irg-2 (infection response gene 2; ORF C49G7.5) encodes a 278-residue Caenorhabditis elegans protein of unknown molecular function. Its mRNA is a transcriptional readout of the intestinal innate immune response: it is strongly and specifically induced upon infection with the virulent Gram-negative pathogen Pseudomonas aeruginosa (strain PA14) but not by an attenuated (gacA) mutant, and its induction requires the bZIP transcription factor ZIP-2 while being independent of the PMK-1/p38 MAPK, DBL-1/TGF-beta and KGB-1/JNK immune pathways. Induction is triggered by pathogen-imposed blockade of host mRNA translation (surveillance immunity) — for example via endocytosed P. aeruginosa Exotoxin A or the chemical translation inhibitor cycloheximide — and irg-2 is co-regulated with the paralogous marker gene irg-1. Beyond acute infection, irg-2 mRNA accumulates with age in a ZIP-2-dependent manner and correlates with mitochondrial damage. The IRG-2 protein carries no recognizable catalytic domain or characterized motif (only a short disordered, polar-residue region near its C-terminus), has evidence only at the transcript level, and has no demonstrated biochemical activity, interacting partner, or subcellular localization. Whether the protein itself contributes to pathogen resistance, as opposed to serving as a downstream reporter of ZIP-2 activation, is not established.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0140367 antibacterial innate immune response | IEP PMID:20133860 bZIP transcription factor zip-2 mediates an early response t... | ACCEPT | Summary: IEP (inferred from expression pattern) annotation: irg-2 is a member of the "infection response gene" class defined by Estes et al. 2010 as genes specifically induced by virulent P. aeruginosa but not by an attenuated gacA mutant. Troemel et al. 2006 independently place irg-2 (as C49G7.5) among the top P. aeruginosa-induced genes at 4 h. The evidence is transcriptional induction during bacterial infection, which the IEP code accurately reflects. Reason: The annotation correctly captures irg-2's defining, well-replicated property — strong and specific transcriptional induction during antibacterial (P. aeruginosa) infection — and the IEP evidence code is appropriate for an expression-based inference. Retained as a core aspect of the gene. Note the boundary of this evidence: it demonstrates induction, not that the IRG-2 protein is functionally required for the response (see knowledge_gaps). Supporting Evidence: PMID:20133860 We focused on genes that are induced in C. elegans by infection with the bacterial pathogen Pseudomonas aeruginosa, but are not induced by an isogenic attenuated gacA mutant. PMID:17096597 We tested induction of the top five genes upregulated by P. aeruginosa versus E. coli at 4 h |
| GO:0003674 molecular_function | ND GO_REF:0000015 | ACCEPT | Summary: ND (no data) annotation at the molecular_function root: no molecular function has been experimentally determined for IRG-2. The protein has no recognizable catalytic domain or characterized motif, and — unlike its co-regulated paralog irg-1 (which carries a predicted NADAR/YbiA-like domain) — offers no sequence feature from which to even hypothesize an activity. Reason: The ND annotation honestly reflects the current state of knowledge: IRG-2 is molecular-function dark. This is a genuine biology knowledge gap (see knowledge_gaps), not a curation defect to be repaired by asserting an unsupported activity. |
| GO:0050829 defense response to Gram-negative bacterium | IEP PMID:20133860 bZIP transcription factor zip-2 mediates an early response t... | ACCEPT | Summary: IEP annotation specifying that irg-2's induction occurs in response to a Gram-negative bacterium. P. aeruginosa, the pathogen that induces irg-2, is Gram-negative, so this is a more specific and appropriate child of the antibacterial response term, supported by the same expression evidence. Reason: Accurate and appropriately specific: irg-2 is induced by the Gram-negative pathogen P. aeruginosa, and the IEP evidence code matches the expression-based source. As with the other IEP term, this reflects induction rather than a demonstrated protein-level effector requirement. Supporting Evidence: PMID:20133860 This screen identified zip-2, a bZIP transcription factor that is required for inducing irg-1, as well as several other genes, and is important for defense against infection by P. aeruginosa. |
| GO:0045087 innate immune response | HEP PMID:16968778 A conserved role for a GATA transcription factor in regulati... | ACCEPT | Summary: HEP (inferred from high-throughput expression pattern) annotation from the Shapira et al. 2006 genome-wide study of intestinal infection responses, in which the endodermal GATA factor ELT-2 governs a suite of P. aeruginosa-induced genes. This is a broader parent of the antibacterial-response terms and provides independent high-throughput expression support that irg-2 is part of the intestinal innate immune transcriptional program. Reason: Valid and appropriate. C. elegans has only innate immunity, so this general term correctly classifies irg-2's immune involvement, and the HEP evidence code matches the high-throughput expression source. It is retained as a higher-level classification alongside the more specific antibacterial terms. Supporting Evidence: PMID:16968778 Gene expression and functional RNAi-based analyses identified the tissue-specific GATA transcription factor ELT-2 as a major regulator of an early intestinal protective response to infection with the human bacterial pathogen Pseudomonas aeruginosa. |
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Download this section (compressed HTML)Q: Does the IRG-2 protein have any enzymatic or direct antimicrobial activity, or is it an inert transcriptional reporter of ZIP-2 activation?
