CASTOR2

CASTOR2 functions as a cytosolic arginine sensor that tunes mTORC1 signaling in response to high arginine concentrations. CASTOR2 homodimerizes and heterodimerizes with CASTOR1, and both proteins interact with the GATOR2 component Mios to inhibit GATOR2–GATOR1 binding; arginine binding induces conformational changes at the ACT2–ACT4 domain interface that release Mios, thereby permitting GATOR2 to activate Rag GTPases and mTORC1 (PMID:26972053, PMID:41506264). Compared with CASTOR1, CASTOR2 has lower arginine sensitivity and responds at high arginine levels, functioning in muscle cells to regulate mTORC1 activity and myogenesis (PMID:41506264). Overexpression of CASTOR2 inhibits mTORC1 activation and suppresses proliferation in KSHV-transformed cells, and KSHV downregulates CASTOR2 expression to sustain mTORC1 signaling (PMID:31305263).

1. 2016 High

   The identification of CASTOR1 as an arginine sensor for mTORC1 established that CASTOR1 homodimerizes and heterodimerizes with CASTOR2, and that arginine binding disrupts the CASTOR1–GATOR2 interaction, linking the CASTOR proteins to nutrient-dependent mTORC1 regulation.

   Evidence Co-immunoprecipitation, biochemical arginine-binding assays, mutagenesis, and cell-based mTORC1 readouts in HEK293T cells

   PMID:26972053

   Open questions at the time

   • Whether CASTOR2 itself binds arginine and at what affinity was not determined
   • The structural basis for arginine-induced dissociation from GATOR2 was unresolved
   • CASTOR2-specific physiological roles remained undefined
2. 2019 Medium

   Demonstrating that CASTOR2 overexpression inhibits mTORC1 and cell proliferation in KSHV-transformed cells established CASTOR2 as a functionally relevant negative regulator of mTORC1 in a disease context.

   Evidence Overexpression and knockdown experiments, colony formation assays, and mTORC1 activity readouts in KSHV-transformed cells

   PMID:31305263

   Open questions at the time

   • CASTOR2's mechanism of mTORC1 inhibition (direct GATOR2 binding vs. indirect) was not biochemically dissected
   • Endogenous CASTOR2 loss-of-function phenotype in non-viral contexts was not tested
   • Whether CASTOR2 acts independently of CASTOR1 in this system was unclear
3. 2026 High

   Biochemical and structural characterization revealed that CASTOR2 binds arginine with lower sensitivity than CASTOR1, interacts with the GATOR2 subunit Mios, and undergoes ACT-domain conformational changes upon arginine binding that release Mios, establishing CASTOR2 as a high-arginine-concentration sensor that regulates mTORC1 and myogenesis.

   Evidence Biochemical binding assays, structural analysis of ACT2–ACT4 conformational changes, Co-IP with Mios/GATOR1/GATOR2, mutagenesis, and C2C12 myogenesis assays

   PMID:41506264

   Open questions at the time

   • Full atomic-resolution structure of the CASTOR2–Mios complex is not yet available
   • In vivo physiological consequences of CASTOR2 loss in muscle tissue have not been demonstrated
   • How CASTOR1 and CASTOR2 coordinate their differential arginine sensitivities within the same cell remains uncharacterized

• The in vivo tissue-specific roles of CASTOR2, the structural basis of differential arginine sensitivity between CASTOR1 and CASTOR2, and the physiological significance of CASTOR1–CASTOR2 heterodimers versus homodimers remain unresolved.
• No animal knockout models for CASTOR2 have been reported
• Heterodimerization stoichiometry and its functional consequences are undefined
• Whether CASTOR2 senses other amino acids or metabolites has not been tested