Type III Secretion System

The Type III Secretion System (T3SS), also known as the injectisome, is a macromolecular protein-export apparatus (~3.5–4.0 MDa, composed of more than 20 distinct proteins in over 200 subunit copies) that spans the inner cytoplasmic membrane, periplasmic peptidoglycan layer, and outer membrane of Gram-negative bacteria—and projects an extracellular hollow needle to puncture the plasma or vacuolar membrane of target eukaryotic host cells. In a single continuous step across three biological membranes (bacterial inner membrane, bacterial outer membrane, and eukaryotic host membrane), the T3SS translocates unfolded bacterial effector proteins directly from the bacterial cytoplasm into the host cytosol at rates of 7–60 effector molecules per second.

Encoded primarily on horizontally acquired pathogenicity islands (such as SPI-1 and SPI-2 in Salmonella enterica, the locus of enterocyte effacement [LEE] in enteropathogenic and enterohemorrhagic Escherichia coli) or on large virulence plasmids (such as the pCD1/pYV plasmid in Yersinia pestis and pWR100 in Shigella flexneri), the virulence-associated injectisome shares a conserved core of nine inner-membrane and cytoplasmic export proteins (SctR, SctS, SctT, SctU, SctV, SctJ, SctN, SctO, SctQ) with the ancestral bacterial flagellar basal body, from which it evolved via exaptation for inter-kingdom protein translocation.

To unify the historically divergent nomenclature used across Yersinia (Ysc), Salmonella (Inv/Spa and Ssa), Shigella (Mxi/Spa), Pseudomonas (Psc), and plant pathogens (Hrc), structural biologists use the unified Secretion and Cellular Translocation (Sct) nomenclature. Cryo-electron tomography and single-particle cryo-EM reconstructions divide the injectisome into four coaxial subcomplexes:

Because the inner channel of the SctF needle is only 15–20 Å (1.5–2.0 nm) in diameter, fully folded globular proteins cannot pass through it; substrates must be threaded through the lumen in an unfolded or partially α-helical conformation (with the N-terminus leading). Furthermore, the injectisome builds its own extracellular needle and tip by secreting its structural subunits in a strict three-tier temporal hierarchy:

1. Tier 1 — Early Substrates (Inner Rod SctI and Needle Monomer SctF): Once the membrane rings (SctC–SctD–SctJ) and export apparatus (SctRSTUV) assemble, the injectisome secretes SctI and SctF monomers, which polymerize at the distal tip of the growing needle alongside a secreted molecular ruler protein (SctP, such as YscP in Yersinia or InvJ in Salmonella).
2. First Specificity Switch (SctP Molecular Ruler → SctU Autoproteolysis): The length of the extracellular needle (~45 nm in Salmonella SPI-1; ~60 nm in Yersinia Ysc) scales linearly with the amino acid length of the SctP ruler. When the needle reaches its programmed length, the N-terminus of SctP tethered at the growing tip stretches its C-terminal domain against the cytoplasmic SctU (YscU / SpaS) switch protein—which has already undergone intramolecular autoproteolysis at its conserved NPTH motif (Asn–Pro–Thr–His). This interaction flips the first specificity switch: arresting further needle elongation and transitioning the machine to secrete Tier 2 middle substrates.
3. Tier 2 — Middle Substrates (Hydrophilic Tip SctA and Hydrophobic Translocon SctE / SctB): Five copies of the hydrophilic tip protein SctA (LcrV in Yersinia, SipD in Salmonella, IpaD in Shigella) cap the distal end of the needle in a quiescent state, while a cytoplasmic gatekeeper complex (SctW, e.g., YopN–TyeA or InvE/MxiC) blocks premature secretion of late virulence effectors into the extracellular medium.
4. Tier 3 — Host-Cell Contact and Late Virulence Effector Translocation: Physical contact between the SctA tip and host plasma membrane lipids (cholesterol and sphingomyelin) or integrins triggers insertion of the hydrophobic SctE–SctB translocon pore (~2.5–3.5 nm inner diameter) into the host membrane. A conformational signal propagates down the needle filament to release the cytoplasmic SctW gatekeeper, opening the export channel for rapid translocation of late virulence effectors driven jointly by SctN ATPase chaperone stripping/unfolding and the inner-membrane protonmotive force (ΔpH and ΔΨ).

Although the structural nanosyringe (Sct core) is conserved across Gram-negative clades, the translocated effector repertoire (ranging from 6 core Yops in Yersinia to >40 effectors in Salmonella and >70 effectors in plant-pathogenic Pseudomonas syringae and Ralstonia) is highly modular and drives opposite cellular phenotypes depending on the pathogen's ecological niche:

• Cytoskeletal Ruffling and Forced Invasion (Salmonella SPI-1 & Shigella Mxi-Spa): Intestinal Salmonella enterica uses its SPI-1 T3SS to inject SopE/SopE2 (guanine nucleotide exchange factors that activate host Rho-family GTPases Cdc42 and Rac1), SopB (phosphoinositide phosphatase), and SipA/SipC (actin-nucleating and bundling proteins) into non-phagocytic intestinal epithelial cells—triggering dramatic actin membrane ruffling (macropinocytosis) that engulfs the bacterium; SptP (a GTPase-activating protein and tyrosine phosphatase) then reverses Cdc42/Rac1 activation to restore normal epithelial architecture once the bacterium is inside.
• Intravacuolar Niche Maintenance (Salmonella SPI-2): Once inside the acidified Salmonella-containing vacuole (SCV) of macrophages, low pH (~5.0), low Mg²⁺, and low phosphate activate the PhoP/PhoQ and SsrA/SsrB two-component systems to express a second, distinct injectisome (SPI-2 T3SS), which translocates SifA, SseF, SseG, and PipB2 across the vacuolar membrane to remodel host kinesin/dynein motors and block lysosomal fusion.
• Phagocytosis Blockade and Immune Paralysis (Yersinia pestis / Y. pseudotuberculosis): Extracellular Yersinia uses its plasmid-encoded Ysc T3SS to paralyze macrophages and neutrophils rather than invade them: YopH (one of the most catalytically rapid protein tyrosine phosphatases known, k_cat > 1,000 s⁻¹) dephosphorylates focal adhesion kinase (FAK) and p130Cas; YopE (Rho GAP) and YopT (cysteine protease that cleaves theprenylated C-terminus of RhoA) collapse the actin cytoskeleton; YopJ/YopP (acetyltransferase) acetylates IKKβ and MAPKKs to block NF-κB/MAPK pro-inflammatory signaling and trigger macrophage pyroptosis/apoptosis.
• Pedestal Formation (E. coli EPEC/EHEC LEE Island): Extracellular enteropathogenic and enterohemorrhagic E. coli inject their own receptor—Tir (Translocated Intimin Receptor)—into the enterocyte plasma membrane, where its extracellular loop binds bacterial outer-membrane Intimin while its cytosolic domain recruits host Nck and N-WASP–Arp2/3 to polymerize raised actin attaching-and-effacing (A/E) pedestals beneath the adherent bacterium.

Did the Type III Secretion System evolve from the bacterial flagellum, or did the flagellum evolve from the T3SS?

Phylogenomic analyses across bacterial phyla demonstrate that the bacterial flagellum is ancestral and the virulence injectisome (vT3SS) evolved from the flagellar export apparatus:

1. Flagellar systems are distributed broadly across deeply branching bacterial phyla (including Firmicutes/Bacillota, Thermotogota, Aquificota, Spirochaetota, and Gracilicutes) and enable free-living chemotactic motility independent of eukaryotes.
2. In contrast, virulence injectisomes are restricted to Gram-negative clades (predominantly Proteobacteria/Pseudomonadota, Chlamydiota, and Verrucomicrobiota) that interact intimately with eukaryotic hosts (animals, plants, fungi, or amoebae), which emerged over a billion years after bacteria.
3. During the evolution of the injectisome from a flagellar ancestor, the rotary motor stator (MotA/MotB) and helical propeller (FliC flagellin) were lost, while an outer-membrane secretin ring (SctC) was recruited via horizontal module shuffling from Type II / Type IV pilus systems to breach the Gram-negative outer membrane.

How can researchers or therapeutics disarm the T3SS without killing commensal gut bacteria?

Because the T3SS is essential for virulence in pathogens (Salmonella, Shigella, EHEC, Yersinia, Pseudomonas aeruginosa) but is absent in beneficial commensal gut bacteria (such as Bacteroides, Bifidobacterium, and Lactobacillus) and non-essential for bacterial growth in vitro, targeting the T3SS represents a leading "anti-virulence" strategy:

• Monoclonal Antibodies Against the Needle Tip (SctA): Because the hydrophilic tip complex (PcrV in Pseudomonas aeruginosa, LcrV in Yersinia pestis, IpaD in Shigella) is surface-exposed at the distal tip of the needle and required for translocon pore insertion, anti-PcrV and anti-LcrV antibodies sterically block effector translocation and protect against lethal challenge in preclinical and clinical respiratory/plague models.
• Small-Molecule Salicylidene Acylhydrazides and Phenoxyacetamides: Inhibit needle assembly (SctF polymerization) or SctN ATPase activity without exerting bactericidal selective pressure on the broader microbiome.

1. Galán, J. E., & Wolf-Watz, H. (2006). "Protein delivery into eukaryotic cells by type III secretion machines." Nature, 444(7119), 567–573. https://doi.org/10.1038/nature05272
2. Deng, W., Marshall, N. C., Rowland, J. L., et al. (2017). "Assembly, structure, function and regulation of type III secretion systems." Nature Reviews Microbiology, 15(6), 323–337. https://doi.org/10.1038/nrmicro.2017.20
3. Hu, J., Worrall, L. J., Vuckovic, M., et al. (2018). "Cryo-EM analysis of the T3S injectisome reveals the structure of the needle and open secretin." Nature Communications, 9(1), 3840. https://doi.org/10.1038/s41467-018-06298-8
4. Abby, S. S., & Rocha, E. P. C. (2012). "The non-flagellar type III secretion system evolved from the bacterial flagellum and diversified into host-cell adapted systems." PLoS Genetics, 8(9), e1002983. https://doi.org/10.1371/journal.pgen.1002983