The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.
You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.
We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.
We are interested in where in or outside the cell the gene product carries out its function.
We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.
Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.
The requested identity is verified: P52612 is the E. coli K-12 FliI flagellar type III secretion ATPase, encoded by fliI (ordered locus b1941/JW1925; historical aliases flaAIII/flaC). Its description, ATPase α/β-chain-family assignment, AAA+-like/Walker ATPase domains, and role in flagellar export are mutually consistent. No literature concerning a different same-symbol gene was incorporated.
FliI catalyzes Mg²⁺-dependent ATP hydrolysis:
ATP + H₂O → ADP + inorganic phosphate.
Its primary biological function is not to synthesize ATP and not to form the transmembrane channel. Rather, cytoplasmic FliI assembles with FliH and FliJ into an ATPase complex at the cytoplasmic face of the flagellar basal body. This complex delivers flagellar export substrates, loads them onto the export machinery, and uses nucleotide-dependent conformational changes to activate and regulate the FlhA/FlhB/FliPQR transmembrane export gate. Sustained translocation is normally powered predominantly by ion motive force, particularly proton motive force in enteric bacteria. Thus, ATP hydrolysis facilitates substrate handling and gate activation but is not tightly stoichiometrically coupled to every exported polypeptide segment. Detailed mechanistic evidence comes predominantly from the highly conserved Salmonella enterica ortholog and should be treated as strong ortholog inference rather than direct P52612 measurement. (minamino2014thebacterialflagellar pages 1-2, terashima2018invitroreconstitution pages 8-9, minamino2022insightintodistinct pages 1-2)
| Annotation aspect | Best-supported conclusion | Evidence type/organism | Key quantitative detail | Confidence |
|---|---|---|---|---|
| Identity and family | P52612/b1941 is FliI, the flagellum-specific ATPase of the E. coli K-12 flagellar type III secretion system; it is a Walker-type AAA+ ATPase evolutionarily and structurally related to F₁-ATPase α/β subunits. | Target accession/locus identity; structural and comparative evidence, principally the Salmonella ortholog (imada2007structuralsimilaritybetween pages 1-1) | ADP-bound FliI structure solved at 2.4 Å; three domains include Walker A/B-containing ATPase and C-terminal domains. | High |
| Catalytic reaction and specificity | FliI catalyzes ATP + H₂O → ADP + Pi; Mg²⁺–ATP is the experimentally supported nucleotide substrate. Its physiological protein clients are flagellar export substrates, but these are cargo—not chemical substrates of the ATPase reaction. | Biochemical and reconstitution evidence, principally Salmonella ortholog (terashima2018invitroreconstitution pages 8-9, minamino2014thebacterialflagellar pages 2-3) | Reconstituted export used 5 mM ATP; catalytically inactive E211Q still bound Mg²⁺–ATP but did not support transport. | High for ATPase reaction; moderate for exact E. coli kinetics |
| Oligomeric organization | ATP-bound FliI assembles into a homohexameric ring with six intersubunit catalytic sites; FliJ occupies the central pore and FliH dimers bind the FliI N-terminal domains. | Structural, biochemical, and review synthesis; principally Salmonella ortholog (minamino2014thebacterialflagellar pages 1-2, imada2007structuralsimilaritybetween pages 1-1, minamino2022insightintodistinct pages 1-2) | Assembled ATPase complex stoichiometry: FliH₁₂–FliI₆–FliJ₁; six catalytic sites. | High for conserved architecture |
| Cellular localization | FliI is a soluble cytoplasmic protein that cycles between a cytoplasmic FliH₂–FliI carrier complex and a basal-body-associated, peripheral-membrane ATPase complex on the cytoplasmic face of the inner membrane; it is not an integral membrane protein. | Localization and interaction evidence, principally Salmonella ortholog (minamino2014thebacterialflagellar pages 4-5, halte2021proteinexportvia pages 4-6, minamino2022insightintodistinct pages 1-2) | Cytoplasmic carrier: FliH₂–FliI₁; basal complex: FliH₁₂–FliI₆–FliJ₁. | High for system-level localization; moderate for direct P52612 measurements |
