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NLRX1 Has a Tail to Tell

Tsan Sam Xiao1,* and Jenny P.-Y. Ting2,* 1Structural Immunobiology Unit, Laboratory of Immunology, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892, USA 2Department of Microbiology and Immunology, Lineberger Comprehensive Cancer Center, Center of Translational Immunology and the Inflammatory Diseases Institute, Chapel Hill, NC 27599, USA *Correspondence: [email protected] (T.S.X.), [email protected] (J.P.-Y.T.) DOI 10.1016/j.immuni.2012.03.002

In this issue of Immunity, Hong et al. (2012) report the first structural analysis of the C-terminal fragment of an NLR (nucleotide-binding domain [NBD] and leucine-rich repeat [LRR]-containing) protein, NLRX1. This fragment forms a hexamer and binds RNA.

The NLR (nucleotide-binding domain membrane (MOM) protein (Moore et al., lecular interactions at the trimer interface. [NBD] and leucine-rich repeat [LRR]-con- 2008), another group has shown its Whether the NLRX1 NBD promotes taining) proteins are a large protein family localization in the mitochondrial matrix further oligomerization of NLRX1 is that plays important roles in a wide range (Arnoult et al., 2009). Different serologic unknown. Interestingly, the NLRX1 NBD of biological processes, a prime function reagents were used in these localization has a classic Walker A motif that binds of which is the detection of pathogen- studies, which might impact the outcome. ATP but a poor Walker B motif. It is associated molecular patterns (PAMPs). Additionally, one has to entertain the possible that an uncommon ATP binding NLRX1 is the first NLR protein shown to possibility that these varied localizations motif endows different oligomerization be localized at the mitochondria. It has may be relevant to the multiple functions function for the NLRX1 NBD. been associated with multiple functions, of NLRX1 reported to date. A second finding in this paper relates including negatively impacting antiviral Hong et al. (2012) in this issue of to how NLRs detect PAMPs, a central inflammatory response via the MAVS- Immunity presents the first crystallo- unresolved issue in the field. One of the RIG-I sensing pathway (Moore et al., graphic analysis of the C-terminal frag- most studied NLRs is NLRP3, which 2008; Xia et al., 2011) or lipopolisacchar- ment of NLRX1 that includes two a-helical mediates inflammasome response to ide-elicited response via the TRAF6-IKK domains flanking a middle LRR section. numerous PAMPs and DAMPs (damage- signaling pathway (Allen et al., 2011; Xia Several important and surprising features associated molecular patterns). An et al., 2011) and positively controlling of the NLRX1 are revealed in this study. emerging consensus is that NLRP3 is NF-kB and JNK signaling pathway to acti- A key finding is that the NLRX1 LRR and unlikely to directly bind to such a wide vate reactive oxygen species (ROS) pro- its associated helical domains form a variety of molecular structures. Rather, it duction in response to TNF-a, poly(I:C), hexameric platform composed of a trimer might ‘‘sense’’ PAMPs and DAMPs and pathogens (Abdul-Sater et al., 2010; of three dimers, through extensive intra- through indirect detection by an interme- Tattoli et al., 2008). Gene deletion of molecular and intermolecular interactions diate moieties that remains elusive. A exons 4 and 5 within the Nlrx1 gene re- (Figure 1). These higher-order oligomers recent large-scale analysis of plant NLRs sulted in elevated type I interferon (IFN-I) may promote cooperative association has been illuminating regarding the inter- and IL-6 response to viral infection in cells between NLRX1 and its target proteins, section of NLRs and PAMPs. This work and whole animals and in a modest thereby ensuring stringent regulation of shows that only 2 of 30 NLRs directly reduction of basal ROS in innate immune innate immune responses. Although the interact with PAMPs (pathogen effectors) cells (Allen et al., 2011). A different gene- NLR nucleotide-binding domain is typi- (Mukhtar et al., 2011). By contrast, nearly targeted strain showed reduced ROS cally considered an oligomerization half of the host cell proteins that interact production to Chlamydia, although the motif, an early report that first described with NLRs are direct targets of pathogen deleted genetic sequence was not oligomerization of the founding NLR effectors. These findings led to the con- described (Abdul-Sater et al., 2010). A protein CIITA revealed that both NBD clusion that a majority of pathogen effec- third group studied a strain that has a and LRR domains undergo homotypic tors interact with host proteins that in deletion of the first four exons of Nlrx1 (NBD-NBD, LRR-LRR) and heterotypic turn activate plant NLRs, while a minority (Rebsamen et al., 2011). These authors (NBD-LRR) association (Linhoff et al., directly associate with NLRs. Support for did not detect an effect of this deletion 2001). In agreement with this, Hong et al. the latter is emerging in mammalian cells on either IFN expression or TNF-a amplifi- (2012) show that LRR-LRR interaction although direct binding between NLRs cation of NF-kB and JNK signaling. In occurs through the C-terminal a-helical and their respective ligands have yet to addition to possibly mediating different domain (referred to as LRRCT by these be demonstrated in a purified recombi- functions, the precise site of cellular local- authors)-mediated intra- and intermolec- nant system. ization varies in different reports. Although ular interactions at the dimer interface Hong et al. (2012) used a purified re- one group has found endogenous NLRX1 and the N-terminal a-helical domain combinant C-terminal fragment of localization as a mitochondrial outer (referred to as LRRNT)-mediated intermo- NLRX1 to demonstrate its binding to

Immunity 36, March 23, 2012 ª2012 Elsevier Inc. 311 Immunity Previews

ROS production? One report showed that endogenous NLRX1 constitutively interacts with MAVS (Moore et al., 2008). Does RNA induce conformational changes or oligomerization states of NLRX1 so that NLRX1 releases MAVS to allow for RIG-1 signaling? The hexameric full-length NLRX1 model shows close proximity of its RNA binding surface with the modeled NBD. It is conceivable that RNA binding at LRR and MAVS binding at NBD are mutually exclusive, thus providing a mechanism to release the repression of the RIG-1 pathway upon RNA exposure to both RIG-1 and NLRX1. The above working models can now be tested with the information pro- vided by Hong et al. (2012).

REFERENCES

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