9.A.69 The Intermembrane Phospholipid Translocase (IMPL-T) Family
The outer membranes of gram-negative bacteria are asymmetric bilayers in which lipopolysaccharides (LPSs) and phospholipids are localized in the outer- and inner-leaflet, respectively. This asymmetry is important for membrane integrity. In Escherichia coli, the Mla transport pathway (TC# 3.A.1.27.3) maintains this asymmetry by removing phospholipids from the outer-leaflet (Malinverni and Silhavy 2009). The MlaD component of this system is a mammalian cell entry (MCE) domain protein, and E. coli has two other MCE domain proteins of unknown function (PqiB and YebT). Nakayama and Zhang-Akiyama 2016 showed that these two proteins are components of novel transport pathways that contribute to membrane integrity. The pqiAB operon is regulated by SoxS and RpoS. The yebST operon contains pqiAB homologous genes. Nakayama and Zhang-Akiyama 2016 found a third member of the pqi operon, ymbA (pqiC). A PqiB-PqiC complex bridges the inner- and the outer-membrane, and in other bacteria, pqiBC genes are located in operons together with transporter proteins. Simultaneous deletion of the pqiABC and yebST operons in a Δmla background rendered cells more sensitive to SDS/EDTA, and the SDS/EDTA sensitivity of mla mutants was rescued by additional copies of pqiABC. The yebST operon was induced by a defect in LPS molecules. Thus, PqiABC and YebST are novel transport pathways related to the Mla transport pathway and important for membrane integrity. Nakayama and Zhang-Akiyama 2016 thus proposed that the stress-inducible pqiABC and yebST operons encode transport pathway proteins related to the Mla transport pathway.
E. coli LetAB, a phospholipid transporter is involved in outer membrane integrity. LetA adopts a distinct architecture that is structurally and evolutionarily unrelated to known transporter families. LetA functions as a pump at one end of a ~225 Å long tunnel formed by its binding partner, MCE protein LetB, creating a pathway for lipid transport between the inner and outer membranes (Santarossa et al. 2025). Unexpectedly, the LetA transmembrane domains adopt a fold that is evolutionarily related to the eukaryotic tetraspanin family of membrane proteins, including TARPs and claudins. LetA has no detectable homology to known transport proteins, and defines a new class of membrane transporters. Through a combination of deep mutational scanning, molecular dynamics simulations, AlphaFold-predicted alternative states, and functional studies, Santarossa et al. 2025 presented a model for how the LetA-like family of membrane transporters may use energy from the proton-motive force to drive the transport of lipids across the bacterial cell envelope.
References:
The Paraquot-inducible protein (PqiABC) system of intermembrane phospholipid translocation (Nakayama and Zhang-Akiyama 2016). PqiA has 417 aas and 8 TMSs in the inner membrane; PqiB has 546 aas with several repeats that are stacked on each other to form a trans-periplasmic channel for phospholipid transport, and PqiC (ymbA) has 187 aas and possibly one N-terminal TMS is located in or on the outer membrane (Nakayama and Zhang-Akiyama 2016).
PqiABC of E. coli
The YebST system (renamed the LetAB system) of intermembrane phospholipid translocation (Nakayama and Zhang-Akiyama 2016). LetA is a 427 aa protein with 8 TMSs in the cytoplasmic membrane while LetB is an 877 aa protein; it spans the periplasm and has an N-terminal TMS followed by seven repeats that are stacked on top of each other to form a channel across the periplasm. The first repeat accepts the phospholipid from LetA while the seventh repeat interacts with the outer membrane where it delivers the phospholipid (Isom et al. 2020). Santarossa et al. 2025 determined the structure of E. coli LetAB, a phospholipid transporter involved in outer membrane integrity, and found that LetA adopts a distinct architecture that is structurally and evolutionarily unrelated to known transporter families. LetA functions as a pump at one end of a ~225 Å long tunnel formed by its binding partner, MCE protein LetB, creating a pathway for lipid transport between the inner and outer membranes. Unexpectedly, the LetA transmembrane domains adopt a fold that is evolutionarily related to the eukaryotic tetraspanin family of membrane proteins, including TARPs and claudins. LetA has no detectable homology to known transport proteins, and defines a new class of membrane transporters. Through a combination of deep mutational scanning, molecular dynamics simulations, AlphaFold-predicted alternative states, and functional studies, Santarossa et al. 2025 presented a model for how the LetA-like family of membrane transporters may use energy from the proton-motive force to drive the transport of lipids across the bacterial cell envelope.
LetAB (YebST) of E. coli
YebT of 899 aas with possibly 1 N-terminal TMS plus 8 C-terminal TMSs of lower hydropphobicity. However, TC BLAST against 9.A.69.1.2 indicates that there are four repeats, each of about 220 aas in length.
YebT of Desulfomarina profundi
Mammalian cell entry related domain protein, MCERD, of 890 aas and 1 N-terminal TMS. This and other members of this family show a low degree of sequence similarity with the cholesterol-dependent cytolysin, inerolysin, of 519 aas (TC# 1.C.12.1.17). The latter has 1 N-terminal TMS.
MCERD of Psychromonas ingrahamii