| Substrate delivery | Cytoplasmic FliH₂–FliI dynamically delivers selected flagellar proteins and chaperone–cargo complexes to the FlhA docking platform; demonstrated clients include FlgN–FlgK and FliT–FliD, whereas FliC–FliS was not detected as a comparable FliH₂–FliI client in the cited synthesis. | Protein-interaction and mechanistic evidence, largely Salmonella ortholog (minamino2014thebacterialflagellar pages 4-5, halte2021proteinexportvia pages 4-6) | Rapid association–dissociation with the basal export platform supports sequential substrate transfer; no universal cargo-binding constant is established. | Moderate–high |
| Export-gate activation | ATP binding/hydrolysis and conformational cycling of the FliI₆–FliJ complex activate the FlhA-containing transmembrane export gate; FliJ behaves as a central stalk or “ignition key.” FliI therefore facilitates loading and gate activation rather than acting as the membrane translocation channel itself. | Mutational, structural, and reconstitution evidence, principally Salmonella ortholog (minamino2014thebacterialflagellar pages 5-7, terashima2018invitroreconstitution pages 8-9, minamino2022insightintodistinct pages 1-2) | ATP binding to E211Q FliI₆ produced an inferred ~80° FliJ rotation, but hydrolysis-deficient complexes showed severely impaired activation/export. | Moderate–high; rotational model remains mechanistic interpretation |
| Relationship to proton motive force | FliI ATP hydrolysis promotes efficient gate activation, but sustained export is normally powered chiefly by ion motive force through the membrane export gate. ATPase activity is therefore facilitatory and conditionally bypassable rather than obligatorily coupled one ATP per exported segment. | Genetic and biochemical evidence, principally Salmonella ortholog (minamino2014thebacterialflagellar pages 1-2, halte2021proteinexportvia pages 4-6, halte2021proteinexportvia pages 6-7) | Export can remain processive with extremely infrequent hydrolysis; in vitro, ATP hydrolysis alone drove FlgD/FlgE transport without bulk PMF when FliH₂–FliI and FliJ were supplied (terashima2018invitroreconstitution pages 8-9). | High that ATPase and PMF have distinct roles; moderate on exact energy partition in vivo |
| Catalytic Glu-211 variants | Glu-211 activates water for nucleophilic attack on ATP’s γ-phosphate. E211D hydrolyzes ATP very slowly yet supports appreciable export, whereas E211Q binds ATP and assembles but lacks measurable hydrolysis and strongly impairs export. | Site-directed mutagenesis and export assays, Salmonella ortholog (minamino2014thebacterialflagellar pages 1-2, terashima2018invitroreconstitution pages 8-9, minamino2014thebacterialflagellar pages 2-3) | E211D ATPase activity was about 1/100 of wild type; >80% of cells remained motile and swimming speed was about 50% of wild type in the cited study. | High for Salmonella; high-confidence ortholog inference for catalytic role |
| Residues 401–410 | The C-terminal 401–410 segment is important for coupling catalysis to export-gate activation rather than merely for hexamer formation or ATP turnover. | Deletion-mutant analysis, Salmonella ortholog (minamino2014thebacterialflagellar pages 1-2, minamino2014thebacterialflagellar pages 4-5, minamino2014thebacterialflagellar pages 2-3) | Δ401–410 retained normal hexamer formation and approximately 40% of wild-type ATPase activity but produced little export and no effective flagellar formation. | High for Salmonella; moderate–high for conserved E. coli role |
| 2024 ordered-export development | FliH/FliI were shown to help FlhA impose correct temporal order on flagellar export by supporting FlhA-ring remodeling and correction of substrate-recognition errors during the rod/hook-to-filament transition. This is a refinement of FliI’s role beyond generic ATP supply. | 2024 primary study in Salmonella enterica serovar Typhimurium; ortholog inference for E. coli P52612 (kinoshita2024flihandflii pages 1-2) | Hook completion occurs at approximately 55 nm before switching toward filament-type substrates; FlhA forms a nonameric cytoplasmic ring. | Moderate–high for conserved mechanism; not direct E. coli K-12 evidence |
Table: Evidence-weighted annotation of E. coli K-12 FliI, explicitly distinguishing accession-level identity from detailed mechanistic findings obtained mainly with the closely related Salmonella ortholog.
