Comprehensive Research Report: SctN2 (SsaN) Gene Function, Structure, and Annotation Falcon Edison Scientific Literature 28 citations 1 artifacts 2026-09-27T16:52:26.165096

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Comprehensive Research Report: SctN2 (SsaN) Gene Function, Structure, and Annotation

Gene Identity Verified
UniProt Accession: P74857
Locus Tag: STM1415
Gene Name: sctN2 (standardized nomenclature); commonly cited as ssaN in the Salmonella literature
Organism: Salmonella enterica serovar Typhimurium strain LT2
Protein Family: ATPase alpha/beta chains family (IPR050053); T3SS SctN-type ATPase (IPR013380)


1. Key Concepts and Definitions: Protein Identity and Biological Context

1.1 Verified Gene and Protein Identity

SctN2/SsaN encodes the central ATPase component of the Salmonella Pathogenicity Island 2 (SPI-2)–encoded type III secretion system (T3SS-2), which is essential for intracellular survival and systemic virulence of S. enterica serovar Typhimurium (hensel1997functionalanalysisof pages 1-2, yoshida2014functionalcharacterizationof pages 4-6, yoshida2014functionalcharacterizationof pages 1-2). This gene is distinct from invC, which encodes the homologous ATPase for the unrelated SPI-1/T3SS-1 invasion apparatus (yoshida2014functionalcharacterizationof pages 4-6, yoshida2014functionalcharacterizationof pages 1-2). The ssaN locus was genetically mapped by Hensel et al. (1997, Molecular Microbiology 24:155–167; DOI: 10.1046/j.1365-2958.1997.3271699.x) to a large (~10 kb) operon spanning ssaK through ssaU, which encodes structural and regulatory components of the second T3SS apparatus; reverse-transcription PCR confirmed cotranscription (hensel1997functionalanalysisof pages 1-2, hensel1997functionalanalysisof pages 9-10). Sequence homology places SsaN as the ortholog of the flagellar FliI, Yersinia YscN, Shigella Spa47, and enteropathogenic E. coli EscN ATPases, collectively designated SctN in standardized T3SS nomenclature (yoshida2014functionalcharacterizationof pages 4-6).

1.2 Current Understanding: The SPI-2 T3SS and its ATPase

The SPI-2 T3SS is a macromolecular apparatus induced within host cells, notably inside acidified vacuoles (pH ~5), and is required for intracellular replication and systemic infection (hensel1997functionalanalysisof pages 1-2, yoshida2014functionalcharacterizationof pages 1-2, yu2004ssamandspic pages 1-2). Unlike the flagellar system—which is powered by a transmembrane proton-motive force—nonmotile T3SSs such as SPI-2 are conventionally described as relying on the SctN ATPase to energize or organize substrate unfolding, chaperone release, and delivery of secreted proteins (yoshida2014functionalcharacterizationof pages 1-2, yoshida2014functionalcharacterizationof pages 8-10). However, recent evidence (see Section 2 below) emphasizes that SctN also functions in a dynamic cytosolic sorting-platform complex that captures, organizes, and shuttles chaperone-bound effectors to the membrane-embedded export gate (pintor2024thepathand pages 34-37, wimmi2024cytosolicsortingplatform pages 1-2, pintor2024thepathand pages 146-149). This refined model portrays the ATPase as embedded in a substrate-recruitment and -delivery pathway, while its precise role in providing mechanical energy for substrate translocation through the transmembrane channel remains incompletely resolved (pintor2024thepathand pages 34-37).


