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

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.