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3.A.6 The Type III (Virulence-related) Secretory Pathway (IIISP) Family

Proteins of the IIISP family are found in Gram-negative bacteria and allow secretion of cytoplasmically synthesized proteins across both membranes of the cell envelope (Saier, 2007). They are often concerned with secretion of virulence factors in pathogenic Gram-negative bacteria. These genes are sometimes chromosomally-encoded, in which case they are often found within 'pathogenicity islands' which are inserted DNA segments derived from foreign organismal sources. They may also be plasmid or phage-encoded. Most of these proteins are homologous to proteins concerned with bacterial (both Gram-positive and Gram-negative) flagellar protein export, and the flagellar export machinery has been shown to be capable of secreting virulence factors. They are thus functionally and structurally equivalent, although the constituents generally cluster seperately (Nguyen et al., 2000). Type III systems may transport fully or partially folded substrate proteins (Lee and Schneewind, 2002). The targeting sequence may be in the N-termini of the substrate protein (Hirano et al., 1993; Sorg et al., 2007), but other evidence has led to the suggestions that the system may recognize tertiary structure, or even elements in the mRNA (Petnicki-Ocwieja et al., 2002). The topologies of several of the inner membrane core proteins are known (Berger et al., 2010). Membrane targeting and pore formation by the type III secretion system translocon are discussed by Mattei et al. (2011). Translocation of cell surface-localized effectors can occur via type III secretion systems (Akopyan et al., 2011).

Several findings have illuminated the evolution of flagella. Cut-down versions of flagella in Buchnera and a dispensable ATPase indicate the occurance of simpler versions of the flagellum. Morever, structural evidence for homology between FliG (a component of the flagellar motor) and MgtE (a magnesium transporter) have come to light (Snyder et al., 2009). Examination of the phylogenetic distribution of flagellar genes warns against a simplistic model of early flagellar evolution. The organization and coordinated assembly of the type III secretion export apparatus has been studied in detail (Wagner et al., 2010). 

A high-resolution in situ structure of the intact machine from Shigella was revealed by high-throughput cryoelectron tomography (cryo- ET) showing a cytoplasmic sorting platform consisting of a central hub and six spokes, with a pod-like structure at the terminus of each spoke (Hu et al. 2015). Molecular modeling allowed proposal of a structure of the sorting platform in which the hub consists mainly of a hexamer of the Spa47 ATPase, whereas the MxiN protein comprises the spokes and the Spa33 protein forms the pods. Multiple contacts among those components are essential to align the Spa47 ATPase with the central channel of the MxiA protein export gate to form a unique nanomachine.

The biochemical functions of most of the individual constituents of type IIISP systems are not known. However, one constituent is an ATPase (EC that is believed to allow the coupling of ATP hydrolysis to protein export, and six proteins are found in the inner membrane where they may form a complex that provides the transport pathway. The best characterized systems are derived from Yersinia species. These export Yersinia proteins (YOPS). [N-terminal and internal protein secretion signals as well as mRNA signals have been proposed to target proteins to the secretory apparatus.] The ATPase in this system is YscN while the integral inner membrane proteins are LcrD and YscD, R, S, T and U. They exhibit the following numbers of putative transmembrane spanners: LcrD, 8; YscD, 1; YscR, 4; YscS, 2; YscT, 6; YscU, 4. The YscC protein, an outer membrane protein that may form an oligomeric pore (outer diameter of ~200 Å; inner pore diameter of ~50 Å), belongs to the secretin family (TC #1.B.22). As many as 20 proteins may comprise the secretion apparatus.

As noted above, type III, virulence-related, secretion systems are related to the flagellar secretion apparatus, and the latter may have given rise to the former. Phylogenetic analyses have shown that the flagellar (Fla) and type III secretion systems have evolved with little or no shuffling of protein constituents between systems although lateral transfer of type III systems (but not of Fla systems) between various Gram-negative bacterial species has occurred frequently (Nguyen et al., 2000). The bacterial Fla systems use ATP which is hydrolyzed by FliI to drive export of well over a dozen proteins. These include structural constituents of the basal body, the hook, the hook capping and scaffolding protein, the two hook-filament junctional proteins, a hook-length control protein, flagellin and the flagellin capping protein. Exported regulatory proteins include the antiflagellar σ factor, FlgM, and, a muramidase, FlgJ, that allows penetration of the peptidoglycan layer by the nascent rod. FliI forms a hexameric ring-like structure in the presence of ATP with a central cavity of 2.5-3.0 nm, the same size as the flagellar export channel. The enzyme exhibits cooperativity, and cooperativity is enhanced by the presence of E. coli phospholipids (Claret et al., 2003). The FliM, N, H, I and G proteins form a physical complex, the c-ring. The chaperone protein, FliJ, binds to FliM. This complex serves to energize and regulate secretion, but has several other functions as well (Gonzalez-Pedrajo et al., 2006). FliH and FliI ensure robust and efficient energy coupling of protein export during flagellar assembly (Minamino et al. 2016).

