HUWE1 stimulates mTORC1 activity by enhancing Rheb interaction with mTORC1 and supports de novo pyrimidine synthesis.

The mechanistic target of rapamycin complex 1 (mTORC1), a central regulator of cell growth, is activated by Rheb small GTPase. Our recent studies have demonstrated that polyubiquitinated Rheb enhances its interaction with mTORC1, resulting in the activation of mTORC1. Here, we demonstrate that the HECT, UBA, and WWE domain containing E3 ubiquitin protein ligase 1 (HUWE1), an E3 ubiquitin ligase, preferentially interacts with ubiquitinated Rheb and facilitates Rheb's binding to mTORC1 and its subsequent activation. The ablation of HUWE1 results in reduced ubiquitination of Rheb and decreased mTORC1 activity in cultured cells and mouse liver. HUWE1 is also necessary for Rheb to interact with carbamoyl-phosphate synthetase 2, aspartate transcarbamoylase, dihydroorotase (CAD), a key enzyme in pyrimidine biosynthesis, and for CAD activation through the activation of the mTORC1-S6K1 pathway. Moreover, HUWE1 maintains CAD expression by increasing its transcript in cells and liver tissues. Therefore, HUWE1 acts as a key organizer of the ubiquitinated Rheb complex, playing a vital role in enhancing mTORC1 activity and pyrimidine synthesis by increasing both CAD activity and expression.