Mitochondria also participate in the homeostasis of intracellular Ca 2+5. They are central for the energy production of the cell upon oxidation of tricarboxylic acid (TCA) cycle intermediates which involves the creation and harnessing of a membrane potential across the inner mitochondrial membrane 4. Mitochondria are fundamental to eukaryotic cell function. These processes are essential to maintain functional mitochondria when cells experience metabolic or environmental stresses 3. However, and in contrast to most bacteria, mitochondria are highly dynamic, and their morphology is balanced between two tightly regulated opposite events, fission and fusion. Consistent with this bacterial origin, mitochondria are delimitated by a double membrane with an inner membrane characterized by the presence of cardiolipin and absence of cholesterol they house a small circular genome referred as mitochondrial DNA (mtDNA), and a functional machinery for protein synthesis 2. Mitochondria originated from an endosymbiont α- Proteobacterium related to the genus Rickettsia more than 1.45 billion years ago 1. Our work provides an example of T6SS manipulation of eukaryotic cells via alteration of the mitochondria. VgrG4 abrogates the NEDDylation of cullin-1 by inactivation of Ubc12, the NEDD8-conjugating enzyme. The degradation of IκBα is triggered by the ubiquitin ligase SCF β-TrCP, which requires the modification of the cullin-1 subunit by NEDD8.

NLRX1-induced ROS limits NF-κB activation by modulating the degradation of the NF-κB inhibitor IκBα. Ca 2+ elevation also induces the activation of the innate immunity receptor NLRX1 to produce reactive oxygen species (ROS). VgrG4 induces the transfer of Ca 2+ from the ER to the mitochondria, activating Drp1 (a regulator of mitochondrial fission) thus leading to mitochondrial network fragmentation. VgrG4 colocalizes with the endoplasmic reticulum (ER) protein mitofusin 2.

Here, we show that the trans-kingdom antimicrobial T6SS effector VgrG4 from Klebsiella pneumoniae triggers the fragmentation of the mitochondrial network.

T6SS effectors can also be injected into eukaryotic cells to facilitate bacterial survival, often by targeting the cytoskeleton. Bacteria can inhibit the growth of other bacteria by injecting effectors using a type VI secretion system (T6SS).

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