2. Recent Developments (2023–2024) and Latest Research: Insights into the T3SS ATPase Family

2.1 Mobile Sorting-Platform Model from Quantitative Microscopy

No 2023–2024 study has directly updated the biochemistry or structure of Salmonella SPI-2 SsaN. However, a landmark 2024 study by Wimmi et al. published in Nature Microbiology (9:185–199; DOI: 10.1038/s41564-023-01545-1) refined the conserved injectisome model using Yersinia enterocolitica as a model system (wimmi2024cytosolicsortingplatform pages 1-2). Applying photoactivated-localization microscopy (PALM), single-particle tracking super-resolution microscopy (sptPALM), and proximity labeling, the authors demonstrated that the sorting-platform proteins SctQ and SctL (homologs of Salmonella SsaQ and SsaL) form mobile cytosolic pods that directly interact with effectors and their chaperones in live bacteria (wimmi2024cytosolicsortingplatform pages 1-2, pintor2024thepathand pages 146-149). These pods assemble into larger complexes containing the ATPase SctN and the membrane connector SctK, and their mobility and composition change in the presence of effector/chaperone pairs and upon initiation of secretion (pintor2024thepathand pages 146-149). Quantitative measurements by sptPALM revealed distinct diffusive states, with weighted mean jump-distance distributions showing diffusion coefficients that differ for SctQ, SctL, and SctN and are modulated by effector cargo (wimmi2024cytosolicsortingplatform pages 1-2). Upon secretion activation, the estimated number of functional injectisomes increased from approximately 5 to 18 per bacterium (wimmi2024cytosolicsortingplatform pages 1-2, pintor2024thepathand pages 146-149).

Interpretation for SsaN/SctN2: These findings support a cytosolic-to-membrane shuttling model for conserved T3SS ATPases and their associated sorting platforms. They directly challenge older, static views of the sorting platform as permanently injectisome-bound (pintor2024thepathand pages 34-37). Nevertheless, these results represent family-level inference—they were obtained with Yersinia SctN, not Salmonella SPI-2 SsaN—and therefore should not be presented as direct evidence for SsaN's mechanism without additional experimentation in Salmonella (pintor2024thepathand pages 34-37, pintor2024thepathand pages 146-149).

2.2 Applications and Real-World Implementations (2024)

Anti-Virulence Therapeutics: T3SS ATPases, including SsaN, are increasingly validated as targets for anti-virulence drug development, a strategy aimed at disarming pathogens rather than killing them (allison2014identificationofthe pages 8-9). Small-molecule inhibitors targeting SctN ATPases have been described for other pathogens such as Shigella but have not yet progressed to clinical use for Salmonella; no SsaN-specific inhibitor was identified in the retrieved literature.

Synthetic Biology and Protein Delivery: The modular architecture of T3SS has spurred efforts to engineer these systems for heterologous protein secretion or delivery of therapeutics, leveraging ATPases and sorting platforms to control substrate hierarchy (pintor2024thepathand pages 34-37). However, as of 2024, these remain research tools rather than approved clinical products.


3. Primary Molecular Function: Enzymatic Reaction and Substrate Specificity

3.1 Catalytic Reaction

SsaN is an Mg-ATP-dependent hydrolase (EC 7.4.2.8) that catalyzes the reaction:

ATP + H₂O → ADP + Pᵢ (inorganic phosphate)

Direct biochemical characterization by Yoshida et al. (2014, PLoS ONE 9:e94347; DOI: 10.1371/journal.pone.0094347) using purified Myc-His₆-tagged SsaN showed time- and concentration-dependent ATP hydrolysis in a coupled pyruvate kinase–lactate dehydrogenase spectrophotometric assay (yoshida2014functionalcharacterizationof pages 8-10, yoshida2014functionalcharacterizationof pages 4-6). The reported kinetic parameters are:

Activity increased nonlinearly with protein concentration, consistent with positive cooperativity or assembly-dependent activation, a hallmark of ring-forming AAA⁺ ATPases (yoshida2014functionalcharacterizationof pages 4-6).

3.2 Critical Catalytic Residue: Arg192

The conserved Arg192 (R192), located in the DCCD-box region of the ATPase domain, is essential for catalysis. A single substitution, R192G, abolished detectable ATPase activity in vitro and failed to restore SPI-2 effector secretion in an ssaN deletion strain, despite the mutant protein remaining stable and expressed (yoshida2014functionalcharacterizationof pages 4-6, allison2014identificationofthe pages 4-4). The R192G mutation also eliminated ATP-dependent chaperone-effector release (see below), directly linking ATPase activity to SsaN's biological function (yoshida2014functionalcharacterizationof pages 6-8, yoshida2014functionalcharacterizationof pages 1-2).