The supplied accession, organism, locus, and product description all converge on the same protein: the flagellar export ATPase FliI of E. coli K-12. The reported InterPro assignments—AAA+ ATPase, ATPase α/β chains, F₁/V₁/A₁ nucleotide-binding fold, and T3SS FliI/YscN—fit the experimentally established architecture. FliI has an N-terminal interaction domain, a central nucleotide-binding ATPase domain containing Walker A and Walker B motifs, and a C-terminal domain involved in oligomeric and functional coupling. An ADP-bound FliI structure was determined at 2.4 Å and showed close structural correspondence to F₁-ATPase α/β subunits. This explains the ATPase α/β-family annotation but does not mean FliI is a conventional membrane ATP synthase subunit. (imada2007structuralsimilaritybetween pages 1-1)
The functional analogy extends to higher-order architecture: six FliI molecules form a ring, while FliJ occupies its central pore in a stalk-like position. FliH dimers associate with the FliI N-terminal domains. The basal ATPase assembly is generally represented as FliH₁₂–FliI₆–FliJ₁, analogous in broad organization—not identical chemistry—to the soluble catalytic sector of rotary ATPases. Six ATPase active sites occur at FliI–FliI interfaces. (minamino2014thebacterialflagellar pages 1-2, minamino2022insightintodistinct pages 1-2)
The chemical substrate is ATP, experimentally used as Mg²⁺–ATP. Hydrolysis produces ADP and phosphate. The catalytic glutamate corresponding to Glu-211 activates water for attack on ATP’s γ-phosphate. The conservative E211D substitution lowers ATPase activity to approximately 1/100 of wild type, whereas E211Q can bind Mg²⁺–ATP and assemble into a ring but lacks effective hydrolysis. These results distinguish nucleotide binding and oligomerization from catalysis. (terashima2018invitroreconstitution pages 8-9, minamino2014thebacterialflagellar pages 2-3)
No robust evidence retrieved here establishes physiologically important hydrolysis of another nucleoside triphosphate by E. coli P52612. Accordingly, annotation should remain ATP-specific ATPase rather than a broad NTPase unless direct P52612 kinetics demonstrate otherwise.