FlhB, with 4 putative TMSs, appears to gate the flagellar export pathway and determines the substrates transported (Fraser et al., 2003). An oligomeric form of it may comprise all or part of the channel for protein export. After translocation across the cytoplasmic membrane, the proteins diffuse sequentially down the channel formed by the secretion apparatus at the nascent flagellum, and they assemble at its distal end. The export apparatus includes three cytoplasmic proteins (FliH, I and J) and six integral cytoplasmic membrane proteins (FlhA and B; FlhO, P, Q and R), but a secretin, present in type III secretion systems, is absent. Early and late flagellar subunits dock at the ATPase, FliI, and are probably distinguished (Stafford et al., 2007) not by late chaperones but by N-terminal export signals of the subunits themselves. FlhA and FlhB form a docking platform for the 'soluble' components of the export apparatus FliH, FliI and FliJ. The C-terminal cytoplasmic domain of FlhA (FlhAC) is required for protein export. Minamino et al. (2010) showed that FlhAC not only forms a part of the docking platform for the FliH-FliI-FliJ-export substrate complex but also is directly involved in the translocation of export substrates into the central channel of the growing flagellar structure.

Type IIISP systems often secrete their products, or some of their products directly into the host cell cytoplasm without an intermediary stage in the extracellular millieu. This fact implies the existence of a pore complex that spans the host cell cytoplasmic membrane and is contiguous with the bacterial secretion apparatus. The Yersinia protein that is believed to provide this function is YopB. YopB (401 aas; spP37131) exhibits two hydrophobic domains (residues 168-208 and 224-258) that may span the host cell cytoplasmic membrane. The protein probably forms an oligomeric pore complex. This protein is homologous to PepB of Pseudomonas aeruginosa (gbAF035922), which may serve the same function. It may show some similarity with bacterial toxins such as IpaB of S. flexneri (580 aas; spP18011) and SipB of S. typhimurium (593 aas; gbU25631) as well as with a diverse group of eukaryotic proteins. These proteins comprise the Bacterial Type III-Target Cell Pore (IIITCP) family (TC #1.C.36).

The pathway for protein transport from the bacterial cell surface is not well defined. One report (Jin and He, 2001) suggests that the pilus of a Pseudomonas syringae type III protein secretion system functions as a conduit for protein delivery. Another report (Sekiya et al., 2001) suggests that in the enteropathogenic E. coli type III secretion system, the EspA protein forms a filamentous structure that assembles as a physical bridge between the bacterial surface and the host cell surface, where the EspB/EspD proteins form the pore in the host membrane. EspA may thus provide the pathway for protein transfer between bacterial and animal cells. A hydrophilic protein forms a complex on the distal end of the injectisome needle, the tip complex, and serves as an assembly platform for the two hydrophobic translocators, EspB/D (Mueller et al., 2008). The structure of the needle protein is divergent from the flagellar filament protein (Galkin et al., 2010).

The 'injectisome', consisting of more than 20 different proteins, has been viewed as a result of T3SS crystal structures of the major oligomeric inner membrane ring, the helical needle filament, the needle tip protein, the associated ATPase, and outer membrane pilotin (Moraes et al., 2008). Two reports have demonstrated that the FliH-FliI complex facilitates only the initial entry of export substrates into the gate, with the energy of ATP hydrolysis being used to disassemble and release the FliH-FliI complex from the protein about to be exported. The rest of the successive unfolding/translocation process of the substrates is driven by the proton motive force (Minamino and Namba, 2008; Paul et al., 2008).

