Structural basis for the homotypic fusion of chlamydial inclusions by the SNARE-like protein IncA

Gino Cingolani,Ravi K Lokareddy,Deanna L Greco,Fabienne Paumet,Erik Ronzone,Jordan Wesolowski,Anna Lobley,Juan R Perilla,Alexander J Bryer,Michael Mccauley

doi:10.1038/s41467-019-10806-9

Abstract

Many intracellular bacteria, including Chlamydia, establish a parasitic membrane-bound organelle inside the host cell that is essential for the bacteria’s survival. Chlamydia trachomatis forms inclusions that are decorated with poorly characterized membrane proteins known as Incs. The prototypical Inc, called IncA, enhances Chlamydia pathogenicity by promoting the homotypic fusion of inclusions and shares structural and functional similarity to eukaryotic SNAREs. Here, we present the atomic structure of the cytoplasmic domain of IncA, which reveals a non-canonical four-helix bundle. Structure-based mutagenesis, molecular dynamics simulation, and functional cellular assays identify an intramolecular clamp that is essential for IncA-mediated homotypic membrane fusion during infection.

Highlights

Many intracellular bacteria, including Chlamydia, establish a parasitic membrane-bound organelle inside the host cell that is essential for the bacteria’s survival
At high multiplicities of infection (MOI), cells contain multiple inclusions that fuse together to form one large inclusion per cell. This homotypic fusion event is important for the pathogenicity of C. trachomatis because natural non-fusogenic IncA mutants are replicationdefective in humans and cause significantly milder disease compared with patients infected with normal fusogenic strains[12,13,14]
Cells infected with an IncA-deficient strain of C. trachomatis displayed multiple inclusions at a high multiplicity of infection, further establishing that IncA is required for the homotypic fusion of inclusions[11,16]

Summary

Introduction

Many intracellular bacteria, including Chlamydia, establish a parasitic membrane-bound organelle inside the host cell that is essential for the bacteria’s survival. Inclusion membranes are decorated with ~60 transmembrane Inc proteins that are known to directly interact with host cell components and play a critical role in sustaining Chlamydia’s life cycle[2,3,4] Despite their importance, Incs have remained relatively uncharacterized and little is known about their function at the molecular level. At high multiplicities of infection (MOI), cells contain multiple inclusions that fuse together to form one large inclusion per cell This homotypic fusion event is important for the pathogenicity of C. trachomatis because natural non-fusogenic IncA mutants are replicationdefective in humans and cause significantly milder disease compared with patients infected with normal fusogenic strains[12,13,14]. Our work sheds light on a class of bacterial transmembrane proteins that control membrane fusion during infection, which is critical for Chlamydia pathogenicity

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Results

Conclusion