3.A.6.1.3
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.