During assembly of the T3SS, as well as the evolutionarily related flagellar apparatus, a post-translational cleavage event within the inner membrane proteins EscU/FlhB is required to promote a secretion-competent state. These proteins have long been proposed to act as a part of a molecular switch, which would regulate the appropriate chronological secretion of the various T3SS apparatus components during assembly and subsequently the transported virulence effectors. Zarivach et al. (2008) showed that a surface type II beta-turn in the stabliized Escherichia coli protein EscU undergoes auto-cleavage by a mechanism involving cyclization of a strictly conserved asparagine residue. Structural and in vivo analysis of point and deletion mutations illustrates the subtle conformational effects of auto-cleavage in modulating the molecular features of a highly conserved surface region of EscU, a potential point of interaction with other T3SS components at the inner membrane. Deane et al. (2008) have determined the crystal structure of the cytoplasmic complex of a homologue, Spa40 of Shigella flexneri. Riordan & Schneewind (2008) suggest that auto-cleavage of YscU in Yersinia promotes interaction with YschU and recruitment of ATPase complexes that initiate secretin.

The assembly of a type III secretion injectisome culminates in the formation of the needle. In Yersinia, this step requires not only the needle subunit (YscF), but also the small components YscI, YscO, YscX and YscY. Diepold et al. (2012) found that these elements act after the completion of the transmembrane export apparatus. YscX and YscY co-purified with the export apparatus protein YscV. YscY is probably present in multiple copies. YscO and YscX are required for export of the early substrates, YscF, YscI and YscP, but they were only exported after the substrate specificity switch had occurred. Unlike its flagellar homologue FliJ, YscO was not required for assembly of the ATPase, YscN.  No export of the reporter substrate, YscP(1-137)-PhoA, into the periplasm was observed in absence of YscI, YscO or YscX, confirming that these proteins are required for export of the first substrates. In contrast, YscP(1-137)-PhoA accumulated in the periplasm in the absence of YscF, suggesting that YscF is not required for the function of the export apparatus, but that its polymerization opens the secretin YscC channel (Diepold et al. 2012). 

The flagellar type III export apparatus utilizes ATP and the proton motive force (PMF) to transport flagellar proteins to the distal end of the growing flagellar structure for self-assembly. The transmembrane export gate complex is a H+-protein antiporter, of which activity is greatly augmented by an associated cytoplasmic ATPase complex. Minamino et al. 2016 reported that the export gate complex can use the sodium motive force (SMF) in addition to the PMF to drive protein export. Protein export was considerably reduced in the absence of the ATPase complex and a pH gradient across the membrane, but Na+ increased it dramatically. Phenamil, a blocker of Na+ translocation, inhibited protein export. Overexpression of FlhA increased the intracellular Na+ concentration in the presence of 100 mM external NaCl but not in its absence, suggesting that FlhA acts as a Na+ channel. In wild-type cells, however, neither Na+ nor phenamil affected protein export, indicating that the Na+ channel activity of FlhA is suppressed by the ATPase complex. Minamino et al. 2016 proposed that the export gate by itself is a dual fuel engine that uses both the PMF and the SMF for protein export, and that the ATPase complex switches this dual fuel engine into a PMF-driven export machinery to become much more robust against environmental changes in external pH and Na+ concentration. 

(Gaytán et al. 2016). The core architecture of the T3SS consists of a multi-ring basal body embedded in the bacterial envelope, a periplasmic inner rod, a transmembrane export apparatus in the inner membrane, and cytosolic components including an ATPase complex and the C-ring. Two distinct hollow appendages are assembled on the extracellular face of the basal body creating a channel for protein secretion: an approximately 23 nm needle, and a filament that extends up to 600 nm. This filamentous structure allows E. coli pathogens to get through the host cells mucus barrier. Upon contact with the target cell, a translocation pore is assembled in the host membrane through which the effector proteins are injected. Assembly of the T3SS is strictly regulated to ensure proper timing of substrate secretion. The different type III substrates coexist in the bacterial cytoplasm, and their hierarchical secretion is determined by specialized chaperones in coordination with two molecular switches and the so-called sorting platform (Gaytán et al. 2016).  Type III small hydrophobic export apparatus components SpaP and SpaR nucleate assembly of the needle complex and form the central 'cup' substructure of a Salmonella Typhimurium secretion system. Gaytán et al. 2016 presented evidence that a SpaP pentamer forms a 15 Å wide pore. They provided a map of SpaP interactions with the export apparatus components SpaQ, SpaR, and SpaS. They also refined the current view of export apparatus assembly, consolidated transmembrane topology models for SpaP and SpaR, and suggested interactions of the periplasmic domains of SpaP and SpaR with the inner rod protein PrgJ. Their results indicated how the export apparatus and needle filament are connected to create a continuous conduit for substrate translocation. 

