Regulatory Function of Phosphorylation and Ubiquitination on The TORC1 Signaling Pathway

TORC1 is a highly conserved kinase complex in eukaryotes that regulates cellular metabolism and growth by sensing signals such as nutrients, growth factors, and energy levels. Dysregulation of the TORC1 signaling pathway has been associated with metabolic disorders, neurodegenerative diseases, cancer, and aging. In this review, we compare the structure and function of TORC1 in yeast and mammalian cells. TORC1 in yeast consists of Tor1/Tor2, Kog1, Lst8, and Tco89, while mTORC1 in mammalian consists of mTOR (homolog of yeast TOR1/2), RAPTOR (homolog of yeast Kog1), mLST8 (homolog of yeast Lst8), PRAS40 and DEPTOR. We then review the critical roles of phosphorylation and ubiquitination in transducing external signals to TORC1 and regulating the downstream signaling pathways. Yeast TORC1 phosphorylates and activates Sch9 and Ypk3 to promote protein translation. Yeast TORC1 also regulates stress response, nitrogen metabolism, and autophagy by phosphorylating and inhibiting Tap42, Atg13, and Npr1. Mammalian AMPK, CK1 alpha, Rheb, and AKT can phosphorylate mTORC1, thereby regulating the activity of mTORC1. mTORC1 regulates protein and lipid metabolism by phosphorylating downstream effector proteins, including S6K1, 4E-BP1, and LIPIN1. mTORC1 also inhibits autophagy by phosphorylating ULK1, TFEB, and ATG13. In addition, E3 ubiquitin ligases, including RNF167, RNF186, SCF, etc., either cause protein degradation or promote protein interactions through different forms of polyubiquitination, thus precisely modulating the mTORC1 signaling pathways. The crosstalk between phosphorylation and ubiquitination involved in the mTORC1 signaling pathway is also summarized. An in-depth understanding of the effects of phosphorylation and ubiquitination on the TORC1 signaling pathway can provide new insights for drug target discovery.