RPTOR (regulatory associated protein of mTOR complex 1) is an essential regulatory component of mTORC1, serving as both a molecular adapter and substrate-specific scaffold. RPTOR mediates mTORC1 recruitment to lysosomal membranes through interactions with Rag GTPases in response to amino acid availability 1. As a substrate adapter, RPTOR promotes substrate specificity by binding TOS motif-containing proteins and directing them to the mTOR kinase domain for phosphorylation. The mTORC1 complex containing RPTOR phosphorylates key substrates including RPS6KB1, EIF4EBP1, ULK1, ATG13, and TFEB to regulate protein synthesis, autophagy, and lysosomal biogenesis 2. RPTOR is subject to regulatory phosphorylation by AMPK, which inhibits mTORC1 activity and promotes autophagy through TFEB activation 34. In disease contexts, RPTOR-containing mTORC1 is dysregulated in nonalcoholic fatty liver disease, where inflammatory signals like STING1 can aberrantly activate the complex, restricting beneficial lipophagy and promoting hepatic steatosis 5. The complex also plays roles in cellular senescence through mTORC2 regulation 6. RPTOR's function as a critical node in nutrient sensing and metabolic regulation makes it important for understanding growth control and metabolic diseases.