Protein export via the T3SS is energized by the proton gradient. Erhardt et al. 2017 used a mutational approach to identify proton-binding groups that might function in transport. Conserved proton-binding residues in all the membrane components were tested. The results identified residues R147, R154 and D158 of FlhA. These lie in a small, well-conserved cytoplasmic domain of FlhA, located between transmembrane segments 4 and 5, and this domain forms a multimeric array. A mutation that mimiced protonation of the key acidic residue (D158N) was shown to trigger a global conformational change that affects the other, larger cytoplasmic domain that interacts with the export cargo. The results suggest a transport model based on proton-actuated movements in the cytoplasmic domains of FlhA (Erhardt et al. 2017).

As noted above, the flagellar type III export apparatus utilizes ATP and the PMF) as energy sources, and transports flagellar component proteins from the cytoplasm to the distal growing end of the growing flagellar structure. The apparatus coordinates flagellar protein export with assembly by ordered export of substrates to parallel with their order of the assembly. The export apparatus is composed of a PMF-driven transmembrane export gate complex and a cytoplasmic ATPase complex. Since the ATPase complex is dispensable for flagellar protein export, the PMF is the primary fuel for protein unfolding and translocation (Minamino et al. 2017).

The generalized reaction catalyzed by type IIISP systems is:

Protein (bacterial cytoplasm) + ATP + pmf (or smf) + H+ or Na+ (out) → Protein (out or in the host cell cytoplasm) +  ADP + Pi + H+ or Na+ (in)

References associated with 3.A.6 family:

Aizawa, S.-I. (1996). Flagellar assembly in Salmonella typhimurium. Mol. Microbiol. 19: 1-5. 8821931
Akopyan, K., T. Edgren, H. Wang-Edgren, R. Rosqvist, A. Fahlgren, H. Wolf-Watz, and M. Fallman. (2011). Translocation of surface-localized effectors in type III secretion. Proc. Natl. Acad. Sci. USA 108: 1639-1644. 21220342
Berger, C., G.P. Robin, U. Bonas, and R. Koebnik. (2010). Membrane topology of conserved components of the type III secretion system from the plant pathogen Xanthomonas campestris pv. vesicatoria. Microbiology 156: 1963-1974. 20378646
Claret, L., S.R. Calder, M. Higgins, and C. Hughes. (2003). Oligomerization adn activation of the Flil ATPase central to bacterial flagellum assembly. Mol. Microbiol. 48: 1349-1355. 12787361
Cornelis, G.R. (1998). The Yersinia deadly kiss. J. Bacteriol. 180: 5495-5504. 9791096
Cornelis, G.R. and F. Van Gijsegem. (2000). Assembly and function of type III secretory systems. Annu. Rev. Microbiol. 54: 735-774. 11018143
Cornelis, G.R., A. Boland, A.P. Boyd, C. Geuijen, M. Iriarte, C. Neyt, M.-P. Sory, and I. Stainier. (1998). The virulence plasmid of Yersinia, and antihost genome. Microbiol. Mol. Biol. Rev. 62: 1315-1352. 9841674
Deane, J.E., S.C. Graham, E.P. Mitchell, D. Flot, S. Johnson, and S.M. Lea. (2008). Crystal structure of Spa40, the specificity switch for the Shigella flexneri type III secretion system. Mol. Microbiol. 69: 267-276. 18485071
Dehoux, P., R. Flores, C. Dauga, G. Zhong, and A. Subtil. (2011). Multi-genome identification and characterization of chlamydiae-specific type III secretion substrates: the Inc proteins. BMC Genomics 12: 109. 21324157
Diepold, A., U. Wiesand, and G.R. Cornelis. (2011). The assembly of the export apparatus (YscR,S,T,U,V) of the Yersinia type III secretion apparatus occurs independently of other structural components and involves the formation of an YscV oligomer. Mol. Microbiol. 82: 502-514. 21923772
Diepold, A., U. Wiesand, M. Amstutz, and G.R. Cornelis. (2012). Assembly of the Yersinia injectisome: the missing pieces. Mol. Microbiol. 85: 878-892. 22788867
Dietsche, T., M. Tesfazgi Mebrhatu, M.J. Brunner, P. Abrusci, J. Yan, M. Franz-Wachtel, C. Schärfe, S. Zilkenat, I. Grin, J.E. Galán, O. Kohlbacher, S. Lea, B. Macek, T.C. Marlovits, C.V. Robinson, and S. Wagner. (2016). Structural and Functional Characterization of the Bacterial Type III Secretion Export Apparatus. PLoS Pathog 12: e1006071. 27977800
Erhardt, M., P. Wheatley, E.A. Kim, T. Hirano, Y. Zhang, M.K. Sarkar, K.T. Hughes, and D.F. Blair. (2017). Mechanism of type-III protein secretion: Regulation of FlhA conformation by a functionally critical charged-residue cluster. Mol. Microbiol. 104: 234-249. 28106310
Erlandson, K.J., E. Or, A.R. Osborne, and T.A. Rapoport. (2008). Analysis of polypeptide movement in the SecY channel during SecA-mediated protein translocation. J. Biol. Chem. 283: 15709-15715. 18359943
Fabiani, F.D., T.T. Renault, B. Peters, T. Dietsche, E.J.C. Gálvez, A. Guse, K. Freier, E. Charpentier, T. Strowig, M. Franz-Wachtel, B. Macek, S. Wagner, M. Hensel, and M. Erhardt. (2017). A flagellum-specific chaperone facilitates assembly of the core type III export apparatus of the bacterial flagellum. PLoS Biol 15: e2002267. 28771474
Fan, F., K. Ohnishi, N.R. Francis, and R.M. McNab. (1997). The FliP and FliR proteins of Salmonella typhimurium, putative components of the type III flagellar export apparatus, are located in the flagellar basal body. Mol. Microbiol. 26: 1035-1046. 9426140
Fraser, G.M., T. Hirano, H.U. Ferris, L.L. Devgan, M. Kihara, and R.M. Macnab. (2003). Substrate specificity of type III flagellar protein export in Salmonella is controlled by subdomain interactions in FlhB. Mol. Microbiol. 48: 1043-1057. 12753195
Galán, J.E. and A. Collmer. (1999). Type III secretion machines: bacterial devices for protein delivery into host cells. Science 284: 1322-1328. 10334981
Galkin, V.E., W.H. Schmied, O. Schraidt, T.C. Marlovits, and E.H. Egelman. (2010). The structure of the Salmonella typhimurium type III secretion system needle shows divergence from the flagellar system. J. Mol. Biol. 396: 1392-1397. 20060835
