Affinage

CASTOR1

Cytosolic arginine sensor for mTORC1 subunit 1 · UniProt Q8WTX7

Length
329 aa
Mass
36.3 kDa
Annotated
2026-04-28
16 papers in source corpus 10 papers cited in narrative 10 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

CASTOR1 is a cytosolic arginine sensor that couples intracellular arginine availability to mTORC1 activation through the GATOR2–GATOR1–Rag GTPase signaling axis. Structurally, CASTOR1 is a homodimer built from four tandem ACT domains per subunit, with ACT1/ACT3 mediating dimerization and ACT2/ACT4 forming the arginine-binding pocket at their interface; in the arginine-free state, a negatively charged surface on CASTOR1 engages two WD40 β-propellers of the GATOR2 subunit MIOS, sequestering GATOR2 and thereby permitting GATOR1-mediated inactivation of Rag GTPases and suppression of mTORC1 (PMID:27487210, PMID:28066558, PMID:40715445). Arginine binding induces ordering of specific loop regions that occlude the MIOS-binding interface, releasing GATOR2 to activate mTORC1, without requiring large-scale conformational rearrangement of the protein (PMID:30503338, PMID:40715445). An arginine-independent regulatory layer exists in which AKT phosphorylates CASTOR1 at Ser14, promoting RNF167-catalyzed K29-linked polyubiquitination and proteasomal degradation of CASTOR1, thereby activating mTORC1 even under low-arginine conditions (PMID:33594058).

Mechanistic history

Synthesis pass · year-by-year structured walk · 6 steps
  1. 2016 High

    Establishing that CASTOR1 is the direct arginine sensor upstream of GATOR2–mTORC1 resolved how cells relay arginine sufficiency to mTORC1 and revealed the structural basis: a homodimer with four ACT domains per subunit, where arginine binds at the ACT2–ACT4 interface and allosterically controls a GATOR2 (MIOS)-binding surface on the opposite face.

    Evidence Three independent crystal structures (1.8–2.5 Å), mutagenesis, and in vitro binding assays across three groups

    PMID:27487210 PMID:27648300 PMID:28066558

    Open questions at the time
    • No structure of the CASTOR1–GATOR2 complex was available to define the binding interface at residue resolution
    • Whether arginine binding causes large-scale conformational change or local rearrangement was unresolved
    • Functional consequence of CASTOR1 regulation in vivo was not yet tested
  2. 2018 High

    Comparison of arginine-bound and apo crystal structures demonstrated that CASTOR1 does not undergo a global conformational change upon arginine binding, restricting the allosteric switch to local loop reordering near the ligand pocket.

    Evidence Crystal structures of apo (2.8 Å) and arginine-bound (2.05 Å) CASTOR1 compared in the same study

    PMID:30503338

    Open questions at the time
    • Which specific loops mediate MIOS occlusion remained undefined
    • No full complex structure to visualize how loop changes translate into loss of GATOR2 binding
  3. 2019 Medium

    The discovery that KSHV-encoded miRNAs directly suppress CASTOR1 expression to activate mTORC1 established that viral pathogens exploit CASTOR1 downregulation as a strategy to hijack nutrient signaling.

    Evidence miRNA target validation via luciferase reporters, miRNA knockdown/overexpression with mTORC1 readouts in KSHV-infected cells

    PMID:31305263

    Open questions at the time
    • Single-lab study; independent replication in other viral systems not performed
    • Whether endogenous host miRNAs similarly regulate CASTOR1 was not explored
  4. 2021 High

    Identification of AKT-mediated Ser14 phosphorylation followed by RNF167-catalyzed K29-linked polyubiquitination and proteasomal degradation of CASTOR1 revealed an arginine-independent mechanism for mTORC1 activation through growth-factor signaling.

    Evidence Reciprocal co-immunoprecipitation, in vitro kinase assay, ubiquitin-linkage typing, phospho-mutant analysis in cells

    PMID:33594058

    Open questions at the time
    • Physiological contexts in which AKT-mediated CASTOR1 degradation dominates over arginine sensing are unclear
    • Whether other kinases or E3 ligases participate in CASTOR1 turnover was not tested
  5. 2025 High

    Cryo-EM of the GATOR2–CASTOR1 complex resolved how two MIOS WD40 β-propellers engage both subunits of a CASTOR1 homodimer and how arginine-induced loop ordering sterically blocks this interface, providing a complete structural mechanism for the arginine-dependent switch.

