CGRRF1 (Q99675) review notes

Identity

• Gene: CGRRF1 (cell growth regulator with RING finger domain protein 1); synonyms CGR19, RNF197; HGNC:15528; UniProt Q99675; 332 aa.
• Domain architecture: N-terminal hydrophobic/transmembrane region (sequence MAAVFLVTLY...GLVLGWFGWDV, ~residues 1-50, multiple hydrophobic stretches consistent with membrane anchoring) and a C-terminal cytosolic RING-type zinc finger (FT ZN_FING 274..309, RING-type; PROSITE PS50089; CDD cd16787 mRING-HC-C3HC5_CGRF1). NMR structure of the RING domain solved (PDB 2EA5, residues 264-318) [file:human/CGRRF1/CGRRF1-uniprot.txt "PDB; 2EA5; NMR; -; A=264-318"].

Origin / discovery

• Originally isolated as "CGR19", a p53-induced cell growth regulatory gene, in differential RT-PCR of rat embryo fibroblasts with a temperature-sensitive p53 allele. Predicted protein contains a RING-finger domain. CGR19 (and CGR11) "are able to inhibit the growth of several cell lines" PMID:8968090. This is the basis for the cell-growth/proliferation annotations. Note: assay is overexpression-based growth inhibition; no mechanism established.

ER membrane E3 ligase / ERAD context

• CGRRF1 was one of 37 human RING-finger + transmembrane proteins computationally selected as candidate ERAD E3 ligases PMID:27485036.
• Its expression is significantly up-regulated by ER stress (thapsigargin, tunicamycin) and induced by ATF6 (N-terminal domain) [PMID:27485036 "The expression of four genes RNFT1, RNF185, CGRRF1 and RNF19B was significantly elevated by both Tg and Tm"; "ATF6 (N-terminal domain) significantly upregulated CGRRF1 and RNF19B expression"]. Peak induction late (~48 h), suggesting indirect UPR pathway.
• Subcellular localization: V5-CGRRF1 partially colocalized with PDI (ER chaperone) in COS-1 cells, indicating ER localization PMID:27485036. UniProt SUBCELLULAR LOCATION lists Nucleus (ECO:0000269|PubMed:22361696) and Endoplasmic reticulum (ECO:0000269|PubMed:27485036).

IMPORTANT caveat on E3 activity (negative result)

• In the same study, in vitro autoubiquitination assays (E1=UBE1, E2=UbcH5c/UBE2D3, ubiquitin) showed CGRRF1 did NOT exhibit autoubiquitination/E3 activity, unlike RNFT1/RNF185/RNF19B PMID:27485036.
• CGRRF1 also did NOT confer ER stress resistance PMID:27485036.
• Interpretation: CGRRF1 has all the structural hallmarks of an ER-membrane RING E3 ligase (RING domain + TM anchor + ER localization + UPR induction), and is strongly predicted to be one, but direct biochemical demonstration of ubiquitin ligase activity is lacking; the one published in vitro test (with a single E2, UbcH5c) was negative. The "ubiquitin protein ligase activity" inference therefore rests on RING-domain homology/structure, not on a positive functional assay. No specific physiological substrate has been demonstrated. Accordingly, MF E3 ligase activity is best treated as supported-by-homology but not core-experimentally-proven; I do not ACCEPT it as a curated experimental function here because it is not present in the existing GOA annotation set and the only direct test was negative.

Protein interactions

• The only MF annotation in GOA is GO:0005515 protein binding (IPI), derived entirely from the HuRI high-throughput binary interactome (Y2H) screen PMID:32296183. UniProt INTERACTION lists ~30 partners, predominantly other multi-pass / single-pass membrane or ER/Golgi proteins (e.g., BET1, CYB5B, MARCHF5, RHBDD1, ZFPL1, YIPF6, JAGN1, SCAMP4). These are systematic Y2H hits, not validated functional partners, and are consistent with membrane-protein co-localization rather than a defined molecular function. Bare "protein binding" is uninformative.

Tissue expression

• Ubiquitously expressed; high in testis and cerebellum PMID:27485036. HPA: low tissue specificity.

Summary of curation decisions

• MF GO:0005515 protein binding (IPI, HuRI) -> MARK_AS_OVER_ANNOTATED (uninformative, HT only).
• CC GO:0005783 endoplasmic reticulum (IDA, PMID:27485036) -> ACCEPT (could be refined to GO:0005789 ER membrane given TM anchor, but IDA supports the parent ER directly).
• CC GO:0005783 endoplasmic reticulum (IEA, GO_REF:0000044, from UniProt SubCell) -> KEEP_AS_NON_CORE (redundant IEA of the experimentally supported ER term).
• CC GO:0005634 nucleus (IEA, from PubMed:22361696 antibody localization) -> KEEP_AS_NON_CORE (a TM-anchored ER membrane protein in the nucleus is biologically questionable; antibody-based, possibly the nucleoplasm signal; keep but non-core).
• CC GO:0005654 nucleoplasm (IDA, HPA) -> KEEP_AS_NON_CORE (HPA immunofluorescence; not consistent with primary ER-membrane function but retained as observed).
• BP GO:0030308 negative regulation of cell growth (IBA) -> KEEP_AS_NON_CORE (phylogenetic transfer rooted in the original p53/overexpression growth-inhibition phenotype; not the molecular core function).
• BP GO:0008285 negative regulation of cell population proliferation (TAS, PMID:8968090) -> KEEP_AS_NON_CORE (original overexpression phenotype; descriptive, not mechanistic core).

Proposed (not in GOA, supported by structure/homology, not added as accepted annotations)

• MF GO:0061630 ubiquitin protein ligase activity and BP GO:0036503 ERAD pathway are strongly suggested by the RING+TM+ER+UPR profile but lack positive functional evidence (the one in vitro test was negative). Raised in suggested_experiments rather than asserted.