Flagellar proteins are often called FliI “substrates” in secretion literature, but chemically they are export cargo, not substrates consumed by the ATPase reaction. FliI participates in handling rod-, hook-, junction-, cap-, and filament-related proteins as they are exported through the flagellar type III secretion system. Documented chaperone–cargo interactions include FlgN–FlgK and FliT–FliD complexes; the same synthesis did not find comparable recruitment of FliC–FliS by soluble FliH₂–FliI. Cargo recognition is therefore selective and distributed among FliI/FliH, FliJ, export chaperones, FlhA, and other export-apparatus components rather than being a simple ATPase active-site specificity. (minamino2014thebacterialflagellar pages 4-5, halte2021proteinexportvia pages 4-6)
FliI exists in the cytoplasm as part of a FliH₂–FliI₁ heterotrimer. This complex binds selected free export substrates or chaperone–cargo complexes and cycles rapidly between the cytoplasmic pool and the flagellar export platform. It is therefore best viewed as a dynamic substrate-delivery and loading module rather than a permanently fixed motor. (minamino2014thebacterialflagellar pages 4-5, minamino2022insightintodistinct pages 1-2)
At the flagellar base, six FliI subunits form a ring with FliJ in the center. FliH connects this assembly to the cytoplasmic C ring through FliN/FliM-associated interactions and also supports association with FlhA. FliI contacts or functionally interacts with FliH, FliJ, the cytoplasmic regions of FlhA and FlhB, and chaperone–cargo complexes. FliI is therefore adjacent to the inner membrane but is not transmembrane. (minamino2014thebacterialflagellar pages 4-5, imada2007structuralsimilaritybetween pages 1-1, halte2021proteinexportvia pages 4-6)
The current model assigns separable but coordinated functions to FliI:
The proposed rotary component is supported by structural analogy and mutational behavior but should not be interpreted as direct proof that FliI performs continuous F₁-like rotation in living E. coli. ATP binding to hydrolysis-defective FliI(E211Q) was associated with an inferred FliJ rotation of approximately 80°, but such events were extremely infrequent without hydrolysis. (minamino2014thebacterialflagellar pages 5-7)
Several observations rule out a simple “one ATP drives one translocation step” model. In Salmonella, E211D retained very low ATPase activity yet more than 80% of cells were motile, with swimming speed around 50% of wild type. Export remained processive despite extremely infrequent ATP hydrolysis. Conversely, deleting FliI residues 401–410 preserved normal hexamer formation and about 40% of ATPase activity but caused severe export and flagellar-assembly defects. ATP turnover rate alone therefore does not determine export rate; productive coupling to gate activation is crucial. (minamino2014thebacterialflagellar pages 1-2, minamino2014thebacterialflagellar pages 2-3)
Reconstitution experiments add an important qualification. In inverted membrane vesicles, 5 mM ATP, 1.5 μM FliH₂/FliI, 0.25 μM FliJ, and flagellar cargo supported export, and ATP hydrolysis could drive FlgD/FlgE transport even when bulk PMF was dissipated with 10 μM CCCP. However, under normal cellular conditions the membrane export gate and ion motive force provide the principal sustained translocation engine, with FliI markedly improving initiation, loading, efficiency, and robustness. (terashima2018invitroreconstitution pages 8-9, halte2021proteinexportvia pages 6-7)
FliI functions on the cytoplasmic side of the inner membrane at the flagellar basal body. It has two operational localizations:
It is not extracellular, periplasmic, or an integral membrane channel. Cargo accepted by the export apparatus passes from the cytoplasm through the inner-membrane gate and then through the narrow central channel of the growing rod, hook, and filament to assemble at the distal tip. (minamino2014thebacterialflagellar pages 4-5, halte2021proteinexportvia pages 4-6, minamino2022insightintodistinct pages 1-2)
FliI belongs to the flagellar type III secretion/flagellar assembly pathway, not directly to chemotactic signal transduction. Its immediate pathway context is:
cytoplasmic flagellar cargo → FliH/FliI/chaperone handling → FlhA docking and FliJ-dependent gate activation → FliP/FliQ/FliR/FlhB/FlhA export gate → axial channel → distal flagellar assembly.
Loss or severe impairment of FliI reduces efficient export of structural subunits, thereby compromising rod, hook, and filament construction and ultimately motility. The phenotype is downstream of assembly failure rather than evidence that FliI directly senses chemoeffectors or generates flagellar rotation.
