Nutrient sensing is a critical cellular process controlling metabolism and signaling. mTOR complex 1 (mTORC1) is the primary signaling hub for nutrient sensing and, when activated, stimulates anabolic processes while decreasing autophagic flux. mTORC1 receives nutrient status signals from intracellular amino acid sensors. One of these sensors, Sestrin-2, functions as an intracellular sensor of cytosolic leucine and inhibitor of mTORC1 activity. Genetic studies of Sestrin-2 have confirmed its critical role in regulating mTORC1 activity, especially in the case of leucine starvation. Sestrin-2 is known to be transcriptionally controlled by several mechanisms; however, the post-translational proteolytic regulation of Sestrin-2 remains unclear. Here, we explored how Sestrin-2 is regulated through the ubiquitin proteasome system. Using an unbiased screening approach of an siRNA library targeting ubiquitin E3 ligases, we identified a RING-type E3 ligase, ring finger protein 186 (RNF186), that critically mediates the Sestrin-2 ubiquitination and degradation. We observed that RNF186 and Sestrin-2 bind each other through distinct C-terminal motifs and that Lys-13 in Sestrin-2 is a putative ubiquitin acceptor site. RNF186 knockdown increased Sestrin-2 protein levels and decreased mTORC1 activation. These results reveal a new mechanism of E3 ligase control of mTORC1 activity through the RNF186-Sestrin-2 axis, suggesting that RNF186 inhibition may be a potential strategy to increase levels of the mTORC1 inhibitor Sestrin-2.
Keywords: E3 ubiquitin ligase; SESN2; autophagy; high-throughput screening (HTS); mechanistic target of rapamycin (mTOR); nutrient sensing; ring finger protein 186 (RNF186); ubiquitylation (ubiquitination).
© 2019 Lear et al.