Structure of a bacterial type III secretion system in contact with a host membrane in situ.

Mikhail Kudryashev,Helen R Saibil,Andrea Nans,Richard D Hayward

doi:10.1038/ncomms10114

Abstract

Many bacterial pathogens of animals and plants use a conserved type III secretion system (T3SS) to inject virulence effector proteins directly into eukaryotic cells to subvert host functions. Contact with host membranes is critical for T3SS activation, yet little is known about T3SS architecture in this state or the conformational changes that drive effector translocation. Here we use cryo-electron tomography and sub-tomogram averaging to derive the intact structure of the primordial Chlamydia trachomatis T3SS in the presence and absence of host membrane contact. Comparison of the averaged structures demonstrates a marked compaction of the basal body (4 nm) occurs when the needle tip contacts the host cell membrane. This compaction is coupled to a stabilization of the cytosolic sorting platform–ATPase. Our findings reveal the first structure of a bacterial T3SS from a major human pathogen engaged with a eukaryotic host, and reveal striking ‘pump-action' conformational changes that underpin effector injection.

Highlights

Many bacterial pathogens of animals and plants use a conserved type III secretion system (T3SS) to inject virulence effector proteins directly into eukaryotic cells to subvert host functions
Sub-tomogram averages show a striking compaction of the basal body and structural stabilization of the sorting platform-ATPase complex upon T3SS needle contact with a host membrane
Our data demonstrate that the T3SS truly behaves as a ‘molecular syringe’, which contracts by compressing the periplasmic domains of the basal body components

Summary

Introduction

Many bacterial pathogens of animals and plants use a conserved type III secretion system (T3SS) to inject virulence effector proteins directly into eukaryotic cells to subvert host functions. Sub-tomogram averages show a striking compaction of the basal body and structural stabilization of the sorting platform-ATPase complex upon T3SS needle contact with a host membrane. To derive the in situ structure of the chlamydial T3SS in contact with host membranes, we visualized C. trachomatis elementary bodies early during entry into cultured cells by cryo-electron tomography, either in association with the host plasma membrane (Fig. 2a) or within early vacuoles (Fig. 2b).

Results

Conclusion