Kinoshita and colleagues showed in Salmonella enterica serovar Typhimurium that FliH/FliI do more than supply ATPase activity: they help the nonameric FlhA cytoplasmic ring impose correct temporal order on exported substrates. Mutations in FlhA’s conserved GYXLI motif indicated that the ATPase complex supports remodeling of FlhA from rod/hook-substrate recognition toward filament-substrate recognition and helps correct recognition errors. The normal transition occurs after completion of an approximately 55-nm hook. This refines FliI’s annotation to include export-order quality control and specificity-switch assistance. The result is highly relevant to E. coli because the apparatus is conserved, but it is ortholog evidence rather than a direct P52612 experiment. Published March 2024: https://doi.org/10.1038/s42003-024-06081-0. (kinoshita2024flihandflii pages 1-2)
Bryant and Fraser identified a C-terminal targeting signal in early flagellar subunits and concluded that this targeting step could occur without the flagellar ATPase and cytoplasmic-ring components. This is conceptually important: not every recognition event is mediated by FliI. Instead, substrates undergo sequential recognition by several parts of the export machinery, with FliI contributing particular delivery, loading, and regulatory steps. Published March 2024: https://doi.org/10.1128/mbio.03067-23.
A 2024 structural review places FliI within an increasingly high-resolution model of flagellar assembly, emphasizing dynamic interactions among the cytoplasmic ATPase, chaperone–cargo complexes, and membrane export platform. It also identifies the 2024 ordered-export study as a significant refinement of FliH/FliI function. Published November 2024: https://doi.org/10.3390/biom14121488. (nakamura2024structureanddynamics pages 18-19)
No direct 2023–2024 biochemical characterization specifically of E. coli K-12 P52612 was identified. The newest detailed mechanistic work remains dominated by Salmonella, whose homologous apparatus is routinely used as the experimentally tractable enteric model.
FliI itself is primarily a research target, not an approved clinical or industrial product target. Current applications include:
These are active research uses. Evidence retrieved here does not support claiming that an FliI-specific inhibitor has reached routine clinical or field implementation.
High-confidence annotation: P52612 is the E. coli K-12 flagellar T3SS ATPase FliI; it hydrolyzes ATP and functions with FliH/FliJ at the cytoplasmic face of the basal export apparatus. Its ATPase-family and F₁-like structural assignments are strongly supported. (imada2007structuralsimilaritybetween pages 1-1, minamino2022insightintodistinct pages 1-2)
High-confidence conserved mechanism: hexameric FliI, FliJ central-stalk association, FliH tethering, substrate delivery, and export-gate activation are supported by convergent structural, genetic, biochemical, and reconstitution evidence. (minamino2014thebacterialflagellar pages 1-2, terashima2018invitroreconstitution pages 8-9, halte2021proteinexportvia pages 4-6)
Principal caveat: many precise residue-level, stoichiometric, and energy-coupling experiments were performed with Salmonella FliI. They are persuasive ortholog evidence for E. coli P52612 but should not be represented as direct measurements of the K-12 protein’s kinetic constants.
Best concise functional annotation:
FliI is a cytoplasmic, basal-body-associated Mg²⁺-ATPase of the E. coli flagellar type III secretion system. It cycles between a soluble FliH₂–FliI substrate-delivery complex and a FliH₁₂–FliI₆–FliJ₁ ATPase ring at the flagellar base, where ATP-dependent conformational changes promote cargo loading, FlhA-containing export-gate activation, and correctly ordered export. The membrane gate then uses ion motive force to translocate flagellar structural proteins for distal assembly.
References
(minamino2014thebacterialflagellar pages 1-2): Tohru Minamino, Yusuke V. Morimoto, Miki Kinoshita, Phillip D. Aldridge, and Keiichi Namba. The bacterial flagellar protein export apparatus processively transports flagellar proteins even with extremely infrequent atp hydrolysis. Scientific Reports, Dec 2014. URL: https://doi.org/10.1038/srep07579, doi:10.1038/srep07579. This article has 106 citations and is from a peer-reviewed journal.
(terashima2018invitroreconstitution pages 8-9): Hiroyuki Terashima, Akihiro Kawamoto, Chinatsu Tatsumi, Keiichi Namba, Tohru Minamino, and Katsumi Imada. in vitro reconstitution of functional type iii protein export and insights into flagellar assembly. mBio, Jul 2018. URL: https://doi.org/10.1128/mbio.00988-18, doi:10.1128/mbio.00988-18. This article has 39 citations and is from a domain leading peer-reviewed journal.
