An N-terminal addressing sequence targets NLRX1 to the mitochondrial matrix

Abstract

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.

Inflammasomes and its importance in viral infections

“…However, the interaction between NLRX1 and MAVS remains doubtful owing to the localisation of these two proteins in distinct mitochondrial subdomains that are separated by two membranes. Further, the overexpression of NLRX1 in mitochondria leads to ROS induction likely due to the interaction with UQCRC2, a matrix-facing component of the mitochondrial respiratory chain complex III that has a crucial role in ROS generation [104]. NLRX1 leads to ROS induction in synergy with proinflammatory stimuli such as Poly (I:C), Shigella, and TNF-a, which subsequently potentiates NF-jB and JNK pathways [103].…”

NLRX1/NOD5 deficiency does not affect MAVS signalling

“…3 Interestingly, a recent study proposed that NLRX1/NOD5 is imported to the mitochondrial matrix, making NLRX1/NOD5 interaction with the outer mitochondrial membrane protein MAVS difficult to explain. 4 To clarify the role of NOD5 in vivo, we generated NOD5-deficient mice by replacing the first four coding exons with a neomycin selection cassette. Mutant mice are viable and born at the expected Mendelian ratios (data not shown).…”

Beyond the inflammasome: regulatory NOD-like receptor modulation of the host immune response following virus exposure

“…Subsequent evaluation indicated that NLRX1 inhibits type I IFN production through its interaction with MAVS and attenuation of the RIG-I and melanoma differentiation-associated protein 5 (MDA-5) signalling pathways (Moore et al, 2008). However, the localization data conflict with other studies that suggest NLRX1 is associated with the inner mitochondrial membrane and matrix (Arnoult et al, 2009;Tattoli et al, 2008). While the exact localization of NLRX1 around or within the mitochondria is still an area of debate, it is possible that NLRX1 behaves similarly to other NLR family members that shuttle from the cytosol to the mitochondrial matrix.…”

“…While some ligands have been identified, it is highly likely that the full repertoire of viral PAMPs that signal through NLRX1 have yet to be fully elucidated. It is considered unique among the NLR family as it has been found to be associated with the mitochondria in multiple studies (Arnoult et al, 2009;Moore et al, 2008;Tattoli et al, 2008). Initially, Moore et al (2008) showed that NLRX1 is most likely associated with the mitochondrial outer membrane (Moore et al, 2008).…”

“…Subsequent studies provided additional mechanistic insight that revealed, upon stimulation, that NLRX1 dissociates from TRAF6 to interact with the Ik B kinase (IKK) complex and downregulate NF-k B signalling (Xia et al, 2011). While NLRX1 appears to negatively regulate type I IFN and NF-k B signalling, it also seems to augment ROS production through interactions with the mitochondrial protein ubiquinol-cytochrome c reductase core protein II (UQCRC2; Arnoult et al, 2009). Functionally, HeLa cells transfected with NLRX1 significantly increased ROS production induced by TNF-a, Shigella sp., poly(I : C), Chlamydia trachomatis and TNF-a/ cycloheximide (Abdul-Sater et al, 2010;Singh et al, 2015;Tattoli et al, 2008).…”