ITFG2

ITFG2 functions as a lysosomal scaffold subunit and a cytoplasmic inhibitor of ubiquitin-mediated protein degradation. As a core component of the KICSTOR complex (with KPTN, C12orf66, and SZT2), ITFG2 forms a heterodimer with KPTN that docks onto the SZT2 crescent-shaped scaffold at lysosomes, where KICSTOR recruits the GATOR1 complex to enable nutrient-sensing (amino acid, glucose, S-adenosylmethionine) inhibition of mTORC1 via the Rag GTPases (PMID:28199306, PMID:35776786, PMID:41198956). Independent of mTORC1, ITFG2 binds the E3 ubiquitin ligase NEDD4-2 and competitively inhibits its interaction with multiple substrates—ATP5b, SERCA2a, and Nav1.5—thereby protecting these proteins from ubiquitination and degradation in cardiomyocytes under ischemic stress (PMID:38848780, PMID:39477020, PMID:39864577). ITFG2 also plays a cell-intrinsic role in B cell differentiation and migration, as Itfg2-deficient mice exhibit defective germinal center responses, impaired plasma cell homing to bone marrow, and reduced serum IgG (PMID:23997217).

1. 2013 High

   The first in vivo function of ITFG2 was established: it is required cell-intrinsically for B cell differentiation, germinal center maturation, and plasma cell migration to bone marrow, resolving whether it acts as a surface receptor (it does not) or an intracellular regulator of immune cell fate.

   Evidence Itfg2 knockout mice, bone marrow transplantation, immunization, in vitro migration assays, flow cytometry

   PMID:23997217

   Open questions at the time

   • Molecular mechanism by which ITFG2 controls B cell migration is unknown
   • Whether the B cell phenotype relates to mTORC1 signaling was not tested
   • No interacting partners identified in this context
2. 2017 High

   ITFG2 was identified as a subunit of the KICSTOR complex that localizes to lysosomes and recruits GATOR1 to inhibit mTORC1 upon nutrient deprivation, establishing the mechanistic basis for how upstream nutrient signals reach GATOR1 and the Rag GTPases.

   Evidence Reciprocal co-immunoprecipitation, lysosomal fractionation, genetic knockout in human cells and mouse tissues, epistasis analysis with GATOR1/GATOR2/Rag components

   PMID:28199306

   Open questions at the time

   • Structural organization of KICSTOR and the ITFG2-KPTN interface were not resolved
   • Whether ITFG2 has functions outside the KICSTOR-mTORC1 axis was unclear
   • Relationship between KICSTOR and the B cell phenotype of ITFG2 deficiency was not addressed
3. 2021 Medium

   A genome-wide CRISPR screen independently confirmed ITFG2 as a negative regulator of mTORC1, showing that its loss confers resistance to PI3K-AKT inhibition by sustaining mTOR signaling in cancer cells.

   Evidence Whole-genome CRISPR/Cas9 knockout screen with individual gene knockout validation and mTOR signaling readouts in cancer cell lines

   PMID:33685991

   Open questions at the time

   • Whether resistance operates solely through KICSTOR-GATOR1 or involves additional pathways was not dissected
   • Clinical relevance of ITFG2 loss in drug resistance remains correlative
4. 2022 High

   The KICSTOR complex was shown to serve as the docking platform for SAMTOR, linking S-adenosylmethionine sensing to GATOR1-mediated mTORC1 inhibition and broadening the nutrient inputs that signal through ITFG2-containing KICSTOR.

   Evidence Crystal structures of SAMTOR in apo and SAM-bound states, in vitro binding assays, mutagenesis

   PMID:35776786

   Open questions at the time

   • Which KICSTOR subunit directly contacts SAMTOR was not resolved
   • In vivo physiological relevance of SAM sensing through KICSTOR not tested
5. 2024 Medium

   A distinct mTORC1-independent function was uncovered: ITFG2 binds NEDD4-2 and inhibits its ubiquitin ligase activity toward multiple cardiac substrates (ATP5b, SERCA2a), protecting mitochondrial function and calcium homeostasis in ischemic cardiomyocytes.

   Evidence Co-immunoprecipitation, ubiquitination assays, AAV9-mediated overexpression/knockdown in myocardial infarction mouse model, primary cardiomyocyte hypoxia assays

   PMID:38848780 PMID:39477020

   Open questions at the time

   • All NEDD4-2 studies come from a single laboratory; independent replication is needed
   • Whether ITFG2 acts on NEDD4-2 as part of KICSTOR or independently is unknown
   • Structural basis for the ITFG2-NEDD4-2 interaction has not been determined
6. 2025 High

   Cryo-EM resolved the architecture of KICSTOR, showing that the ITFG2-KPTN heterodimer binds the C-terminal region of the SZT2 scaffold, and revealed that FBXO2-mediated ubiquitination of KPTN disrupts both the KPTN-ITFG2 and KPTN-SZT2 interfaces, providing a regulatory mechanism for KICSTOR disassembly.

   Evidence Cryo-electron microscopy, computational modeling, co-immunoprecipitation, ubiquitination assays, mTORC1 signaling readouts

   PMID:41198956 PMID:41401028

   Open questions at the time

   • High-resolution structure of ITFG2 itself within the complex is not fully resolved
   • Whether FBXO2-mediated disassembly occurs under physiological nutrient conditions is untested
7. 2025 Medium

   ITFG2 inhibition of NEDD4-2 was extended to a third substrate, Nav1.5, stabilizing sodium channel expression and current density in post-infarction cardiomyocytes and reducing arrhythmia susceptibility.

   Evidence Co-immunoprecipitation, patch-clamp electrophysiology, ubiquitination assay, AAV9 overexpression/knockdown in MI mouse model

   PMID:39864577

   Open questions at the time

   • All three NEDD4-2 substrate studies are from one group; independent confirmation lacking
   • Whether ITFG2 is a general NEDD4-2 inhibitor or substrate-selective is unresolved

• Key unresolved questions include: whether the B cell and cardiomyocyte functions of ITFG2 are mechanistically connected to KICSTOR-mTORC1 signaling or represent independent activities; the structural basis of ITFG2 interaction with NEDD4-2; and whether ITFG2 has additional NEDD4-2-independent roles outside the KICSTOR complex.
• No study has tested whether ITFG2's immune and cardiac functions depend on KICSTOR or mTORC1
• Structural basis for ITFG2-NEDD4-2 binding is unknown
• Tissue-specific versus universal functions of ITFG2 have not been systematically mapped