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.