Activation of Shigella flexneri type 3 secretion requires a host-induced conformational change to the translocon pore

Abstract

Type 3 secretion systems (T3SSs) are conserved bacterial nanomachines that inject virulence proteins (effectors) into eukaryotic cells during infection. Due to their ability to inject heterologous proteins into human cells, these systems are being developed as therapeutic delivery devices. The T3SS assembles a translocon pore in the plasma membrane and then docks onto the pore. Docking activates effector secretion through the pore and into the host cytosol. Here, using Shigella flexneri, a model pathogen for the study of type 3 secretion, we determined the molecular mechanisms by which host intermediate filaments trigger docking and enable effector secretion. We show that the interaction of intermediate filaments with the translocon pore protein IpaC changed the pore’s conformation in a manner that was required for docking. Intermediate filaments repositioned residues of the Shigella pore protein IpaC that are located on the surface of the pore and in the pore channel. Restricting these conformational changes blocked docking in an intermediate filament-dependent manner. These data demonstrate that a host-induced conformational change to the pore enables T3SS docking and effector secretion, providing new mechanistic insight into the regulation of type 3 secretion.

The type 3 secretion system requires actin polymerization to open translocon pores

“…The interaction of intermediate filaments with IpaC induces a conformational change in the pore that is required for efficient docking (Fig. 1a, and 13,16 ). We therefore investigated whether actin polymerization-induced effector translocation depended on this interaction.…”

“…Docking establishes a continuous channel from the bacterial cytoplasm to the eukaryotic cytosol and enables the direct delivery of bacterial effectors into the host cytosol. For S. flexneri, Salmonella enterica serovar Typhimurium, and Yersinia pseudotuberculosis , docking depends on the interaction of a cytosolic domain of the translocon pore with host intermediate filaments 13,16 and, at least in S. flexneri , a conformational change in the translocon pore induced by the interaction of the pore with intermediate filaments 16 (Fig. 1a).…”

The Shigella Type III Secretion System: An Overview from Top to Bottom

“…Translocon formation requires input from a second translocator protein, which for Shigella is IpaC [ 87 , 88 , 89 ]. It has been shown in Shigella that IpaC is recruited to the needle tip upon bacterial contact with phospholipid membranes containing sphingolipids and cholesterol [ 36 ] and this signals the onset of secretion induction and translocon formation.…”

Structural Insights of Shigella Translocator IpaB and Its Chaperone IpgC in Solution

“…The translocation pore in Shigella is formed by two proteins: Invasion plasmid antigens B and C (IpaB and IpaC) ( Blocker et al., 1999 ; Matteï et al., 2011 ), which are also T3S substrates themselves. A tightly- controlled multi-step folding pathway is followed, comprising i) translation of the nascent polypeptides, ii) maintenance in the bacterial cytoplasm in a secretion competent state, iii) delivery to the secretion apparatus, iv) traversing through the narrow T3S needle channel, v) maintenance on the distant end of the machinery (tip of the needle), vi) insertion and polymerization into the host cell membrane and rearrangement of the translocation pore to efficiently dock the T3S needle ( Olive et al., 2007 ; Stensrud et al., 2008 ; Cheung et al., 2015 ; Barta et al., 2018 ; Russo et al., 2019 ).…”