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Type III protein secretion complex. Assembly of the YscR,S,T,U,V complex occurs independently of other structural components and involves the formation of a YscV oligomer (Diepold et al., 2011). A C-terminal region of Yersinia pestis YscD binds the outer membrane secretin YscC (Ross and Plano, 2011). YopBD-mediated translocation of T3SS cargo, but not YopBD pore formation, leads to activation of host pathways influencing inflammation, cell death, and response to stress (Kwuan et al. 2013).

Type IIISP system of Yersinia enterocolitica species (22 subunits) LcrD + YscA - R:
LcrD - P0C2V3
YscA - A1JU92
YscB - A1JU93
YscC - Q93KT1
YscD - Q93KT0
YscE - Q93KS9
YscF - Q93KS8
YscG - Q93KS7
YscH - Q93KS6
YscI - Q93KS5
YscJ - Q7BFA4
YscK - Q93KS4
YscL - Q93KS3
YscM - A1JUA4
YscN - P40290
YscO - Q93KT7
YscP - Q93KT6
YscQ - Q9ZA78
YscR - Q9ZA77
YscS - Q7BFA7
YscT - Q93KT5
YscU - Q93KT4

Type III secretion system, SpaLMNOPQRS.  Required for surface presentation of invasion plasmid antigens. Required for invasion and for secretion of the three IPA proteins. A cryo-EM structure of the isolated Shigella T3SS needle complex has been pulished (Lunelli et al. 2020). The inner membrane (IM) region of the basal body adopts 24-fold rotational symmetry and forms a channel system that connects the bacterial periplasm with the export apparatus cage. The secretin oligomer adopts a heterogeneous architecture with 16- and 15-fold cyclic symmetry in the periplasmic N-terminal connector and C-terminal outer membrane ring, respectively. Two out of three IM subunits bind the secretin connector via a β-sheet augmentation. The cryo-EM map also reveals the helical architecture of the export apparatus core, the inner rod, the needle and their intervening interfaces (Lunelli et al. 2020).

Type III secretion system of Shigella flexneri

Type III secretion system, SpaLMNOPQRS-PrgHIJK.  SpaP forms a pentameric 15 Å wide pore.  It interacts with SpaQ, R and S as well as the inner rod protein, PrgJ (Dietsche et al. 2016). The small hydrophobic export apparatus components, SpaP and SpaR, nucleate assembly of the needle complex and form the central "cup" substructure of this secretion system. The ortholog of SpaQ in E. coli (SctS, of 86 aas and 2 TMSs like SpaQ) functions to ensures the fittings between the dynamic and static components of the T3SS (Tseytin et al. 2019). The Salmonella flagellar protein export machinery consists of a transmembrane export gate complex and a cytoplasmic ATPase complex. The gate complex has two intrinsic and distinct H+-driven and Na+-driven engines to drive the export of flagellar structural proteins (Minamino et al. 2021). Salmonella wild-type cells preferentially use the H+-driven engine under a variety of environmental conditions. The Na+-driven engine is activated by the interaction of the FlgN chaperone with FlhA when the ATPase complex is non-functional due to either of two single-residue substitutions in FlhA. Thus, it is likely that the FlgN-FlhA interaction generates a conformational change in FlhA that allows it to function as a Na+ channel. Minamino et al. 2021 proposed that this type of activation would be useful for flagellar construction under conditions in which the proton motive force is severely restricted. In enteropathogenic E. coli, assembly of the T3SS is initiated by the association of three proteins, SctR, SctS, and SctT, which create an entry portal to the translocation channel within the bacterial inner membrane. Using the T3SS, Tseytin et al. 2022 investigated the role of two structural construction sites formed within the SctRST complex that are likely to act as seals, preventing leakage of ions and metabolites rather than as substrate gates. Two residues in SctS, Pro23, and Lys54, are critical for the proper activity of the T3SS. Pro23 may be critical for the physical orientation of the SctS transmembrane domains that create the tip of the SctRST complex and for their positioning with regard to other T3SS substructures. SctS Lys54, which was previously suggested to mediate the SctS self-oligomerization, is critical for T3SS activity due to its essential role in SctS-SctT hetero-interactions (Tseytin et al. 2022). SctR: 217 aas and 4 TMSs, 2 N-terminal and 2 C-terminal; SctS: 86 aas and 2 TMSs; SctT: 255 aas and 6 TMSs in a 3 + 1 + 2 TMS arrangement.  


Type III secretion system, SpaLMNOPQRS-PrgHIJK of Samonella enterica Typhimurium
SpaL, ATPase also called InvC or SpaI; 431 aas
SpaM, 147 aas
SpaN, 336 aas (also called InvJ
SpaO, 303 aas, 0 TMSs
SpaP, 224 aas
SpaQ, 86 aas, 2 TMSs
SpaR, 263 aas, 6 TMSs
SpaS, 256 aas, 6 TMSs
PrgI, 80 aas
PrgJ, 101 aas, inner rod protein
PrgK, 252 aas, lipoprotein

Flagellar protein export system.  Infrequent ATP hydrolysis by the FliI6FliJ ring is sufficient for gate activation, allowing processive translocation of export flagellar protein substrates for efficient flagellar assembly (Minamino et al. 2014). FliO has been identified as a flagellar basal body chaparone protein (Fabiani et al. 2017). The flagellar protein export apparatus switches its substrate specificity when hook length has reached approximately 55 nm, and the hydrophilic C-terminal domain of FlhB is involved in this switching process (Inoue et al. 2019). A positively chargef region of Salmonella FliI is required for ATPase formation and efficient flagellar protein export (Kinoshita et al. 2021).

Flagellar subunit export system of Salmonella typhimurium (10 subunits)

Chlamydial type III secretion complex, CdsCDJLNQRSTUV, FliF, FliI, FlhA (Peters et al., 2007).  Genome analyses have indicated which proteins are substrates (Dehoux et al. 2011).

Type III SP of Chlamydia trachomatis:
CdsC - O84681
CdsD - O84671
CdsJ - O84563
CdsL - O84565
CdsN - O84676
CdsQ - O84679
CdsR - O84566
CdsS - O84567
CdsT - O84568
CdsU - O84093
DsdV - O84092
FliF - O84724
FliI - O84722
FlhA - O84063