Gaytán, M.O., V.I. Martínez-Santos, E. Soto, and B. González-Pedrajo. (2016). Type Three Secretion System in Attaching and Effacing Pathogens. Front Cell Infect Microbiol 6: 129. 27818950
González-Pedrajo, B., T. Minamino, M. Kihara, and K. Namba. (2006). Interactions between C ring proteins and export apparatus components: a possible mechanism for facilitating type III protein export. Mol. Microbiol. 60: 984-998. 16677309
Hirano, T., T. Minamino, K. Namba, and R.M. Macnab. (2003). Substrate specificity classes and the recognition signal for Salmonella type III flagellar export. J. Bacteriol. 185: 2485-2492. 12670972
Hu, B., D.R. Morado, W. Margolin, J.R. Rohde, O. Arizmendi, W.L. Picking, W.D. Picking, and J. Liu. (2015). Visualization of the type III secretion sorting platform of Shigella flexneri. Proc. Natl. Acad. Sci. USA 112: 1047-1052. 25583506
Hueck, C.J. (1998). Type III protein secretion systems in bacterial pathogens of animals and plants. Microbiol. Mol. Biol. Rev. 62: 379-433. 9618447
Jin, Q. and S.-Y. He. (2001). Role of the Hrp pilus in Type III protein secretion in Pseudomonas syringae. Science 294: 2556-2558. 11752577
Karaolis, D.K.R., S. Somara, D.R. Maneval Jr., J.A. Johnson, and J.B. Kaper. (1999). A bacteriophage encoding a pathogenicity island, a type-IV pilus and a phage receptor in cholera bacteria. Nature 399: 375-379. 10360577
Koster, M., W. Bitter, and J. Tommassen. (2000). Protein secretion mechanisms in Gram-negative bacteria. Int. J. Med. Microbiol. 290: 325-331. 11111906
Kutsukake, K., T. Minamino, and T. Yokoseki. (1994). Isolation and characterization of FliK-independent flagellation mutants from Salmonella typhimurium. J. Bacteriol. 176: 7625-7629. 8002586
Kwuan, L., W. Adams, and V. Auerbuch. (2013). Impact of Host Membrane Pore Formation by the Yersinia pseudotuberculosis Type III Secretion System on the Macrophage Innate Immune Response. Infect. Immun. 81: 905-914. 23297383
Lee, V.T. and O. Schneewind. (2002). Yop fusions to tightly folded protein domains and their effects on Yersinia enterocolitica type III secretion. J. Bacteriol. 184: 3740-3745. 12057971
Matteï, P.J., E. Faudry, V. Job, T. Izoré, I. Attree, and A. Dessen. (2011). Membrane targeting and pore formation by the type III secretion system translocon. FEBS J. 278: 414-426. 21182592
Mecsas, J. and E.J. Strauss. (1996). Molecular mechanisms of bacterial virulence: type III secretion and pathogenicity islands. Emerging Infect. Dis. 2: 271-288. 8969244
Minamino T., Shimada M., Okabe M., Saijo-Hamano Y., Imada K., Kihara M. and Namba K. (2010). Role of the C-terminal cytoplasmic domain of FlhA in bacterial flagellar type III protein export. J Bacteriol. 192(7):1929-36. 20118266
Minamino, T. and R.M. Macnab. (2000). Interactions among components of the Salmonella flagellar export apparatus and its substrates. Mol. Microbiol. 35: 1052-1064. 10712687
Minamino, T., and K. Namba (2008). Distinct roles of the FliI ATPase and proton motive force in bacterial flagellar protein export. Nature 451: 485-8. 18216858
Minamino, T., M. Kinoshita, and K. Namba. (2017). Fuel of the Bacterial Flagellar Type III Protein Export Apparatus. Methods Mol Biol 1593: 3-16. 28389941
Minamino, T., M. Kinoshita, Y. Inoue, Y.V. Morimoto, K. Ihara, S. Koya, N. Hara, N. Nishioka, S. Kojima, M. Homma, and K. Namba. (2016). FliH and FliI ensure efficient energy coupling of flagellar type III protein export in Salmonella. Microbiologyopen 5: 424-435. 26916245
Minamino, T., T. Iino, and K. Kutsukake. (1994). Molecular characterization of the Salmonella typhimurium flhB operon and its protein products. J. Bacteriol. 176: 7630-7637. 8002587
Minamino, T., Y.V. Morimoto, M. Kinoshita, P.D. Aldridge, and K. Namba. (2014). The bacterial flagellar protein export apparatus processively transports flagellar proteins even with extremely infrequent ATP hydrolysis. Sci Rep 4: 7579. 25531309
Minamino, T., Y.V. Morimoto, N. Hara, P.D. Aldridge, and K. Namba. (2016). The Bacterial Flagellar Type III Export Gate Complex Is a Dual Fuel Engine That Can Use Both H+ and Na+ for Flagellar Protein Export. PLoS Pathog 12: e1005495. 26943926
Moraes, T.F., T. Spreter, and N.C. Strynadka. (2008). Piecing together the Type III injectisome of bacterial pathogens. Curr. Opin. Struct. Biol. 18: 258-266. 18258424
