| Claim/Observation | Evidence type | Key quantitative data | Experimental context | Source (paper, year, DOI URL) |
|---|---|---|---|---|
| **Target identity verified:** **abu-1** corresponds to **AC3.3 / ABU-1** in *C. elegans* and is a representative member of the **ABU (Activated in Blocked UPR)** family | Gene/protein identification from primary paper and reporter studies | ABU family initially described as **9 highly related genes** with shared sequence identity sufficient for cross-targeting by RNAi in conserved 3' regions | *C. elegans* ER-stress genetics; abu-1 used as representative family member | Urano et al., 2002, *J Cell Biol*, https://doi.org/10.1083/jcb.200203086 (pqac-00000001, pqac-00000002) |
| **Protein architecture:** ABU-1 is a predicted **type I single-pass membrane protein** with **N-terminal signal peptide**, **luminal domain**, **one transmembrane segment**, and **short C-terminal cytosolic tail** | Sequence analysis; heterologous expression with TM-deletion test | TM deletion caused **secretion** of ABU-1, whereas full-length protein stayed membrane-associated and required detergent extraction | Structural/biochemical characterization in mammalian COS1 cells plus *C. elegans* sequence analysis | Urano et al., 2002, *J Cell Biol*, https://doi.org/10.1083/jcb.200203086 (pqac-00000000, pqac-00000001, pqac-00000015) |
| **Family relationship:** ABU proteins are related to the broader **pqn/prion-like Q/N-rich** family and are considered a non-canonical ER-stress/proteostasis module | Family-level review/microarray interpretation | ABU1–9 predicted TM proteins; ABU-10/11 predicted luminal in one later family analysis | ER stress/longevity literature synthesis in *C. elegans* | Viswanathan et al., 2005, *Dev Cell*, https://doi.org/10.1016/j.devcel.2005.09.017 (pqac-00000006) |
| **Subcellular localization:** ABU-1 localizes to the **endomembrane system/ER** rather than the plasma membrane | GFP fusion reporter; colocalization with ER marker; biochemical fractionation | ges-1::abu-1::gfp showed **punctate vesicular pattern** in intestine, tending to cluster near the **apical** surface; FLAG-ABU-1 colocalized with **ribophorin I** | Transgenic *C. elegans* intestine reporter and COS1 cell expression | Urano et al., 2002, *J Cell Biol*, https://doi.org/10.1083/jcb.200203086 (pqac-00000000, pqac-00000001, pqac-00000015) |
| **Basal expression pattern:** abu-1 is constitutively expressed in the **pharynx/head** and at low basal levels in intestine | Promoter/reporter assay | Strong pharyngeal/head expression from **L3–L4 to young adult**; low intestinal baseline that becomes stress inducible | abu-1::gfp reporter in living worms | Urano et al., 2002, *J Cell Biol*, https://doi.org/10.1083/jcb.200203086 (pqac-00000003, pqac-00000004) |
| **ER-stress regulation:** abu-1 is preferentially induced when the canonical **IRE-1/XBP-1 UPR** is blocked | Microarray; Northern blot; stress reporters | **abu-1/AC3.3 base-2 log fold induction** after tunicamycin: **1.45 ± 0.29 in xbp-1 mutants vs 0.16 ± 0.46 in N2** | Tunicamycin-treated worms comparing wild type and **xbp-1** mutants | Urano et al., 2002, *J Cell Biol*, https://doi.org/10.1083/jcb.200203086 (pqac-00000001, pqac-00000010) |
| **Stress-inducible tissue response:** ER stress induces abu-1 expression in intestine | GFP reporter under chemical stress | abu-1::gfp induced by **tunicamycin** and **cadmium** in intestine, especially in **xbp-1** mutants | Chemical ER stress in transgenic worms | Urano et al., 2002, *J Cell Biol*, https://doi.org/10.1083/jcb.200203086 (pqac-00000000, pqac-00000014, pqac-00000015) |
| **Loss of abu-1 function causes ER stress:** ABU-1 normally helps maintain ER proteostasis | RNAi knockdown with ER-stress reporter | **abu-1(RNAi)** induced the ER stress reporter **hsp-4::gfp** in otherwise normal animals | Feeding RNAi in worms carrying hsp-4::gfp | Urano et al., 2002, *J Cell Biol*, https://doi.org/10.1083/jcb.200203086 (pqac-00000002, pqac-00000003, pqac-00000011, pqac-00000015) |