    Evidence Cryo-electron microscopy of human GATOR2 bound to CASTOR1 (arginine-free), with functional validation

    PMID:40715445

    Open questions at the time
    • Dynamic intermediates of CASTOR1 dissociation from GATOR2 upon arginine binding have not been captured
    • Whether CASTOR1 regulates GATOR2 enzymatic activity or only its interaction with GATOR1 is unresolved
  6. 2026 High

    Discovery that CASTOR2, a CASTOR1 paralog, shares the same GATOR2-inhibitory mechanism but responds to high rather than low arginine concentrations established a two-sensor system that tunes mTORC1 across a broad range of arginine availability.

    Evidence Biochemical binding assays, mutagenesis, and mTORC1 activity measurements in C2C12 cells comparing CASTOR1 and CASTOR2

    PMID:41506264

    Open questions at the time
    • The physiological significance of dual-sensor regulation in specific tissues or developmental contexts is unexplored
    • Whether CASTOR1–CASTOR2 heterodimers form and function is unknown

Open questions

Synthesis pass · forward-looking unresolved questions
  • How CASTOR1 integrates with parallel amino-acid sensing branches (e.g., Sestrin2/leucine, SAMTOR/SAM) at the level of GATOR2 complex stoichiometry and dynamics, and the in vivo consequences of CASTOR1 loss in normal physiology and disease, remain incompletely understood.
  • No structural model of GATOR2 simultaneously engaged by multiple upstream sensors
  • Conditional knockout studies in specific tissues (beyond KRAS-driven lung tumors) have not been reported in peer-reviewed literature
  • Whether CASTOR1 has functions independent of GATOR2–mTORC1 signaling is untested

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0140299 molecular sensor activity 5 GO:0098772 molecular function regulator activity 4
Localization
GO:0005829 cytosol 2
Pathway
R-HSA-162582 Signal Transduction 5
Partners
AKT1CASTOR2MIOSRNF167

Evidence

Reading pass · 10 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2016 CASTOR1 is a homodimeric cytosolic arginine sensor that binds arginine at the interface of two ACT domains; arginine binding allosterically controls the adjacent GATOR2-binding site, triggering dissociation of CASTOR1 from GATOR2 and downstream activation of mTORC1. Crystal structure at 1.8 Å revealed structural homology to the lysine-binding regulatory domain of prokaryotic aspartate kinases. X-ray crystallography (1.8 Å), in vitro binding assays, mutagenesis Nature High 27487210
2016 CASTOR1 binds arginine in a pocket carved between its NTD and CTD (ACT) domains; a surface patch on CASTOR1-NTD opposite the arginine-binding site mediates direct physical interaction with GATOR2 subunit Mios; mutation of this patch disrupts CASTOR1's inhibition of GATOR2. X-ray crystallography, in vitro pull-down assay, normal mode analysis, mutagenesis Cell discovery High 28066558
2016 CASTOR1 comprises four tandem ACT domains arranged like the C-terminal allosteric domains of aspartate kinases; ACT1 and ACT3 mediate homodimerization, while ACT2 and ACT4 form the arginine-binding pocket; key residues validated by mutagenesis and biochemical assays. X-ray crystallography (2.5 Å), mutagenesis, biochemical binding assays Cell discovery High 27648300
2018 Comparison of arginine-bound and apo crystal structures of CASTOR1 revealed near-identical overall conformations with differences confined to two loop regions, indicating CASTOR1 does not undergo large conformational changes upon arginine binding. X-ray crystallography (2.05 Å bound; 2.8 Å apo), structural comparison Biochemical and biophysical research communications High 30503338
2021 E3 ubiquitin ligase RNF167 ubiquitinates CASTOR1 with K29-linked polyubiquitin chains, leading to its proteasomal degradation. AKT phosphorylates CASTOR1 at Ser14, which increases CASTOR1 binding to RNF167 and promotes its ubiquitination/degradation, while simultaneously decreasing CASTOR1 affinity for MIOS (GATOR2 subunit), thereby activating mTORC1 independent of arginine levels. Co-immunoprecipitation, ubiquitination assay, phospho-mutagenesis, in vitro kinase assay, cell-based loss-of-function/gain-of-function Nature communications High 33594058
2025 Cryo-EM structure of human GATOR2 bound to CASTOR1 (in absence of arginine) shows that two MIOS WD40 domain β-propellers of the GATOR2 cage engage both subunits of a single CASTOR1 homodimer at a negatively charged MIOS-binding interface distal to the arginine pocket; arginine-triggered loop ordering in CASTOR1 blocks this MIOS-binding interface, explaining how arginine binding switches off CASTOR1-GATOR2 interaction to activate mTORC1. Cryo-electron microscopy, structural analysis, functional validation Nature structural & molecular biology High 40715445
2019 KSHV-encoded miRNAs miR-K4-5p (and miR-K1-5p) directly target the CASTOR1 3'-UTR to suppress CASTOR1 expression, thereby relieving CASTOR1-mediated inhibition of GATOR2 and activating mTORC1; knockdown of these miRNAs restored CASTOR1 expression and attenuated mTORC1 activation. miRNA target validation, luciferase reporter assay, miRNA knockdown/overexpression, mTORC1 activity assays The Journal of clinical investigation Medium 31305263
2026 CASTOR1 and its paralog CASTOR2 both bind arginine and interact with GATOR2 subunit Mios to inhibit its binding to GATOR1, but CASTOR1 responds to low arginine levels while CASTOR2 responds to high arginine concentrations; arginine binding induces conformational changes at the ACT2-ACT4 interface causing dissociation from Mios. Biochemical binding assays, structural/conformational analysis, cell-based mTORC1 activity assays (C2C12 cells), mutagenesis Molecular cell High 41506264
2022 CASTOR1 overexpression inhibits mTOR signaling pathway activation, and this inhibition of mTOR mediates CASTOR1's regulatory effect on microglia M1/M2 polarization; treatment with mTOR activator MHY1485 attenuated the anti-M1 effect of CASTOR1 overexpression, placing CASTOR1 upstream of mTOR in microglial polarization. Overexpression, pharmacological rescue (mTOR activator), cytokine/marker assays in microglia Metabolic brain disease Medium 36454504
2025 CASTOR1 loss activates mTORC1 signaling (elevated 4EBP1 and S6 phosphorylation) and also augments AKT and ERK activation, revealing crosstalk between the PI3K/AKT/mTORC1 and KRAS/ERK pathways; CASTOR1 genetic ablation in a KRAS-driven GEMM accelerated lung tumor initiation and progression. Genetically engineered mouse model (KRAS G12D GEMM with CASTOR1 KO), phospho-protein analysis, organoid cultures bioRxivpreprint Medium 40313924

