TtgA is the N-terminally lipidated periplasmic adaptor or membrane fusion protein subunit of the tripartite TtgABC RND efflux system in Pseudomonas putida KT2440. It is anchored to the inner membrane and helps couple the inner-membrane transporter TtgB to the outer-membrane exit duct TtgC, thereby supporting export of toxic hydrophobic compounds. Evidence from KT2440 and closely related Pseudomonas systems links TtgABC to tolerance against antibiotics, organic solvents, and monoterpenoid toxicants, although the exact substrate range of the KT2440 pump remains incompletely resolved.
Definition: Binding-based adaptor activity by a periplasmic membrane-fusion protein subunit that bridges an inner-membrane RND transporter and an outer-membrane exit duct, stabilizing a tripartite efflux complex that exports toxic compounds across the cell envelope.
Justification: Current GO annotations tend to over-annotate membrane-fusion/adaptor subunits like TtgA as transmembrane transporter activity even though these proteins do not themselves form the transport pore. A dedicated adaptor term would better represent the molecular role of RND pump MFP subunits.
Parent term: transmembrane transporter binding
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005886 plasma membrane | IEA GO_REF:0000120 | ACCEPT | Summary: This localization is appropriate for TtgA. UniProt identifies TtgA as a lipid-anchored inner-membrane protein, and the membrane-fusion/adaptor subunit of the TtgABC pump is expected to remain associated with the plasma membrane while projecting into the periplasm. Reason: TtgA is not a soluble periplasmic protein; it is a lipoprotein tethered to the inner membrane. GO:0005886 is therefore an informative and biologically correct cellular component term for this gene product. Supporting Evidence: file:PSEPK/ttgA/ttgA-uniprot.txt SUBCELLULAR LOCATION: Cell inner membrane {ECO:0000305}; Lipid-anchor {ECO:0000255|PROSITE-ProRule:PRU00303} |
| GO:0016020 membrane | IEA GO_REF:0000002 | REMOVE | Summary: This annotation is too broad. TtgA is membrane-associated, but GO:0005886 already captures the specific and relevant location. Reason: Retaining the generic membrane term adds little value once the more specific plasma membrane annotation is accepted. The broad InterPro-derived term obscures the fact that TtgA is specifically anchored to the bacterial inner membrane. |
| GO:0022857 transmembrane transporter activity | IEA GO_REF:0000002 | MODIFY | Summary: This is an over-annotation for TtgA itself. TtgA is the periplasmic adaptor subunit of the TtgABC complex, not the inner-membrane transport pore or the energy-coupling RND transporter. Its role is to stabilize coupling between the transporter and the outer-membrane channel. Reason: The tripartite pump as a whole enables transmembrane transport, but that activity should not be assigned directly to the non-transmembrane adaptor subunit. A better molecular-function proxy is GO:0044325 transmembrane transporter binding, with complex membership captured separately by GO:1902495. Proposed replacements: transmembrane transporter binding Supporting Evidence: file:PSEPK/ttgA/ttgA-uniprot.txt Probable periplasmic linker protein component of the TtgABC file:PSEPK/ttgA/ttgA-uniprot.txt Belongs to the membrane fusion protein (MFP) (TC 8.A.1) family. |
| GO:0046677 response to antibiotic | IEA GO_REF:0000118 | KEEP AS NON CORE | Summary: The annotation is plausible because TtgABC-family pumps contribute to antibiotic tolerance, but it is narrower than the best-supported biological picture. TtgABC systems also handle organic solvents and monoterpenoid toxicants, so antibiotic response is only one facet of a broader xenobiotic-defense role. Reason: Experimental work in Pseudomonas links TtgABC to antibiotic efflux, but the broader core function is export of multiple toxic hydrophobic compounds. Keeping this term as non-core preserves a supported substrate class without mistaking it for the full functional scope. Supporting Evidence: file:PSEPK/ttgA/ttgA-uniprot.txt GO; GO:0046677; P:response to antibiotic; IEA:TreeGrafter. |
| GO:0055085 transmembrane transport | IEA GO_REF:0000120 | MODIFY | Summary: The general process term is directionally correct but too broad. The best-supported process is transport of toxic xenobiotic compounds rather than abstract transmembrane transport. Reason: TtgA functions in a multidrug or solvent-efflux context, so GO:0042908 xenobiotic transport better captures the biological process than the parent term GO:0055085. Proposed replacements: xenobiotic transport Supporting Evidence: file:PSEPK/ttgA/ttgA-uniprot.txt GO; GO:0042908; P:xenobiotic transport; IEA:UniProtKB-ARBA. file:PSEPK/ttgA/ttgA-deep-research-falcon.md encodes the **membrane fusion protein (MFP) / periplasmic adaptor** subunit |
