ANKRD9

ANKRD9 is a metabolically regulated ankyrin-repeat protein that functions as the substrate-recognition subunit of a CUL5–ELOB–ELOC–RNF7 cullin-RING E3 ubiquitin ligase, targeting the purine-biosynthesis enzymes IMPDH1 and IMPDH2 for ubiquitination and proteasomal degradation (PMID:30293565). Its control of IMPDH2 is coupled to cellular nutrient and guanosine status: under basal conditions ANKRD9 is sequestered in vesicle-like structures away from cytosolic IMPDH2, but upon nutrient limitation it relocalizes and co-assembles with IMPDH2 into rod-like filaments, a vesicle-to-rod transition and IMPDH2 binding that requires its conserved Cys109–Cys110 motif; guanosine addition reverses rod formation and restores the vesicular pattern (PMID:31337707). Through this IMPDH2-degrading activity ANKRD9 acts as a negative regulator of skeletal myogenesis, where its overexpression suppresses myoblast proliferation and differentiation and its knockdown increases muscle mass, effects reversed by restoring IMPDH2 (PMID:41691811). In intestinal enterocytes ANKRD9 regulates purine-biosynthesis enzymes to sustain ATP synthesis, and its loss in mice lowers intestinal ATP, alters Golgi morphology, delays ApoB/chylomicron trafficking, and produces lipid accumulation with a lean phenotype, linking purine metabolism to dietary fat absorption (PMID:41826336).

1. 2018 High

   Established ANKRD9's biochemical identity by defining it as the substrate receptor of a specific CUL5-based E3 ligase and identifying IMPDH1/2 as its degradation substrates, answering what molecular activity the protein carries.

   Evidence Quantitative proteomics, complex reconstitution, and in vitro ubiquitylation assays defining CUL5-ELOB-ELOC-RNF7 assembly and IMPDH ubiquitination

   PMID:30293565

   Open questions at the time

   • Structural basis of IMPDH recognition by the ankyrin repeats not defined
   • Whether substrates beyond IMPDH1/2 exist not addressed
2. 2019 High

   Showed that ANKRD9-mediated IMPDH2 regulation is metabolically gated through a guanosine/nutrient-responsive vesicle-to-rod relocalization requiring the Cys109-Cys110 motif, explaining how substrate engagement is controlled.

   Evidence Live imaging, subcellular fractionation, siRNA knockdown, Cys109/Cys110 mutagenesis, ribavirin and guanosine treatments

   PMID:31337707

   Open questions at the time

   • Mechanism by which the Cys109-Cys110 motif senses metabolic state unknown
   • Composition and identity of the vesicle-like structures not defined
   • Whether rod incorporation promotes or inhibits IMPDH2 degradation not fully resolved
3. 2009 Medium

   Provided the earliest functional context by linking ANKRD9 expression to nutritional and hormonal state and lipid/fatty acid oxidation disorders, hinting at a metabolic role before its molecular function was known.

   Evidence Expression profiling in riboflavin-deficient chicken liver, hormonal/fasting/re-feeding regulation, GFP-tagged cytoplasmic localization

   PMID:19788857

   Open questions at the time

   • Mechanistic link between ANKRD9 and lipid metabolism inferred, not tested
   • No functional perturbation performed
4. 2026 Medium

   Demonstrated a physiological output of the ANKRD9-IMPDH2 axis by showing ANKRD9 negatively regulates skeletal myogenesis through IMPDH2 degradation, with epistatic rescue confirming IMPDH2 as the relevant effector.

   Evidence Overexpression/knockdown in chicken myoblasts, in vivo siRNA, co-IP, ubiquitination assays, and IMPDH2 re-expression rescue

   PMID:41691811

   Open questions at the time

   • Single-lab study without independent replication
   • Chicken-specific; relevance to mammalian muscle not tested here
5. 2026 High

   Connected ANKRD9 to whole-animal physiology by showing it couples purine-driven ATP synthesis to Golgi dynamics and chylomicron trafficking in enterocytes, establishing a role in dietary fat absorption.

   Evidence Ankrd9 knockout mice with intestinal ATP measurements, Golgi imaging, ApoB/chylomicron trafficking assays, and body composition analysis

   PMID:41826336

   Open questions at the time

   • Direct mechanistic link from IMPDH2/purine enzymes to Golgi and ApoB trafficking not fully resolved
   • Whether the ATP defect alone explains the trafficking phenotype unclear
6. 2026 Low

   Added associative metabolic support that ANKRD9 suppresses IMP accumulation via the purine pathway in myoblasts.

   Evidence ANKRD9 overexpression in chicken myoblasts with ELISA for IMP plus transcriptomics/metabolomics

   PMID:41769753

   Open questions at the time

   • Key claim rests on a single ELISA readout; not independently confirmed
   • Transcriptomic/metabolomic associations not causally tested

• How ANKRD9's metabolic sensing, IMPDH2 filament dynamics, and downstream control of Golgi/lipoprotein trafficking are mechanistically integrated remains unresolved.
• No structural model of ANKRD9-IMPDH recognition
• Causal chain from ATP/purine flux to Golgi and chylomicron trafficking undefined
• Human in vivo relevance of myogenic and intestinal roles not established