to that of its membrane partners, LptFG, at the step where ATP binding drives the closure of the LptB dimer and the collapse of the LPS-binding cavity in LptFG that moves LPS to the Lpt periplasmic bridge consisting of LptC, A and D (from inside to out) and then to the outer membrane insertase, LptE. Defects caused by changing residue E86 are suppressed by mutations altering either the LPS structure or TMSs in LptG. These suppressors fix defects in the coupling helix of LptF, but not of LptG. These observations support a transport mechanism in which the ATP-driven movements of LptB and those of the substrate-binding cavity in LptFG are bi-directionally coordinated through the rigid-body coupling, with LptF's coupling helix being important in coordinating cavity collapse with LptB dimerization (Lundstedt et al. 2020). The TMS of LptC participates in LPS extraction by the LptB2 FGC transporter (Wilson and Ruiz 2022). A small molecule, IMB-0042, inhibits the interaction of LPS transporter proteins, LptA and LptC. This give rise to filament morphology, impaired OM integrity, and an accumulation of LPS in the periplasm (Dai et al. 2022). Macrocyclic peptide (MCP) antibiotics have potent antibacterial activity and represent a new class of antibiotics (Zampaloni et al. 2024), and LptB2FGC is target. Pahil et al. 2024 showed that novel antibiotics trap a substrate-bound conformation of the LPS transporter that stalls this machine. The inhibitors accomplish this by recognizing a composite binding site made up of both the Lpt transporter and its LPS substrate. The identity of an unusual mechanism of lipid transport inhibition reveals a druggable conformation of the Lpt transporter and provides the foundation for extending this class of antibiotics to other Gram-negative pathogens (Pahil et al. 2024).  Residues within the LptC transmembrane helix are critical for E. coli LptB(2) FG ATPase (Cina et al. 2024).

LptA-G of E. coli:
LptA (YhbN; OstA(s)) (P0ADV1)
LptB (YhbG; ATPase) (P0A9V1)
LptC (YrbG) (P0ADW0)
LptD (OstA; Imp) (P31554)
LptE (RlpB) (P0ADC1)
LptF (YjgP) (P0AF98)
LptG (YjgQ) (P0ADC6)

OstA homologue (Bhat et al. 2011).

OstA homologue of Myxococcus xanthus

OstA of 842 aas

OstA of Rhodopseudomonas palustris

OstA of 753 aas

OstA of Helicobacter pylori

OstA 0f 680 aas

OstA of Hydrogenobaculum sp.

OstA of 880 aas

OstA of Chlorobium luteolum

OstA of 894 aas

OstA of Nonlabens dokdonensis

OstA of 833 aas

OstA of Methylacidiphilum infernorum

OstA homologue of 991 aas

OstA homologue of Leptospira interrogans

OstA of 975 aas

OstA of Brachyspira hyodysenteriae

OstA of 537 aas

OstA of Halobacteroides halobius

Putative LptF-LptG-LptD (OstA) fusion protein of 1040 aas (may be an artifact due to a sequencing error, and may also be a contaminant, accounting for its occurance in a Firmicute. However, it shows an N-terminal domain resembling ABC membrane proteins (3.A.1.152) and a hydrophilic C-terminal domain resembling members of porin family 1.B.42. NCBI BLAST results show that there are several homologues of the same "fused" protein in several species of Halothermothrix, Halanaerobium and Candidatus Frackibacter.

OstA of Halothermothrix orenii

LPS assembly protein, LptD, of 691 aas and possibly two TMSs, N- and C-terminal.

LptD of Thermoanaerobaculales bacterium (marine sediment metagenome)

Permease [Mesotoga sp. SC_NapDC of 1443 aas and possibly 8 TMSs, 7 at the N-terminus in a 3 + 4 TMS arrangement plus possibly 1 TMS at the C-terminus. It has the ABC-type membrane protein domain at the N-terminus followed by other hydrophilic domains, possibly involved in LPS transport from the inner membrane to the outermembrane including an LptD domain. 

Permease of Mesotoga sp. SC_NapDC

LPS-assembly protein, LptD, of 799 aas and 1 N-terminal TMS.

LptD of Bacteriovorax sp. (wastewater metagenome)

OstA homologue of 1069 aa

OstA of Treponema denticola