ttg2E (PP_0962; Q88P90) encodes the MlaB ortholog — a small (100-residue) cytoplasmic STAS-domain protein that is the auxiliary regulatory/assembly subunit of the inner-membrane ABC transporter MlaFEDB (the "Ttg2" system) in Pseudomonas putida KT2440. It is not itself an enzyme or a transporter; instead it binds the MlaF ATPase on the cytoplasmic face of the inner membrane, where it is required for both the correct assembly and the ATP-hydrolytic activity of the transporter. The MlaFEDB complex powers retrograde transport of mislocalized glycerophospholipids from the outer membrane (OM) back to the inner membrane (IM), thereby maintaining OM lipid asymmetry. This membrane-barrier function is the molecular basis for the historically observed "toluene-tolerance" phenotype and for low-level intrinsic resistance to lipid-permeant antibiotics.
The verification requested in the task was performed and passed:
ttg2E and is annotated "Toluene-tolerance protein." The "ttg" (toluene-tolerance genes) label originates from a transposon screen in P. putida that first identified ttg loci required to survive solvent exposure (Kim et al. 1998, PMID 9658016), in which Ttg1/Ttg2 were identified as ABC-transporter homologs. The label is a phenotype-based historical name, not a description of the biochemical activity.STAS_2; InterPro IPR002645 / IPR036513; SUPFAM SSF52091 SpoIIaa-like; Gene3D 3.30.750.24). A ~100-aa protein consisting solely of a STAS domain is the diagnostic architecture of MlaB.| Locus | Gene | Len (aa) | Pfam | Mla component |
|---|---|---|---|---|
| PP_0958 | mlaF | 269 | PF00005 (ABC ATPase) | MlaF (ATPase / NBD) |
| PP_0959 | mlaE | 266 | PF02405 | MlaE (permease / TMD) |
| PP_0960 | mlaD | 161 | PF02470 (MCE) | MlaD (substrate-binding) |
| PP_0961 | ttg2D | 215 | PF05494 | MlaC (periplasmic shuttle) |
| PP_0962 | ttg2E | 100 | PF13466 (STAS) | MlaB (regulatory subunit) |
This gene order (F‑E‑D‑C‑B) is identical to the canonical E. coli mlaFEDCB operon, unambiguously assigning ttg2E = MlaB. The "Ttg2 = Mla" equivalence is independently confirmed by the demonstration that P. aeruginosa Ttg2D is the periplasmic phospholipid-binding (MlaC-type) component of this ABC system (Yero et al. 2021, PMID 33837253).
No conflicting-gene ambiguity was encountered. All lines of evidence converge on a single, well-characterized system.
MlaB is a cytoplasmic STAS (Sulphate Transporter and Anti-Sigma-factor antagonist) domain protein. Direct analysis of the P. putida sequence (Q88P90) shows a 100-residue protein consisting of a single STAS domain (residues 13–100) with no signal peptide and no transmembrane segment (longest hydrophobic stretch = 6 residues, well below the ~15–20 needed for a membrane helix) — i.e. a small, soluble, cytoplasmic protein. In the assembled transporter it clamps onto the nucleotide-binding (ATPase) domain MlaF (typically two MlaB copies per MlaFEDB complex). Functional dissection of the homologous E. coli complex showed that the transporter is built from canonical components MlaF and MlaE plus auxiliary proteins MlaD and MlaB, and that "MlaB plays critical roles in both the assembly and activity of the transporter" (Thong et al. 2016, PMID 27529189). A subsequent explosion of cryo-EM structures of MlaFEDB (apo, phospholipid-, ADP- and AMP-PNP-bound; 3.3–4.1 Å) resolves MlaB as an integral subunit of the complex, and mutagenesis mapped residues that "regulate the activity and structural stability of the MlaFEDB complex" (Tang et al. 2021, PMID 33199922; reviewed in Ekiert et al. 2022, PMID 35981415). Thus the precise role of ttg2E is:
It has no substrate specificity of its own (it is not a catalytic or transport subunit); its "substrate" is effectively its protein partner MlaF.
The MlaFEDB transporter is the inner-membrane engine of the cell-envelope-spanning Mla pathway, which comprises three assemblies: the OM lipoprotein complex MlaA–OmpC/F, the periplasmic phospholipid shuttle MlaC (= Ttg2D here), and the IM ABC transporter MlaFEDB (containing MlaB) (Abellon-Ruiz 2023, PMID 36459067). The pathway removes glycerophospholipids that become mislocalized in the OM outer leaflet and returns them to the inner membrane — i.e. retrograde (OM→IM) phospholipid transport — using energy from ATP hydrolysis to maintain OM lipid asymmetry (Thong et al. 2016, PMID 27529189). MlaB's function is therefore executed within this pathway by enabling the ATPase that drives the shuttle.
By preserving OM lipid asymmetry (LPS outer leaflet / phospholipid inner leaflet), the Ttg2/Mla system keeps the OM an effective permeability barrier. This explains the associated phenotypes:
- Organic-solvent (toluene) tolerance in P. putida: ttg2 disruption yields toluene-hypersensitive mutants, and membrane integrity/repair — not solvent efflux per se — was identified as the key protective mechanism (Kim et al. 1998, PMID 9658016).
- Intrinsic antibiotic resistance: in P. aeruginosa the Ttg2 system contributes to low-level resistance against antibiotics that permeate via a lipid-mediated pathway (Yero et al. 2021, PMID 33837253).
| Claim | Evidence type | Source |
|---|---|---|
| ttg2E = MlaB (STAS subunit) | Genomic/operon mapping + domain annotation (UniProt/Pfam/InterPro) | This analysis; UniProt Q88P90 |
| Ttg2 system = Mla lipid-asymmetry ABC transporter in Pseudomonas | Crystal structure + biochemistry of Ttg2D=MlaC | Yero et al. 2021, PMID 33837253 |
| MlaB required for assembly + ATPase activity of MlaFEDB | Genetic + biochemical reconstitution (E. coli homolog) | Thong et al. 2016, PMID 27529189 |
| MlaB is an integral subunit of MlaFEDB; regulates complex activity/stability | Cryo-EM (3.3–4.1 Å) + mutagenesis (E. coli homolog) | Tang et al. 2021, PMID 33199922; Ekiert et al. 2022, PMID 35981415 |
| ttg2E is soluble/cytoplasmic (single STAS domain, no TM/signal) | Sequence/bioinformatic analysis of Q88P90 | This analysis (UniProt Q88P90) |
| Pathway = retrograde OM→IM phospholipid transport maintaining OM asymmetry | Review of structural/biochemical literature | Abellon-Ruiz 2023, PMID 36459067; Thong 2016 |
| ttg2 needed for toluene tolerance via membrane integrity | Tn5 mutagenesis in P. putida | Kim et al. 1998, PMID 9658016 |
| Ttg2/Mla → intrinsic antibiotic resistance | Gene knockouts in P. aeruginosa | Yero et al. 2021, PMID 33837253 |
| ttg2/Mla is distinct from RND solvent-efflux pumps (TtgABC etc.) | Authoritative review of solvent-tolerance mechanisms | Ramos et al. 2002, PMID 12142492 |
Evidence spans direct experiment (genetic reconstitution, crystallography, knockouts) in closely related orthologs and bioinformatic inference (operon synteny, conserved domain architecture) for the KT2440 protein specifically. No experimental study on the KT2440 MlaB protein itself was found; the annotation is by strong orthology.