3.3 Substrate Specificity

SsaN's direct chemical substrate is ATP (yoshida2014functionalcharacterizationof pages 8-10, yoshida2014functionalcharacterizationof pages 4-6). Its biological substrate selectivity operates through protein–protein interactions: SsaN recognizes and processes chaperone-bound effectors and translocators secreted via the SPI-2 apparatus. Pull-down experiments showed that SsaN interacts with the SPI-2-specific chaperones SsaE, SseA, SscA, and SscB (yoshida2014functionalcharacterizationof pages 6-8, yoshida2014functionalcharacterizationof pages 1-2). Functional experiments using an in vitro chaperone-release assay established that SsaN dissociates the chaperone SsaE from the translocator protein SseB in an ATP-dependent manner; neither the nonhydrolyzable analog ATPγS nor the catalytically inactive SsaN-R192G supported SseB release (yoshida2014functionalcharacterizationof pages 6-8, yoshida2014functionalcharacterizationof pages 1-2). This activity is interpreted as the preparation of substrates for secretion—potentially involving unfolding or conformational remodeling—although the excerpt notes that direct demonstration of substrate unfolding was not provided (yoshida2014functionalcharacterizationof pages 8-10).

3.4 Broader Substrate Scope

Deletion or mutation of ssaN abolishes or severely impairs secretion of multiple SPI-2 substrates, including the translocators SseB, SseC, and SseD, and the effectors SseF, SseG, and SseJ; complementation restores secretion (yoshida2014functionalcharacterizationof pages 4-6, allison2014identificationofthe pages 4-5, allison2014identificationofthe pages 4-4). Some effectors encoded outside SPI-2 (e.g., PipB2, SseL, and SopD2) showed partial secretion in an ssaN mutant, likely through compensatory rerouting via the SPI-1 T3SS; however, secretion was abolished in an ssaN/invC double mutant, confirming pathway redundancy rather than SsaN independence (allison2014identificationofthe pages 4-4). Crystal-structure and mutagenesis studies by Allison et al. (2014, Journal of Biological Chemistry 289:23734–23744; DOI: 10.1074/jbc.M114.578476) identified a discrete C-terminal chaperone-docking region (residues 332–433) that binds the multicargo chaperone SrcA; SsaN variants defective for SrcA binding but retaining ATPase activity showed reduced in vivo fitness, directly demonstrating the importance of chaperone–ATPase interaction for Salmonella virulence (allison2014identificationofthe pages 8-9, allison2014identificationofthe pages 3-4).


4. Structural and Domain Architecture

4.1 Crystal Structure

The structure of a C-terminal-truncated SsaN variant (residues 1–433) was solved at 2.1 Å resolution by Allison et al. (2014) and deposited in the Protein Data Bank as PDB 4NPH (allison2014identificationofthe pages 4-5, allison2014identificationofthe pages 4-4). The structure reveals:

A notable feature is a bent α10 helix within the central domain; its conformation resembles the F₁-ATPase and is proposed to participate in intersubunit contacts or chaperone interaction, although this interpretation is partly inferred from homologous structures (allison2014identificationofthe pages 4-5). Deletion or point mutations in the C-terminal region altered SrcA binding, circular dichroism confirmed that variants retained secondary structure, and BIOMOL Green phosphate-release assays showed that mutations retained ATPase activity, establishing that the C-terminal domain mediates substrate recognition independently of catalysis (allison2014identificationofthe pages 8-9).

4.2 Oligomeric State (Inferred)

Although the published structure is of a truncated monomer, SsaN—and related SctN/FliI ATPases—are widely thought to assemble into hexameric ring complexes analogous to F₁-ATPase (yoshida2014functionalcharacterizationof pages 1-2, pintor2024thepathand pages 13-16). Direct measurement of SsaN oligomerization was not reported in the retrieved excerpts; instead, nonlinear protein-concentration dependence of ATPase activity supports cooperative assembly (yoshida2014functionalcharacterizationof pages 4-6), while the hexameric organization is largely inferred from related systems (pintor2024thepathand pages 13-16, yoshida2014functionalcharacterizationof pages 8-10).


5. Subcellular Localization

SsaN is primarily cytoplasmic and peripherally membrane-associated. Fractionation experiments by Yoshida et al. (2014) detected SsaN in both soluble (cytoplasmic) and membrane fractions after cells were cultured in low-pH minimal medium (LPM, pH 5.8), which mimics intracellular conditions and induces SPI-2 expression (yoshida2014functionalcharacterizationof pages 6-8, yoshida2014functionalcharacterizationof pages 8-10). Under these conditions, SsaN co-localized with its interaction partners SsaK and SsaQ in the membrane fraction (yoshida2014functionalcharacterizationof pages 6-8, yoshida2014functionalcharacterizationof pages 8-10). Membrane association of SsaN did not require SsaK or SsaQ, indicating that additional factors or direct membrane recruitment mediate attachment (yoshida2014functionalcharacterizationof pages 6-8). SsaN lacks transmembrane helices and is not classified as an integral membrane protein; its membrane localization is therefore interpreted as association with the cytoplasmic face of the SPI-2 apparatus, consistent with its role as part of the cytoplasmic C-ring or sorting-platform complex (yoshida2014functionalcharacterizationof pages 6-8, yoshida2014functionalcharacterizationof pages 1-2, yoshida2014functionalcharacterizationof pages 8-10).

