The intracellular retinoic acid-inducible gene I-like receptors (RLRs) sense viral ribonucleic acid and signal through the mitochondrial protein mitochondrial antiviral signalling (MAVS) to trigger the production of type I interferons and proinflammatory cytokines. In this study, we report that RLR activation promotes elongation of the mitochondrial network. Mimicking this elongation enhances signalling downstream from MAVS and favours the binding of MAVS to stimulator of interferon genes, an endoplasmic reticulum (ER) protein involved in the RLR pathway. By contrast, enforced mitochondrial fragmentation dampens signalling and reduces the association between both proteins. Our finding that MAVS is associated with a pool of mitofusin 1, a protein of the mitochondrial fusion machinery, suggests that MAVS is capable of regulating mitochondrial dynamics to facilitate the mitochondria-ER association required for signal transduction. Importantly, we observed that viral mitochondria-localized inhibitor of apoptosis, a cytomegalovirus (CMV) antiapoptotic protein that promotes mitochondrial fragmentation, inhibits signalling downstream from MAVS, suggesting a possible new immune modulation strategy of the CMV.

NLRX1 is the only member of the Nod-like receptor (NLR) family that is targeted to the mitochondria, and its overexpression induces the generation of reactive oxygen species (ROS), thus impacting on NFκB- and JNK-dependent signaling cascades. In addition, NLRX1 has been shown to interact with MAVS (also known as IPS-1, VISA and Cardif) at the mitochondrial outer membrane and to modulate antiviral responses. Here we report that NLRX1 has a functional leader sequence and fully translocates to the mitochondrial matrix via a mechanism requiring the mitochondrial inner-membrane potential, ΔΨm. Importantly, we failed to detect NLRX1 at the mitochondrial outer membrane. We also show that the leader sequence of NLRX1 is removed, which generates a mature protein lacking the first 39 amino acids through a maturation process that is common for mitochondrial-matrix proteins. Finally, we identified UQCRC2, a matrix-facing protein of the respiratory chain complex III, as an NLRX1-interacting molecule, thus providing a molecular basis for the role of NLRX1 in ROS generation. These results provide the first identification of a protein belonging to the NLR family that is targeted to the mitochondrial matrix.