# Chlamydial membrane vesicles deliver the beta barrel outer membrane protein OmpA to mitochondria to inhibit apoptosis

## TL;DR

Chlamydia bacteria use membrane vesicles to deliver a protein called OmpA to mitochondria, preventing cell death and helping the bacteria survive inside human cells.

## Contribution

This study reveals a novel mechanism by which Chlamydia inhibits apoptosis via vesicle-mediated delivery of OmpA to mitochondria.

## Key findings

- Chlamydia uses membrane vesicles to transport OmpA to mitochondria during infection.
- OmpA interacts with BAK and prevents apoptosis, mimicking the action of the human protein VDAC2.
- Chlamydia-derived vesicles protect uninfected cells from apoptosis when added externally.

## Abstract

Chlamydiae are obligate intracellular bacteria that inhibit mitochondrial apoptosis to maintain integrity of the host cell. We have previously reported that a chlamydial outer membrane β-barrel protein, OmpA, can during ectopic expression inhibit mitochondrial apoptosis through direct interaction with the BCL-2-family effectors BAX and BAK. We here show that OmpA from Chlamydia trachomatis (Ctr) uses membrane vesicles for its delivery to the outer mitochondrial membrane during Ctr infection. Using a number of imaging and fractionation techniques, we show that OmpA during infection reaches mitochondria and is inserted into mitochondrial membranes. Chlamydia derived vesicles (CDV) from Ctr-infected cells contained OmpA as well as other outer membrane proteins and LPS. When added to uninfected cells, CDVs fused with mitochondrial membranes, causing the interaction of OmpA with BAK and the cytosolic retro-translocation of BAX. CDV addition to uninfected cells also protected the cells against apoptosis. We previously showed that OmpA works in co-ordination with VDAC2 to block apoptosis and here propose a structural model of this BAK inhibition by OmpA that reenacts the inhibition of BAK by VDAC2. The results provide evidence that OmpA from Chlamydia, as well as the structurally similar ortholog from the related Simkania, specifically exploits its relationship to mitochondrial porins to protect the infected cell against apoptosis and to enable intracellular growth of the bacteria in human cells.

In this study, we investigated how a protein from Chlamydia called OmpA—which interferes with apoptosis, a form of controlled cell death—can reach mitochondria. Chlamydia live inside human cells within a membrane-bound compartment called an inclusion. Apoptosis is activated at mitochondria, and we previously found that OmpA expressed in cells can travel to mitochondria and block this process. Here, we show that in infected cells Chlamydia use vesicles derived from their own membranes to transport OmpA across the inclusion and through the host cell to mitochondria. Mitochondria are thought to have evolved from ancient bacteria and contain unique membrane proteins called beta barrels, found only in their outer membrane and in certain bacteria. OmpA shares this structure, which allows it to access and interact with mitochondrial proteins. We propose that OmpA binds to BAK, a pro-apoptotic protein at mitochondria, in a similar way to the human beta-barrel protein VDAC2, which naturally restrains BAK activity and cell death. By mimicking this interaction, OmpA prevents the cell from initiating apoptosis, enabling Chlamydia to survive longer inside human cells. Our findings reveal a strategy by which bacterial pathogens exploit the evolutionary origins of mitochondria to manipulate cell death and promote their growth.