| **Functional placement:** ABU-1 protects animals specifically when the canonical UPR is impaired | RNAi + survival assay | abu-1(RNAi) killed about **50%** of ER-stressed **ire-1** and **xbp-1** mutant animals | ER stress induced in UPR-defective backgrounds | Urano et al., 2002, *J Cell Biol*, https://doi.org/10.1083/jcb.200203086 (pqac-00000002) |
| **Genetic interaction with ERAD:** abu-1 acts partly redundantly with **sel-1** | Double perturbation genetics; phenotypic analysis | Combined **abu-1 RNAi + sel-1 inactivation** increased lethality and caused prominent dark intestinal granules/vesicles | ER quality-control stress in worms | Urano et al., 2002, *J Cell Biol*, https://doi.org/10.1083/jcb.200203086 (pqac-00000003, pqac-00000004, pqac-00000014) |
| **Recent pathway placement (2023):** abu-family genes remain markers/effectors of **non-canonical ER proteostasis stress** | RNA-seq/RT-qPCR in aging/dietary restriction study | In **eat-2; acs-20 vs eat-2**, **138 genes** were upregulated using **FC >2, adjusted p <0.01**; ERUPR was top GO term and **abu genes** were among validated induced transcripts | Dietary restriction/epidermal lipid metabolism perturbation linked to ER proteostasis | Wang et al., 2023, *Nat Commun*, https://doi.org/10.1038/s41467-023-43613-4 (pqac-00000008) |
| **Innate immunity role:** pqn/abu genes, including **abu-1**, act in a **CED-1-dependent** host-defense pathway | Genetics, overexpression, RNAi, infection assays | In **ced-1** mutants, by **48 h** more than **50%** of animals showed infected pharynges; abu-1 overexpression rescued susceptibility to live *Salmonella* | *S. enterica* infection; pharyngeal invasion and survival assays | Haskins et al., 2008, *Dev Cell*, https://doi.org/10.1016/j.devcel.2008.05.006 (pqac-00000013, pqac-00000016, pqac-00000017) |
| **abu-1 is functionally protective in infection:** reducing abu-1 increases pathogen invasion; increasing ABU-1 improves defense | RNAi knockdown and transgenic overexpression | Figure-based evidence shows **abu-1 RNAi increased Salmonella pharyngeal invasion**; **ABU-1 overexpression** rescued **ced-1(e1735)** susceptibility | Live bacterial infection, confocal and survival assays | Haskins et al., 2008, *Dev Cell*, https://doi.org/10.1016/j.devcel.2008.05.006 (pqac-00000013, pqac-00000016, pqac-00000017) |
| **Neuronal immune regulation:** noncanonical **pqn/abu** genes are negatively regulated by neuronal **OCTR-1** signaling | Genetic analysis; genome-wide expression profiling | octr-1 mutants showed enhanced resistance to *P. aeruginosa* and increased expression of noncanonical UPR/immune genes including pqn/abu cohort (gene-set level rather than abu-1-specific value in excerpt) | Sensory-neuron control of peripheral immunity | Sun et al., 2011, *Science*, https://doi.org/10.1126/science.1203411 (pqac-00000012) |
| **Current best functional interpretation:** ABU-1 is not an enzyme or transporter with known substrate; instead it is best supported as an **ER/endomembrane membrane protein involved in proteostasis quality control**, especially under **blocked canonical UPR** and during **infection-associated stress** | Synthesis of localization, induction, and genetic interaction evidence | No catalytic activity or transported substrate demonstrated; strongest evidence is from localization, reporter induction, synthetic sickness/lethality, and infection phenotypes | Integrative conclusion from primary studies | Supported collectively by Urano et al., 2002; Haskins et al., 2008; Sun et al., 2011; Wang et al., 2023 (pqac-00000002, pqac-00000013, pqac-00000012, pqac-00000008) |


*Table: This table compiles the strongest primary evidence for the identity, structure, localization, stress regulation, genetic interactions, and immunity-related roles of C. elegans ABU-1/AC3.3. It is designed to support a precise functional annotation centered on ER/endoplasmic membrane proteostasis rather than an enzymatic or transporter activity.*