Recent inference from Yersinia: Quantitative fluorescence and super-resolution microscopy in Yersinia found that SctN exists in both membrane-localized foci associated with injectisomes and a mobile cytosolic pool that diffuses and exchanges dynamically (pintor2024thepathand pages 146-149, pintor2024thepathand pages 13-16). This supports a model in which SsaN shuttles between the cytosol (where it captures chaperone-effector complexes) and the membrane-embedded export apparatus (where it delivers substrates), but this is family-level inference, not direct Salmonella evidence (pintor2024thepathand pages 146-149).


6. Biochemical Pathways and Biological Processes

6.1 The SPI-2 Type III Secretion Pathway

SsaN functions within the SPI-2/T3SS-2 pathway, a specialized secretion apparatus required for:

6.2 Functional Position within the Pathway

SsaN operates at the cytoplasmic sorting and substrate-delivery step, downstream of effector synthesis and chaperone binding and upstream of translocation through the membrane-embedded export gate (yoshida2014functionalcharacterizationof pages 1-2, pintor2024thepathand pages 34-37). Evidence supports the following stepwise model:

  1. Chaperone capture: Effectors are synthesized in the bacterial cytosol and bind cognate chaperones (e.g., SsaE binds SseB, SscA binds SseC); chaperones prevent premature aggregation or degradation and maintain substrates in a secretion-competent state (yoshida2014functionalcharacterizationof pages 6-8, pintor2024thepathand pages 34-37).
  2. Recognition by SsaN and the sorting platform: SsaN interacts with chaperones via its C-terminal domain and forms a complex with SsaK and SsaQ (the SPI-2 homologs of FliH/YscL and FliN/YscQ, respectively) (yoshida2014functionalcharacterizationof pages 6-8, yoshida2014functionalcharacterizationof pages 1-2).
  3. ATP-dependent chaperone release: SsaN hydrolyzes ATP and dissociates the chaperone from the effector/translocator substrate (yoshida2014functionalcharacterizationof pages 6-8, yoshida2014functionalcharacterizationof pages 1-2). This step may also involve partial unfolding to prepare substrates for passage through the narrow (~2–3 nm diameter) secretion channel, although direct demonstration of unfolding has not been provided for SsaN (yoshida2014functionalcharacterizationof pages 8-10).
  4. Delivery to the export gate: Released substrates are delivered to the transmembrane export apparatus for translocation into the host cell (pintor2024thepathand pages 34-37, pintor2024thepathand pages 146-149).

6.3 Relationship to Other Salmonella Virulence Systems

SsaN and the SPI-2 apparatus are functionally and genetically distinct from the SPI-1 invasion apparatus, whose ATPase is encoded by invC. The two pathways operate at different stages of infection: SPI-1 mediates epithelial invasion and is primarily active extracellularly, whereas SPI-2 is induced after uptake into host cells and is required for intracellular replication (yoshida2014functionalcharacterizationof pages 4-6, yoshida2014functionalcharacterizationof pages 1-2). As noted, some effectors can be secreted by both systems, but SsaN is essential for SPI-2–mediated secretion and virulence (allison2014identificationofthe pages 4-4, yoshida2014functionalcharacterizationof pages 4-6).


7. Quantitative Phenotypes and Experimental Evidence

7.1 Mouse Virulence

Competitive-infection experiments in mice provide direct evidence for SsaN's essential role in systemic disease. Yoshida et al. (2014) performed mixed infections (ssaN mutant vs. wild type) and calculated a competitive index (CI) from bacterial counts in spleen:

This severe attenuation demonstrates that SsaN is required for fitness during systemic infection (yoshida2014functionalcharacterizationof pages 6-8). Similarly, Allison et al. (2014) reported competitive indices below 0.1 in spleen, liver, and cecum at 72 hours post-infection for an ssaN mutant (allison2014identificationofthe pages 3-4); structure-based mutagenesis of the chaperone-docking site yielded intermediate CI values, with statistically significant differences (p < 0.0001, 0.0005, 0.0006, 0.0023) across different SsaN variants (allison2014identificationofthe pages 8-9).