| GO:1902495 transmembrane transporter complex | TAS PMID:32296906 Investigation of monoterpenoid resistance mechanisms in Pseu... | NEW | Summary: TtgA is described as the lipoprotein/adaptor component of the tripartite TtgABC transporter assembly. The evidence here is a traceable literature statement and UniProt family assignment, not a direct complex-assembly assay. Reason: Complex membership is a more accurate representation of TtgA than assigning the subunit the full transporter activity. TAS is used instead of IMP because the GS1 monoterpenoid paper states the Ttg pump architecture and TtgA/D/G adaptor role, while the mutant phenotypes do not directly demonstrate complex assembly. Supporting Evidence: PMID:32296906 All three efflux pumps consist of three components: an inner membrane protein (TtgB/E/H), which binds the substrates and acts as the extrusion element, an outer membrane protein that reaches into the periplasmic space to form a channel (TtgC/F/I), and a lipoprotein that plays a role in stabilizing the interaction between the other two elements (TtgA/D/G) file:PSEPK/ttgA/ttgA-uniprot.txt Probable periplasmic linker protein component of the TtgABC |
| GO:0042908 xenobiotic transport | IMP PMID:32296906 Investigation of monoterpenoid resistance mechanisms in Pseu... | NEW | Summary: This is the most useful process-level summary of TtgA function. TtgABC-family pumps protect Pseudomonas cells against chemically diverse toxic hydrophobic compounds including antibiotics, solvents, and monoterpenoids. Reason: The phenotype of ttgA loss and the broader TtgABC literature both support a role in transport-based detoxification of xenobiotic compounds. This is more precise than the parent term transmembrane transport and more general than the substrate-specific antibiotic-response annotation. Supporting Evidence: file:PSEPK/ttgA/ttgA-uniprot.txt GO; GO:0042908; P:xenobiotic transport; IEA:UniProtKB-ARBA. |
| GO:0044325 transmembrane transporter binding | TAS PMID:32296906 Investigation of monoterpenoid resistance mechanisms in Pseu... | NEW | Summary: GO currently lacks a term tailored to RND efflux-pump adaptor subunits. As a conservative proxy, transmembrane transporter binding captures the traceable literature statement that the TtgA/D/G lipoprotein class stabilizes the interaction between inner-membrane transporter and outer-membrane channel elements. Reason: TtgA does not itself span the membrane or catalyze transport. TAS is used instead of ISS because the support is a traceable author statement about TtgA/D/G adaptor interactions, not transfer from a characterized ortholog. Until a more specific adaptor term exists, GO:0044325 is the closest informative molecular-function representation. Supporting Evidence: PMID:32296906 a lipoprotein that plays a role in stabilizing the interaction between the other two elements (TtgA/D/G) file:PSEPK/ttgA/ttgA-uniprot.txt Probable periplasmic linker protein component of the TtgABC file:PSEPK/ttgA/ttgA-uniprot.txt Belongs to the membrane fusion protein (MFP) (TC 8.A.1) family. |
Loading supporting content…
Download this section (compressed HTML)Q: What is the native substrate spectrum of the KT2440 TtgABC pump under physiological conditions: antibiotics, aromatic solvents, monoterpenoids, or a broader xenobiotic set?
Suggested experts: Ana Segura, Juan L Ramos, Markus Buchhaupt
Q: Does TtgA merely stabilize the TtgB-TtgC assembly, or does it also influence substrate selectivity and efflux efficiency?
Suggested experts: Ana Segura, Juan L Ramos, Markus Buchhaupt
Experiment: Construct clean ttgA and ttgABC deletion mutants in KT2440, complement them in trans, and measure tolerance or MIC phenotypes for chloramphenicol, tetracycline, toluene, 1,8-cineole, verbenone, and geraniol together with a resazurin efflux assay.
Hypothesis: TtgA is required for efficient efflux of multiple toxic hydrophobic compounds in KT2440.
Type: targeted gene deletion and phenotype profiling
Experiment: Mutate the lipobox cysteine and predicted interfacial residues of TtgA, then test complex formation with TtgB and TtgC by co-immunoprecipitation or crosslinking and compare efflux activity against the wild-type protein.
Hypothesis: The lipidated periplasmic adaptor domain of TtgA is required to couple TtgB and TtgC into a functional tripartite complex.
Type: structure-function analysis of efflux-pump assembly
Loading supporting content…
Download this section (compressed HTML)Loading supporting content…
Download this section (compressed HTML)