ANKRD9

ANKRD9 is a metabolically regulated ankyrin repeat protein that functions as a substrate receptor of a CUL5–ELOB–ELOC–RNF7 E3 ubiquitin ligase complex, directly binding IMPDH1 and IMPDH2 to promote their ubiquitination and proteasomal degradation, thereby controlling purine nucleotide biosynthesis (PMID:30293565, PMID:41691811). Under nutrient-limiting conditions, ANKRD9 transitions from cytoplasmic vesicle-like structures to co-assemble with IMPDH2 into stabilizing rod-like filaments via its conserved Cys109–Cys110 motif, a switch that is reversed by guanosine supplementation (PMID:31337707). In intestinal enterocytes, ANKRD9 couples purine biosynthesis and ATP production to Golgi dynamics and chylomicron trafficking; Ankrd9 knockout in mice reduces intestinal ATP, disrupts Golgi morphology, delays ApoB/chylomicron secretion, and produces a lean phenotype with enterocyte lipid accumulation (PMID:41826336). ANKRD9 expression is itself regulated by metabolic cues including fatty acid oxidation status, thyroid hormone, and fasting–refeeding cycles (PMID:19788857).

1. 2009 Medium

   Establishing that ANKRD9 is a metabolically responsive gene resolved its initial biological context: its transcript levels change with fatty acid oxidation defects, thyroid hormone, and fasting/refeeding, implicating it in lipid/energy metabolism before any molecular function was known.

   Evidence qRT-PCR in lipid-perturbed mouse models and GFP-tagged localization in transfected cells

   PMID:19788857

   Open questions at the time

   • Single-lab GFP overexpression without endogenous localization confirmation
   • No direct functional assay or binding partner identified
   • Mechanism linking metabolic transcriptional regulation to protein function unknown
2. 2018 High

   Identifying ANKRD9 as a CRL5 substrate receptor that directly binds and ubiquitinates IMPDH1/2 established its first molecular function, explaining how it controls purine nucleotide metabolism at the post-translational level.

   Evidence Quantitative proteomics, reconstituted CUL5–ELOB–ELOC–RNF7 complex, in vitro ubiquitylation assay, knockdown with proliferation readouts in human cells

   PMID:30293565

   Open questions at the time

   • Structural basis of ANKRD9–IMPDH2 recognition unresolved
   • Whether ANKRD9 targets additional substrates beyond IMPDH1/2 not tested
   • Physiological signals controlling ANKRD9 E3 ligase activity not defined
3. 2019 High

   Demonstrating that ANKRD9 switches from vesicle-like puncta to IMPDH2-stabilizing rod filaments under nutrient limitation revealed a metabolite-sensitive regulatory mechanism, showing that ANKRD9 can toggle between degrading and protecting its substrate depending on cellular metabolic state.

   Evidence Live-cell fluorescence imaging, ribavirin/guanosine pharmacological perturbation, Cys109/Cys110 site-directed mutagenesis in cultured cells

   PMID:31337707

   Open questions at the time

   • Molecular trigger (specific metabolite or redox state) that initiates the vesicle-to-rod transition not identified
   • Whether the Cys109–Cys110 motif functions via disulfide bonding or another chemistry not resolved
   • Structural organization of ANKRD9 within IMPDH2 rods unknown
4. 2021 Medium

   Showing that miR-29b-1-5p directly targets the ANKRD9 3′UTR to suppress its expression placed ANKRD9 within a post-transcriptional regulatory circuit in muscle development.

   Evidence Luciferase 3′UTR reporter assay and western blotting in chicken primary myoblasts

   PMID:34852967

   Open questions at the time

   • Single-lab finding in chicken myoblasts; conservation in mammals not tested
   • Functional consequences of miR-29b-1-5p–mediated ANKRD9 suppression on IMPDH2 levels not assessed
   • Whether this regulatory axis operates in vivo during muscle growth not demonstrated
5. 2026 High

   In vivo knockout of Ankrd9 in mice revealed its physiological role in coupling purine biosynthesis/ATP production to Golgi integrity and chylomicron secretion in enterocytes, connecting its E3 ligase function to whole-organism lipid metabolism and body composition.

   Evidence Ankrd9 knockout mouse, intestinal ATP quantification, Golgi morphology imaging, ApoB/chylomicron trafficking assays, metabolic phenotyping

   PMID:41826336

   Open questions at the time

   • Whether the Golgi phenotype is a direct consequence of IMPDH2 mis-regulation or involves additional substrates not established
   • Tissue-specific contributions beyond enterocytes (e.g. liver, muscle) not dissected in this study
   • Lipid- and ATP-dependent localization signals on ANKRD9 protein not mapped
6. 2026 Medium

   Confirming the ANKRD9–IMPDH2 ubiquitination axis in a non-mammalian vertebrate (chicken) and linking it to myoblast proliferation and muscle mass broadened the functional scope and demonstrated evolutionary conservation of the pathway.

   Evidence Co-immunoprecipitation, ubiquitination assay, overexpression/knockdown with IMPDH2 rescue, in vivo siRNA knockdown in chicken

   PMID:41691811

   Open questions at the time

   • Cross-species validation in mammalian muscle not performed
   • Whether ANKRD9's effect on muscle differentiation is entirely IMPDH2-dependent or involves other substrates not fully excluded
   • Mechanism by which purine nucleotide levels regulate myoblast differentiation downstream not defined

• Key unresolved questions include the structural basis of ANKRD9–IMPDH2 recognition, the identity of the metabolite or redox signal triggering the vesicle-to-rod transition, whether ANKRD9 ubiquitinates substrates beyond IMPDH1/2, and the tissue-specific physiological roles outside the intestine.
• No crystal or cryo-EM structure of ANKRD9 or its complex with IMPDH2
• Full substrate repertoire of the ANKRD9–CRL5 complex not systematically surveyed
• Tissue-specific conditional knockout studies beyond intestine not reported