Morimoto, Y.V., N. Kami-Ike, T. Miyata, A. Kawamoto, T. Kato, K. Namba, and T. Minamino. (2016). High-Resolution pH Imaging of Living Bacterial Cells To Detect Local pH Differences. MBio 7:. 27923921
Mueller, C.A., P. Broz, and G.R. Cornelis. (2008). The type III secretion system tip complex and translocon. Mol. Microbiol. 68: 1085-1095. 18430138
Nguyen, L., I.T. Paulsen, J. Tchieu, C.J. Hueck, and M.H. Saier, Jr. (2000). Phylogenetic analyses of the constituents of Type III protein secretion systems. J. Mol. Microbiol. Biotechnol. 2: 125-144. 10939240
Ochman, H. and E.A. Groisman. (1995). The evolution of invasion by enteric bacteria. Can. J. Microbiol. 41: 555-561. 7641138
Ohnishi, K., F. Fan, G.J. Schoenhals, M. Kihara, and R.M. MacNab. (1997). The FliO, FliP, FliQ, and FliR proteins of Salmonella typhimurium: putative components for flagellar assembly. J. Bacteriol. 179: 6092-6099. 9324257
Paul, K., M. Erhardt, T. Hirano, D.F. Blair, and K.T. Hughes (2008). Energy source of flagellar type III secretion. Nature 451: 489-92. 18216859
Peters, J., D.P. Wilson, G. Myers, P. Timms, and P.M. Bavoil. (2007). Type III secretion à la Chlamydia. Trends Microbiol. 15(6):241-251. 17482820
Petnicki-Ocwieja, T., D.J. Schneider, V.C. Tam, S.T. Chancey, L. Shan, Y. Jamir, L.M. Schechter, M.D. Janes, C.R. Buell, X. Tang, A. Collmer, and J.R. Alfano. (2002). Genomewide identification of proteins secreted by the Hrp type III protein secretion system of Pseudomonas syringae pv. tomato DC3000. Proc. Natl. Acad. Sci. USA 99: 7652-7657. 12032338
Plano, G.V., J.B. Da,y and F. Ferracci. (2001). Type III export: new uses for an old pathway. Mol. Microbiol. 40: 284-293. 11309112
Radtke, A.L., K.L. Anderson, M.J. Davis, M.J. DiMagno, J.A. Swanson, and M.X. O'Riordan. (2011). Listeria monocytogenes exploits cystic fibrosis transmembrane conductance regulator (CFTR) to escape the phagosome. Proc. Natl. Acad. Sci. USA 108: 1633-1638. 21220348
Riordan, K.E. and O. Schneewind. (2008). YscU cleavage and the assembly of Yersinia type III secretion machine complexes. Mol. Microbiol. 68: 1485-1501. 18452514
Ross, J.A. and G.V. Plano. (2011). A C-terminal region of Yersinia pestis YscD binds the outer membrane secretin YscC. J. Bacteriol. 193: 2276-2289. 21357482
Saier, M.H., Jr. (2006). Protein secretion and membrane insertion systems in gram-negative bacteria. J. Membr. Biol. 214: 75-90. 17546510
Saier, M.H., Jr. (2007). Active transport in communication, protection and nutrition. J. Mol. Microbiol. Biotechnol. 12: 161-164. 17587865
Sekiya, K., M. Ohishi, T. Ogino, K. Tamano, C. Sasakawa, and A. Abe. (2001). Supermolecular structure of the enteropathogenic Escherichia coli type III secretion system and its direct interaction with the EspA-sheath-like structure. Proc. Natl. Acad. Sci. USA 98: 11638-11643. 11562461
Snyder, L.A., N.J. Loman, K. Fütterer, and M.J. Pallen. (2009). Bacterial flagellar diversity and evolution: seek simplicity and distrust it? Trends Microbiol. 17: 1-5. 19081724
Sorg, J.A., B. Blaylock, and O. Schneewind. (2006). Secretion signal recognition by YscN, the Yersinia type III secretion ATPase. Proc. Natl. Acad. Sci. U.S.A. 103(44):16490-16495. 17050689
Stafford G.P., L.D. Evans, R. Krumscheid, P. Dhillon, G.M. Fraser, C. Hughes. Sorting of early and late flagellar subunits after docking at the membrane ATPase of the type III export pathway. J Mol Biol. 374: 877-882.
Wagner, S., L. Königsmaier, M. Lara-Tejero, M. Lefebre, T.C. Marlovits, and J.E. Galán. (2010). Organization and coordinated assembly of the type III secretion export apparatus. Proc. Natl. Acad. Sci. USA 107: 17745-17750. 20876096
Young, G.M., D.H. Schmiel, and V.L. Miller. (1999). A new pathway for the secretion of virulence factors by bacteria, the flagellar export apparatus functions as a protein-secretion system. Proc. Natl. Acad. Sci. USA 96: 6456-6461. 10339609
Zarivach, R., W. Deng, M. Vuckovic, H.B. Felise, H.V. Nguyen, S.I. Miller, B.B. Finlay, and N.C. Strynadka. (2008). Structural analysis of the essential self-cleaving type III secretion proteins EscU and SpaS. Nature 453: 124-127. 18451864