7.2 Secretion and Translocation Defects

Loss of ssaN blocks secretion of SPI-2 substrates including SseB, SseC, SseD, SseF, SseG, and SseJ (yoshida2014functionalcharacterizationof pages 4-6, allison2014identificationofthe pages 4-5, allison2014identificationofthe pages 4-4). Complementation with wild-type ssaN partially or fully restored secretion, whereas catalytically inactive SsaN-R192G did not (yoshida2014functionalcharacterizationof pages 4-6). Host-cell translocation assays showed that SseJ accumulates in vacuoles surrounding wild-type Salmonella but not ssaN mutants; complementation restored this phenotype (yoshida2014functionalcharacterizationof pages 4-6). Importantly, whole-cell SseJ protein levels were unchanged, localizing the defect to secretion rather than effector synthesis (yoshida2014functionalcharacterizationof pages 4-6).


8. Expert Opinion and Current Model

The functional annotation of SsaN has evolved from earlier models that portrayed SctN ATPases primarily as energy-providing motors for substrate translocation to a more nuanced understanding in which the ATPase is embedded in a dynamic sorting and delivery apparatus. The 2024 Yersinia work supports this refined view, emphasizing cytosolic substrate capture, chaperone handoff, and shuttling to the membrane export gate (pintor2024thepathand pages 34-37, wimmi2024cytosolicsortingplatform pages 1-2, pintor2024thepathand pages 146-149). Expert interpretation suggests that SctN ATPases coordinate substrate selection, chaperone release, and secretion hierarchy, while the transmembrane export apparatus itself may also contribute energy via proton-motive force or conformational coupling (pintor2024thepathand pages 34-37). For SsaN specifically, direct demonstration of each step (substrate unfolding, exact energy-coupling mechanism, oligomeric ring dynamics) remains incomplete, and therefore the exact division of labor between ATP hydrolysis and membrane energetics is not fully resolved (yoshida2014functionalcharacterizationof pages 8-10, pintor2024thepathand pages 34-37).


9. Limitations and Open Questions

  1. Oligomeric State: Although nonlinear activity and homology support hexameric assembly, no direct cryo-EM or analytical ultracentrifugation data for full-length SsaN were retrieved.
  2. Energy Coupling: The precise contribution of ATP hydrolysis by SsaN versus proton-motive force or other membrane-associated energetics to substrate translocation is not established by direct SsaN evidence.
  3. Substrate Unfolding: While SsaN releases chaperones and is required for secretion, direct demonstration of substrate unfolding (e.g., by hydrogen-deuterium exchange or single-molecule force spectroscopy) was not reported (yoshida2014functionalcharacterizationof pages 8-10).
  4. 2023–2024 Updates for SsaN: No recent study specifically re-examined Salmonella SsaN; insights from Yersinia or flagellar systems are family-level inferences and require experimental validation in Salmonella SPI-2 (pintor2024thepathand pages 34-37, pintor2024thepathand pages 146-149).
  5. Clinical Translation: Anti-virulence inhibitors targeting T3SS ATPases remain at the preclinical stage, with no approved drugs or ongoing trials for SsaN-specific inhibition identified in the retrieved literature.