Source papers

Stage 0 corpus · 16 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2016 Mechanism of arginine sensing by CASTOR1 upstream of mTORC1. Nature 256 27487210
2016 Structural mechanism for the arginine sensing and regulation of CASTOR1 in the mTORC1 signaling pathway. Cell discovery 47 28066558
2021 RNF167 activates mTORC1 and promotes tumorigenesis by targeting CASTOR1 for ubiquitination and degradation. Nature communications 41 33594058
2019 Kaposi sarcoma-associated herpesvirus miRNAs suppress CASTOR1-mediated mTORC1 inhibition to promote tumorigenesis. The Journal of clinical investigation 35 31305263
2016 Structural insight into the arginine-binding specificity of CASTOR1 in amino acid-dependent mTORC1 signaling. Cell discovery 34 27648300
2024 L-arginine alleviates heat stress-induced mammary gland injury through modulating CASTOR1-mTORC1 axis mediated mitochondrial homeostasis. The Science of the total environment 14 38552976
2018 Crystal structures of arginine sensor CASTOR1 in arginine-bound and ligand free states. Biochemical and biophysical research communications 13 30503338
2025 Structural basis for mTORC1 regulation by the CASTOR1-GATOR2 complex. Nature structural & molecular biology 7 40715445
2025 piR-16404 drives ferroptotic liver injury via CASTOR1/mTORC1/GPX4 dysregulation in HepG2 cells and mice: a novel toxicity mechanism of N, N-dimethylformamide. Archives of toxicology 7 40982001
2022 Downregulation of CASTOR1 Inhibits Heat-Stress-Induced Apoptosis and Promotes Casein and Lipid Synthesis in Mammary Epithelial Cells. Journal of agricultural and food chemistry 6 35442666
2022 Castor1 overexpression regulates microglia M1/M2 polarization via inhibiting mTOR pathway. Metabolic brain disease 6 36454504
2025 Upregulating mTOR/S6 K Pathway by CASTOR1 Promotes Astrocyte Proliferation and Myelination in Gpam-/--induced mouse model of cerebral palsy. Molecular neurobiology 2 40234290
2026 CASTOR1 and CASTOR2 respond to different arginine levels to regulate mTORC1 activity. Molecular cell 0 41506264
2026 Putative identification of CASTOR1 as one of the targets of ganoderic acid a via thermal proteome profiling and molecular docking. Frontiers in pharmacology 0 42028441
2025 CASTOR1: A Novel Tumor Suppressor Linking mTORC1 and KRAS Pathways in Tumorigenesis and Resistance to KRAS-Targeted Therapies in Non-Small Cell Lung Cancer. bioRxiv : the preprint server for biology 0 40313924
2025 Structural basis for mTORC1 regulation by the CASTOR1-GATOR2 complex. Research square 0 40470200