Structure of bacterial phospholipid transporter MlaFEDB with substrate bound.

Cryo-EM establishes MlaB as an assembled MlaFEDB subunit alongside MlaF, MlaE, and MlaD.

"The IM complex, MlaFEDB, consists of four different proteins: MlaD, a membrane anchored protein from the MCE (Mammalian Cell Entry) protein family, which forms a homohexameric ring in the periplasm (Thong et al., 2016; Ekiert et al., 2017); MlaE (also called YrbE), a predicted integral inner membrane ABC permease; MlaF, an ABC ATPase; and MlaB, a STAS (Sulfate Transporter and Anti-Sigma factor antagonist) domain protein with possible regulatory function (Kolich et al., 2020)."

file:PSEPK/ttg2E/ttg2E-uniprot.txt

UniProt record for Pseudomonas putida KT2440 Ttg2E (Q88P90)

file:projects/P_PUTIDA/deep-research/PSEPK__bacterial_mla_intermembrane_phospholipid_transport__ppu02010-deep-research-openscientist.md

OpenScientist PSEPK Mla pathway satisfiability research

PP_0962 is the single-copy K07122 MlaB assignment and lies directly after mlaF-mlaE-mlaD-ttg2D, strongly supporting the MlaB-equivalent identity while leaving its exact target-specific regulatory mechanism untested.

file:PSEPK/ttg2E/ttg2E-deep-research-openscientist.md

OpenScientist functional research for PSEPK Ttg2E/Q88P90

Q88P90 is a small single-STAS-domain protein without a signal peptide or transmembrane helix.

"a 100-residue protein consisting solely of a STAS domain is the diagnostic architecture of **MlaB**"
Operon context strongly supports Ttg2E as the MlaB-equivalent accessory subunit.

"This gene order (F‑E‑D‑C‑B) is identical to the canonical *E. coli* `mlaFEDCB` operon, unambiguously assigning **ttg2E = MlaB**."
Direct experiments on orthologs support assembly and activity roles but do not establish the precise KT2440 mechanism.

"MlaB plays critical roles in both the assembly and activity of the transporter"