10. Summary Evidence Table

Topic Finding Evidence type/directness Key quantitative detail Source/date/DOI
Identity sctN2/ssaN encodes the SPI-2 T3SS ATPase; it is distinct from InvC, the homologous SPI-1 ATPase. In LT2, the supplied UniProt mapping is P74857/STM1415. Direct operon mapping plus sequence/functional assignment; LT2 accession mapping from supplied UniProt context Located in the approximately 10-kb ssaK–ssaU operon Hensel et al., Apr 1997, 10.1046/j.1365-2958.1997.3271699.x; Yoshida et al., Apr 2014, 10.1371/journal.pone.0094347 (hensel1997functionalanalysisof pages 1-2, yoshida2014functionalcharacterizationof pages 4-6)
ATP hydrolysis Purified SsaN hydrolyzes ATP in vitro; activity rises nonlinearly with protein concentration, consistent with cooperative assembly/activity. Direct biochemical assay Apparent Kₘ(ATP) = 0.816 ± 0.02 mM; reported activity/Vmax 0.366 ± 0.06 mmol·min⁻¹·mg⁻¹ Yoshida et al., Apr 2014, 10.1371/journal.pone.0094347 (yoshida2014functionalcharacterizationof pages 8-10, yoshida2014functionalcharacterizationof pages 4-6)
Catalytic Arg192 Substitution R192G abolishes detectable ATPase activity and fails to restore SPI-2 secretion, while the protein remains stable. Direct mutagenesis, enzymology, and genetic complementation R192 lies in the conserved ATPase DCCD-box region Yoshida et al., Apr 2014, 10.1371/journal.pone.0094347 (yoshida2014functionalcharacterizationof pages 4-6)
Structure and domains SsaN has a central ATPase-like catalytic domain and a C-terminal chaperone-recognition domain; the bent α10 region contributes to chaperone docking. Direct X-ray crystallography plus mutagenesis; oligomeric ring model remains partly homolog-based 2.1 Å structure, PDB 4NPH; catalytic domain residues 90–331, C-terminal domain 332–433 Allison et al., Aug 2014, 10.1074/jbc.M114.578476 (allison2014identificationofthe pages 4-5, allison2014identificationofthe pages 4-4)
Apparatus partners SsaN interacts with sorting-platform/C-ring-associated proteins SsaK and SsaQ. Direct FLAG pull-down/protein-interaction evidence Both partners detected with SsaN under SPI-2-inducing conditions Yoshida et al., Apr 2014, 10.1371/journal.pone.0094347 (yoshida2014functionalcharacterizationof pages 6-8, yoshida2014functionalcharacterizationof pages 1-2)
Chaperone partners SsaN binds SPI-2 chaperones SsaE, SseA, SscA, and SscB; its C-terminal surface also docks the multicargo chaperone SrcA. Direct pull-down/binding assays; structure-guided mutagenesis for SrcA Binding-defective SsaN variants retained ATPase activity but reduced in-vivo fitness Yoshida et al., Apr 2014, 10.1371/journal.pone.0094347; Allison et al., Aug 2014, 10.1074/jbc.M114.578476 (yoshida2014functionalcharacterizationof pages 6-8, allison2014identificationofthe pages 8-9)
Localization SsaN occurs in soluble/cytoplasmic and membrane fractions and co-fractionates with SsaK/SsaQ after SPI-2 induction; it is membrane-associated, not demonstrated to be an integral membrane protein. Direct cell fractionation Association observed after growth in SPI-2-inducing low-pH medium; not dependent on SsaK or SsaQ Yoshida et al., Apr 2014, 10.1371/journal.pone.0094347 (yoshida2014functionalcharacterizationof pages 6-8, yoshida2014functionalcharacterizationof pages 8-10)
Chaperone–cargo release SsaN releases translocator SseB from chaperone SsaE in an ATP-hydrolysis-dependent reaction. ATPγS and SsaN-R192G do not support release. Direct reconstituted release assay Release requires hydrolysable ATP and catalytically competent Arg192 Yoshida et al., Apr 2014, 10.1371/journal.pone.0094347 (yoshida2014functionalcharacterizationof pages 6-8, yoshida2014functionalcharacterizationof pages 1-2)
Secretion/translocation Loss of ssaN abolishes or strongly impairs secretion of SPI-2 substrates including SseB, SseC, SseD, SseF, SseG, and SseJ and prevents host-cell SseJ translocation; complementation restores function. Some non-SPI-2 effectors can be rerouted through T3SS-1. Direct deletion, complementation, secretion, and host-cell translocation assays Whole-cell SseJ abundance was unchanged, localizing the defect to secretion rather than synthesis Yoshida et al., Apr 2014, 10.1371/journal.pone.0094347; Allison et al., Aug 2014, 10.1074/jbc.M114.578476 (yoshida2014functionalcharacterizationof pages 4-6, allison2014identificationofthe pages 4-4)
Mouse virulence An ssaN deletion is severely attenuated during systemic infection; plasmid complementation restores competitive fitness. Direct mixed-infection experiment Mutant CI = 0.0476 ± 0.013; complemented strain CI = 1.16 ± 0.21 Yoshida et al., Apr 2014, 10.1371/journal.pone.0094347 (yoshida2014functionalcharacterizationof pages 6-8)
Current family model In Yersinia enterocolitica, mobile cytosolic sorting-platform complexes containing SctN, SctK, SctL, and SctQ bind effectors and shuttle them to injectisomes. This supports—but does not directly prove for SsaN—a dynamic cargo-sorting/delivery role. Family-level inference only; not direct Salmonella SPI-2 evidence Secretion activation increased estimated injectisomes from approximately 5 to 18 per bacterium Wimmi et al., Jan 2024, 10.1038/s41564-023-01545-1 (wimmi2024cytosolicsortingplatform pages 1-2, pintor2024thepathand pages 146-149)

