Novel insights into the mechanism of SepL‐mediated control of effector secretion in enteropathogenic Escherichia coli

Abstract

Type three secretion systems (T3SSs) are virulence determinants employed by several pathogenic bacteria as molecular syringes to inject effector proteins into host cells. Diarrhea‐producing enteropathogenic Escherichia coli (EPEC) uses a T3SS to colonize the intestinal tract. T3S is a highly coordinated process that ensures hierarchical delivery of three classes of substrates: early (inner rod and needle subunits), middle (translocators), and late (effectors). Translocation of effectors is triggered upon host‐cell contact in response to different environmental cues, such as calcium levels. The T3S substrate specificity switch from middle to late substrates in EPEC is regulated by the SepL and SepD proteins, which interact with each other and form a trimeric complex with the chaperone CesL. In this study, we investigated the link between calcium concentration and secretion regulation by the gatekeeper SepL. We found that calcium depletion promotes late substrate secretion in a translocon‐independent manner. Furthermore, the stability, formation, and subcellular localization of the SepL/SepD/CesL regulatory complex were not affected by the absence of calcium. In addition, we demonstrate that SepL interacts in a calcium‐independent manner with the major export gate component EscV, which in turn interacts with both middle and late secretion substrates, providing a docking site for T3S. These results suggest that EscV serves as a binding platform for both the SepL regulatory protein and secreted substrates during the ordered assembly of the T3SS.

Bacterial type III secretion systems: a complex device for the delivery of bacterial effector proteins into eukaryotic host cells

“… 2017 ; Gaytán et al. 2018 ). The SctV–SctW interaction participates in secretion regulation by providing a dedicated binding site for translocator-chaperone pairs and by occluding a binding site for late substrates (Portaliou et al.…”

“… 2017 ; Gaytán et al. 2018 ). For other T3SS, transmission of the host-sensing signal may be of a mechanical nature by propagating a conformational change of protein subunits from the needle tip through the needle and inner rod towards the export apparatus and the cytoplasmic components.…”

The inner rod of virulence‐associated type III secretion systems constitutes a needle adapter of one helical turn that is deeply integrated into the system's export apparatus

“…Autocleavage of SctU at a conserved NPTH motif is required to allow for substrate specificity switching and preventing autocleavage by mutagenesis uncouples specificity switching and needle length control (Shen et al , ; Monjarás Feria et al , ). Besides SctP and autocleaved SctU, the secretion of intermediate substrates also relies on SctW, thought to be an adapter facilitating the binding of chaperone‐intermediate substrate complexes to the cytoplasmic domain of the major export apparatus SctV (Lee et al , ; Shen and Blocker, ; Portaliou et al , ; Gaytán et al , ; Yu et al , ). Upon host cell sensing and full activation of the injectisome, SctW dissociates from SctV and is secreted or degraded.…”

The PopN gate-keeper complex acts on the ATPase PscN to regulate the T3SS secretion switch from early to middle substrates inPseudomonas aeruginosa

“…When the T3SS needle reaches the expected length, fully stretched SctP would promote its dissociation from SctW and SctU and its secretion, which in turn modulates the rate of SctU auto-proteolysis (Ho et al, 2017) facilitating the secretion of translocators and effectors. As already shown, the gate-keeper SctW can also be tethered to the innermembrane protein SctV (Gaytán et al, 2018;Lee et al, 2014;Yu et al, 2018). There, it can fulfill its second function: the regulation of the next secretion switch, from middle to late substrates, which may also involve the sorting platform (Lara-Tejero et al, 2011).…”