ITFG2

ITFG2 is a subunit of the lysosome-associated KICSTOR complex (with KPTN, C12orf66, and SZT2) that couples nutrient status to mTORC1 signaling by recruiting GATOR1 to the lysosomal surface, where GATOR1 acts on the Rag GTPases to enable amino acid- and glucose-dependent mTORC1 inhibition (PMID:28199306). Structurally, ITFG2 forms a heterodimer with KPTN docked onto a crescent-shaped SZT2 scaffold, with GATOR1 engaging SZT2's N-terminal domain via NPRL3; the complex localizes to lysosomes through SZT2/C12orf66 binding to negatively charged lipids, and disrupting the GATOR1–KICSTOR interface hyperactivates mTORC1 and mislocalizes TFE3 independently of nutrient cues (PMID:41198956). This assembly is regulated by ubiquitination: FBXO2 ubiquitinates KPTN to dissolve the KPTN–ITFG2 and KPTN–SZT2 interactions and thereby derepress mTORC1 (PMID:41401028), and loss of ITFG2 sustains mTORC1 signaling to confer resistance to PI3Kα inhibition in PIK3CA-mutant breast cancer cells, placing it as a negative regulator of mTORC1 downstream of PI3K-AKT (PMID:33685991). Independently of KICSTOR, ITFG2 acts in cardiomyocytes as an inhibitor of the E3 ligase NEDD4-2, blocking ubiquitination of ATP5b, SERCA2a, and Nav1.5 to preserve mitochondrial function, calcium homeostasis, and sodium-channel conduction under ischemic stress (PMID:38848780, PMID:39477020, PMID:39864577). ITFG2 is also required cell-intrinsically in the hematopoietic compartment for germinal center formation and normal humoral immunity (PMID:23997217).

1. 2013 Medium

   Established the first in vivo function for ITFG2, showing it is a hematopoietic cell-intrinsic factor required for B cell differentiation and germinal center responses, before any molecular mechanism was known.

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

   PMID:23997217

   Open questions at the time

   • No molecular partner or biochemical activity identified to explain the B cell phenotype
   • Connection to the later-defined KICSTOR/mTORC1 role not established
   • Single lab
2. 2017 High

   Defined ITFG2's core molecular function by identifying it as a subunit of the KICSTOR complex that recruits GATOR1 to lysosomes, linking ITFG2 mechanistically to nutrient-dependent mTORC1 inhibition.

   Evidence Co-IP, subcellular fractionation, siRNA knockdown, and mTORC1 activity assays in human cells plus SZT2-KO mouse tissue

   PMID:28199306

   Open questions at the time

   • ITFG2's specific contribution within KICSTOR not separated from other subunits
   • No structural basis for assembly
   • Relationship to the B cell phenotype unaddressed
3. 2021 Medium

   Placed ITFG2 epistatically as a negative regulator of mTORC1 downstream of PI3K-AKT, with disease relevance: its loss sustains mTORC1 to confer PI3Kα-inhibitor resistance.

   Evidence Genome-wide CRISPR/Cas9 screen with individual knockout and drug-sensitivity validation in PIK3CA-mutant breast cancer cells

   PMID:33685991

   Open questions at the time

   • Does not resolve ITFG2-specific versus complex-wide contribution
   • Mechanism of resistance beyond mTORC1 reactivation not dissected
4. 2022 Medium

   Showed how upstream methionine/SAM sensing feeds into the GATOR1–KICSTOR module, contextualizing the complex ITFG2 belongs to within SAM-responsive mTORC1 regulation.

   Evidence Apo and SAM-bound crystal structures of SAMTOR with mutagenesis and mTORC1 activity assays

   PMID:35776786

   Open questions at the time

   • ITFG2's direct role inferred from complex membership, not probed
   • No structure of ITFG2 within KICSTOR
5. 2024 Medium

   Revealed a KICSTOR-independent function: in cardiomyocytes ITFG2 binds NEDD4-2 and shields its substrates from ubiquitination, protecting mitochondrial and calcium-handling machinery under ischemia.

   Evidence Co-IP, ubiquitination assays, AAV9 overexpression/knockdown in MI mouse models, mitochondrial and calcium functional assays

   PMID:38848780 PMID:39477020

   Open questions at the time

   • Structural basis of the ITFG2–NEDD4-2 interaction unknown
   • Relationship between the NEDD4-2 and KICSTOR functions unresolved
   • Single lab
6. 2025 High

   Provided the structural architecture of KICSTOR and a ubiquitin-based regulatory mechanism, defining how the ITFG2–KPTN heterodimer is positioned on SZT2 and how FBXO2 ubiquitination of KPTN dismantles the complex.

   Evidence Cryo-EM and computational modeling of KICSTOR; Co-IP, lysine-mutagenesis ubiquitination assays, and mTORC1 readouts for FBXO2

   PMID:41198956 PMID:41401028

   Open questions at the time

   • High-resolution structure of ITFG2 itself not detailed
   • Physiological trigger for FBXO2-mediated KPTN ubiquitination unknown
7. 2025 Medium

   Extended the cardiac NEDD4-2 inhibitory role to electrical function, showing ITFG2 preserves Nav1.5 current and conduction velocity to suppress ischemic arrhythmias.

   Evidence Co-IP, ubiquitination assays, patch-clamp, AAV9 overexpression/knockdown in MI mice, optical mapping

   PMID:39864577

   Open questions at the time

   • Whether NEDD4-2 substrate selectivity is direct or indirect not resolved
   • Single lab

• It remains unknown how ITFG2's lysosomal KICSTOR/mTORC1 function relates mechanistically to its NEDD4-2-dependent cardiac role and its B cell-intrinsic requirement.
• No unifying model linking mTORC1 regulation, NEDD4-2 inhibition, and germinal center formation
• ITFG2-specific catalytic or binding determinants not defined