(minamino2022insightintodistinct pages 1-2): Tohru Minamino, Miki Kinoshita, and Keiichi Namba. Insight into distinct functional roles of the flagellar atpase complex for flagellar assembly in salmonella. Frontiers in Microbiology, May 2022. URL: https://doi.org/10.3389/fmicb.2022.864178, doi:10.3389/fmicb.2022.864178. This article has 41 citations and is from a peer-reviewed journal.
(imada2007structuralsimilaritybetween pages 1-1): Katsumi Imada, Tohru Minamino, Aiko Tahara, and Keiichi Namba. Structural similarity between the flagellar type iii atpase flii and f1-atpase subunits. Proceedings of the National Academy of Sciences, 104:485-490, Jan 2007. URL: https://doi.org/10.1073/pnas.0608090104, doi:10.1073/pnas.0608090104. This article has 190 citations and is from a highest quality peer-reviewed journal.
(minamino2014thebacterialflagellar pages 2-3): Tohru Minamino, Yusuke V. Morimoto, Miki Kinoshita, Phillip D. Aldridge, and Keiichi Namba. The bacterial flagellar protein export apparatus processively transports flagellar proteins even with extremely infrequent atp hydrolysis. Scientific Reports, Dec 2014. URL: https://doi.org/10.1038/srep07579, doi:10.1038/srep07579. This article has 106 citations and is from a peer-reviewed journal.
(minamino2014thebacterialflagellar pages 4-5): Tohru Minamino, Yusuke V. Morimoto, Miki Kinoshita, Phillip D. Aldridge, and Keiichi Namba. The bacterial flagellar protein export apparatus processively transports flagellar proteins even with extremely infrequent atp hydrolysis. Scientific Reports, Dec 2014. URL: https://doi.org/10.1038/srep07579, doi:10.1038/srep07579. This article has 106 citations and is from a peer-reviewed journal.
(halte2021proteinexportvia pages 4-6): Manuel Halte and Marc Erhardt. Protein export via the type iii secretion system of the bacterial flagellum. Biomolecules, 11:186, Jan 2021. URL: https://doi.org/10.3390/biom11020186, doi:10.3390/biom11020186. This article has 60 citations.
(minamino2014thebacterialflagellar pages 5-7): Tohru Minamino, Yusuke V. Morimoto, Miki Kinoshita, Phillip D. Aldridge, and Keiichi Namba. The bacterial flagellar protein export apparatus processively transports flagellar proteins even with extremely infrequent atp hydrolysis. Scientific Reports, Dec 2014. URL: https://doi.org/10.1038/srep07579, doi:10.1038/srep07579. This article has 106 citations and is from a peer-reviewed journal.
(halte2021proteinexportvia pages 6-7): Manuel Halte and Marc Erhardt. Protein export via the type iii secretion system of the bacterial flagellum. Biomolecules, 11:186, Jan 2021. URL: https://doi.org/10.3390/biom11020186, doi:10.3390/biom11020186. This article has 60 citations.
(kinoshita2024flihandflii pages 1-2): Miki Kinoshita, Tohru Minamino, Takayuki Uchihashi, and Keiichi Namba. Flih and flii help flha bring strict order to flagellar protein export in salmonella. Communications Biology, Mar 2024. URL: https://doi.org/10.1038/s42003-024-06081-0, doi:10.1038/s42003-024-06081-0. This article has 18 citations and is from a peer-reviewed journal.
(nakamura2024structureanddynamics pages 18-19): Shuichi Nakamura and Tohru Minamino. Structure and dynamics of the bacterial flagellar motor complex. Biomolecules, 14:1488, Nov 2024. URL: https://doi.org/10.3390/biom14121488, doi:10.3390/biom14121488. This article has 32 citations.