Q: Is irg-2 loss-of-function associated with any measurable susceptibility to P. aeruginosa (survival or bacterial burden)?
Q: Where does IRG-2 protein localize within intestinal (and pharyngeal) cells, and is it secreted?
Q: Do irg-1 and irg-2 act redundantly or on distinct targets within the ZIP-2 surveillance program?
Experiment: Generate an irg-2 deletion (and irg-1;irg-2 double) and assay survival and intestinal bacterial burden on P. aeruginosa PA14 versus control bacteria, with transgenic rescue, to test whether IRG-2 protein is required for defense.
Hypothesis: IRG-2 is a functional effector required for wild-type resistance to P. aeruginosa, not merely a downstream reporter.
Experiment: Express and purify recombinant IRG-2 and screen for candidate biochemical activities and for direct antibacterial activity against P. aeruginosa in vitro; determine its structure (experimental or from AlphaFold) to search for cryptic active-site or fold features.
Hypothesis: IRG-2 possesses a discrete molecular activity (enzymatic or antimicrobial) that its lack of an annotated domain has obscured.
Experiment: Use endogenous fluorescent tagging plus affinity/proximity proteomics (AP-MS) of IRG-2 in infected animals to determine subcellular localization and physical interaction partners.
Hypothesis: IRG-2 localizes to a defined compartment (e.g. secretory/apical intestinal) and acts through specific protein partners.
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The molecular function of IRG-2 is undetermined. No catalytic, binding, or antimicrobial activity has been demonstrated, no physical interaction partner is known, and — unlike its co-regulated paralog irg-1, which carries a predicted NADAR/YbiA-like domain — the protein has no recognizable domain from which to even hypothesize an activity.
OPEN BIOLOGY MF_DARK
What is known: irg-2 is a well-established transcriptional readout of the ZIP-2 surveillance-immunity pathway: strongly and specifically induced by virulent P. aeruginosa (PMID:20133860), by translational inhibition (Exotoxin A / cycloheximide; PMID:22520465), and with age in a ZIP-2-dependent manner (PMID:32350153), independent of PMK-1 p38 MAPK (PMID:17096597). The 278-aa protein has only a short disordered/polar-residue region and no catalytic motif, and evidence exists solely at the transcript level (UniProt PE=2).
Significance: irg-2 is one of the canonical downstream effectors invoked to define the ZIP-2 arm of C. elegans surveillance immunity, yet what its protein product actually does — enzyme, antimicrobial effector, or inert reporter — is entirely unknown, leaving the effector output of this well-studied pathway mechanistically blank.
What would resolve it: Recombinant-protein biochemistry and/or structure determination to test for an enzymatic or antimicrobial activity; affinity/proximity proteomics (AP-MS) or yeast two-hybrid to identify partners; endogenous tagging for subcellular localization.
Provenance (the field's own admissions):
Gap: It is unknown whether the IRG-2 protein is functionally required for defense against P. aeruginosa. All evidence linking irg-2 to immunity is transcriptional (IEP/HEP); no irg-2 loss-of-function survival, colonization, or immune phenotype has been reported, so whether IRG-2 is a causal effector or a passive reporter of ZIP-2 activation is unresolved.
OPEN BIOLOGY
What is known: The regulator ZIP-2 is required for defense against P. aeruginosa (PMID:20133860), and irg-2 is a ZIP-2-dependent target (PMID:32350153); but requirement of the regulator does not establish requirement of this particular target. irg-2's association with defense rests entirely on expression correlation, not on perturbation of irg-2 itself.
Significance: Distinguishing effector from reporter is essential before irg-2 can be assigned a causal role in innate immunity, and would clarify whether the ZIP-2 pathway's protective output runs through irg-2 or through other, as-yet-unidentified effectors.
What would resolve it: Loss-of-function (deletion/RNAi) and overexpression of irg-2 with quantitative survival and bacterial-burden assays on P. aeruginosa PA14, ideally with rescue, to test whether IRG-2 protein is required for or sufficient to enhance resistance.
Provenance (the field's own admissions):
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