The HACE1 E3 ligase mediates RAC1‐dependent control of mTOR signaling complexes

HACE1 is a HECT family E3 ubiquitin–protein ligase with broad but incompletely understood tumor suppressor activity. Here, we report a previously unrecognized link between HACE1 and signaling complexes containing mammalian target of rapamycin (mTOR). HACE1 blocks mTORC1 and mTORC2 activities by reducing mTOR stability in an E3 ligase‐dependent manner. Mechanistically, HACE1 binds to and ubiquitylates Ras‐related C3 botulinum toxin substrate 1 (RAC1) when RAC1 is associated with mTOR complexes, including at focal adhesions, leading to proteasomal degradation of RAC1. This in turn decreases the stability of mTOR to reduce mTORC1 and mTORC2 activity. HACE1 deficient cells show enhanced mTORC1/2 activity, which is reversed by chemical or genetic RAC1 inactivation but not in cells expressing the HACE1‐insensitive mutant, RAC1K147R. In vivo, Rac1 deletion reverses enhanced mTOR expression in KRasG12D‐driven lung tumors of Hace1−/− mice. HACE1 co‐localizes with mTOR and RAC1, resulting in RAC1‐dependent loss of mTOR protein stability. Together, our data demonstrate that HACE1 destabilizes mTOR by targeting RAC1 within mTOR‐associated complexes, revealing a unique ubiquitin‐dependent process to control the activity of mTOR signaling complexes. Synopsis HACE1 E3 ligase mediated degradation of RAC1 within mTORC1 and mTORC2 signaling complexes controls the signalling activity of both complexes. HACE1 regulates mTORC1 and mTORC2 activity in an E3 ligase dependent manner. HACE1 targets RAC1 for ubiquitylation when it is associated with mTOR signaling complexes. RAC1 inactivation reduces activity of mTOR signaling complexes. HACE1 E3 ligase mediated degradation of RAC1 within mTORC1 and mTORC2 signaling complexes controls the signalling activity of both complexes.