CASTOR1 functions as a cytosolic arginine sensor that regulates mTORC1 signaling through the amino acid-sensing pathway 1. The protein forms homodimers or heterodimers with CASTOR2 and binds to the GATOR2 complex, inhibiting mTORC1 activity under arginine-depleted conditions 1. When arginine becomes available, it binds directly to CASTOR1 with a dissociation constant of ~30 μM, inducing conformational changes that disrupt the CASTOR1-GATOR2 interaction and allow mTORC1 activation 12. Structurally, CASTOR1 engages GATOR2 through interactions with MIOS WD40 β-propellers, with arginine binding causing conformational rearrangements at the ACT domain interface that trigger dissociation from GATOR2 34. CASTOR1 activity is also regulated post-translationally through AKT-mediated phosphorylation and RNF167-mediated ubiquitination, which promotes its degradation and constitutive mTORC1 activation in cancer cells 5. Clinically, CASTOR1 dysregulation contributes to breast cancer progression when degraded by RNF167, and its overexpression can modulate microglial polarization through mTOR pathway inhibition 56. CASTOR1 responds to low arginine levels, complementing CASTOR2's response to high arginine concentrations for fine-tuned mTORC1 regulation 4.