Table: Compact evidence matrix for the verified Salmonella SPI-2 ATPase SsaN/SctN2, separating direct biochemical, structural, localization, secretion, and virulence findings from recent family-level inference.


References and Data Availability

All cited DOI URLs provide direct access to peer-reviewed publications. Crystal-structure coordinates are available in the Protein Data Bank (PDB 4NPH). Quantitative data presented here (kinetic constants, competitive indices, microscopy counts) are drawn directly from primary literature as cited. The UniProt accession P74857 consolidates cross-references, domain annotations, and orthology relationships.

Date of Report: 2026
Literature Coverage: 1997–2024
Evidence Level: Direct experimental data for Salmonella Typhimurium SsaN; family-level inference from conserved T3SS systems noted explicitly.

References

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  2. (yoshida2014functionalcharacterizationof pages 4-6): Yukie Yoshida, Tsuyoshi Miki, Sayaka Ono, Takeshi Haneda, Masahiro Ito, and Nobuhiko Okada. Functional characterization of the type iii secretion atpase ssan encoded by salmonella pathogenicity island 2. PLoS ONE, 9:e94347, Apr 2014. URL: https://doi.org/10.1371/journal.pone.0094347, doi:10.1371/journal.pone.0094347. This article has 60 citations and is from a peer-reviewed journal.

  3. (yoshida2014functionalcharacterizationof pages 1-2): Yukie Yoshida, Tsuyoshi Miki, Sayaka Ono, Takeshi Haneda, Masahiro Ito, and Nobuhiko Okada. Functional characterization of the type iii secretion atpase ssan encoded by salmonella pathogenicity island 2. PLoS ONE, 9:e94347, Apr 2014. URL: https://doi.org/10.1371/journal.pone.0094347, doi:10.1371/journal.pone.0094347. This article has 60 citations and is from a peer-reviewed journal.

  4. (hensel1997functionalanalysisof pages 9-10): Michael Hensel, Jacqueline E. Shea, Bärbel Raupach, Denise Monack, Stanley Falkow, Colin Gleeson, Toru Kubo, and David W. Holden. Functional analysis of ssaj and the ssak/u operon, 13 genes encoding components of the type iii secretion apparatus of salmonella pathogenicity island 2. Molecular Microbiology, 24:155-167, Apr 1997. URL: https://doi.org/10.1046/j.1365-2958.1997.3271699.x, doi:10.1046/j.1365-2958.1997.3271699.x. This article has 258 citations and is from a domain leading peer-reviewed journal.

  5. (yu2004ssamandspic pages 1-2): Xiu‐Jun Yu, Mei Liu, and David W. Holden. Ssam and spic interact and regulate secretion of salmonella pathogenicity island 2 type iii secretion system effectors and translocators. Molecular Microbiology, 54:604-619, Nov 2004. URL: https://doi.org/10.1111/j.1365-2958.2004.04297.x, doi:10.1111/j.1365-2958.2004.04297.x. This article has 82 citations and is from a domain leading peer-reviewed journal.

  6. (yoshida2014functionalcharacterizationof pages 8-10): Yukie Yoshida, Tsuyoshi Miki, Sayaka Ono, Takeshi Haneda, Masahiro Ito, and Nobuhiko Okada. Functional characterization of the type iii secretion atpase ssan encoded by salmonella pathogenicity island 2. PLoS ONE, 9:e94347, Apr 2014. URL: https://doi.org/10.1371/journal.pone.0094347, doi:10.1371/journal.pone.0094347. This article has 60 citations and is from a peer-reviewed journal.

  7. (pintor2024thepathand pages 34-37): K Pintor. The path and regulation of the type iii secretion effector export. Unknown journal, 2024.

  8. (wimmi2024cytosolicsortingplatform pages 1-2): Stephan Wimmi, Alexander Balinovic, Corentin Brianceau, Katherine Pintor, Jan Vielhauer, Bartosz Turkowyd, Carlos Helbig, Moritz Fleck, Katja Langenfeld, Jörg Kahnt, Timo Glatter, Ulrike Endesfelder, and Andreas Diepold. Cytosolic sorting platform complexes shuttle type iii secretion system effectors to the injectisome in yersinia enterocolitica. Nature Microbiology, 9:185-199, Jan 2024. URL: https://doi.org/10.1038/s41564-023-01545-1, doi:10.1038/s41564-023-01545-1. This article has 25 citations and is from a highest quality peer-reviewed journal.

  9. (pintor2024thepathand pages 146-149): K Pintor. The path and regulation of the type iii secretion effector export. Unknown journal, 2024.

  10. (allison2014identificationofthe pages 8-9): Sarah E. Allison, Brian R. Tuinema, Ellen S. Everson, Seiji Sugiman-Marangos, Kun Zhang, Murray S. Junop, and Brian K. Coombes. Identification of the docking site between a type iii secretion system atpase and a chaperone for effector cargo. Journal of Biological Chemistry, 289:23734-23744, Aug 2014. URL: https://doi.org/10.1074/jbc.m114.578476, doi:10.1074/jbc.m114.578476. This article has 47 citations and is from a domain leading peer-reviewed journal.

  11. (allison2014identificationofthe pages 4-4): Sarah E. Allison, Brian R. Tuinema, Ellen S. Everson, Seiji Sugiman-Marangos, Kun Zhang, Murray S. Junop, and Brian K. Coombes. Identification of the docking site between a type iii secretion system atpase and a chaperone for effector cargo. Journal of Biological Chemistry, 289:23734-23744, Aug 2014. URL: https://doi.org/10.1074/jbc.m114.578476, doi:10.1074/jbc.m114.578476. This article has 47 citations and is from a domain leading peer-reviewed journal.

  12. (yoshida2014functionalcharacterizationof pages 6-8): Yukie Yoshida, Tsuyoshi Miki, Sayaka Ono, Takeshi Haneda, Masahiro Ito, and Nobuhiko Okada. Functional characterization of the type iii secretion atpase ssan encoded by salmonella pathogenicity island 2. PLoS ONE, 9:e94347, Apr 2014. URL: https://doi.org/10.1371/journal.pone.0094347, doi:10.1371/journal.pone.0094347. This article has 60 citations and is from a peer-reviewed journal.

  13. (allison2014identificationofthe pages 4-5): Sarah E. Allison, Brian R. Tuinema, Ellen S. Everson, Seiji Sugiman-Marangos, Kun Zhang, Murray S. Junop, and Brian K. Coombes. Identification of the docking site between a type iii secretion system atpase and a chaperone for effector cargo. Journal of Biological Chemistry, 289:23734-23744, Aug 2014. URL: https://doi.org/10.1074/jbc.m114.578476, doi:10.1074/jbc.m114.578476. This article has 47 citations and is from a domain leading peer-reviewed journal.

  14. (allison2014identificationofthe pages 3-4): Sarah E. Allison, Brian R. Tuinema, Ellen S. Everson, Seiji Sugiman-Marangos, Kun Zhang, Murray S. Junop, and Brian K. Coombes. Identification of the docking site between a type iii secretion system atpase and a chaperone for effector cargo. Journal of Biological Chemistry, 289:23734-23744, Aug 2014. URL: https://doi.org/10.1074/jbc.m114.578476, doi:10.1074/jbc.m114.578476. This article has 47 citations and is from a domain leading peer-reviewed journal.

  15. (pintor2024thepathand pages 13-16): K Pintor. The path and regulation of the type iii secretion effector export. Unknown journal, 2024.

Artifacts

Citations

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  3. wimmi2024cytosolicsortingplatform pages 1-2
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  5. allison2014identificationofthe pages 8-9
  6. yoshida2014functionalcharacterizationof pages 8-10
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  11. allison2014identificationofthe pages 3-4
  12. hensel1997functionalanalysisof pages 1-2
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