MTORC2 Assembly Is Regulated by USP9X-Mediated Deubiquitination of RICTOR

Lidia Wrobel,Farah H Siddiqi,Sandra M Hill,Sung Min Son,Cansu Karabiyik,Hyunjeong Kim,David C Rubinsztein

doi:10.1016/j.celrep.2020.108564

Abstract

The mechanistic target of rapamycin complex 2 (mTORC2) controls cell metabolism and survival in response to environmental inputs. Dysregulation of mTORC2 signaling has been linked to diverse human diseases, including cancer and metabolic disorders, highlighting the importance of a tightly controlled mTORC2. While mTORC2 assembly is a critical determinant of its activity, the factors regulating this event are not well understood, and it is unclear whether this process is regulated by growth factors. Here, we present data, from human cell lines and mice, describing a mechanism by which growth factors regulate ubiquitin-specific protease 9X (USP9X) deubiquitinase to stimulate mTORC2 assembly and activity. USP9X removes Lys63-linked ubiquitin from RICTOR to promote its interaction with mTOR, thereby facilitating mTORC2 signaling. As mTORC2 is central for cellular homeostasis, understanding the mechanisms regulating mTORC2 activation toward its downstream targets is vital for our understanding of physiological processes and for developing new therapeutic strategies in pathology.

Highlights

The mechanistic target of rapamycin coordinates cellular metabolism and growth in response to environmental and intracellular stimuli, including nutrients and growth factors (Kim and Guan, 2019; Saxton and Sabatini, 2017)
Total ubiquitination and Lys63linked ubiquitination, but not Lys48-linked ubiquitination, of RICTOR was significantly increased upon ubiquitin-specific protease 9X (USP9X) knockdown in HEK293 cells (Figures 2B–2D, S2B, and S2C). These results suggest that USP9X-mediated RICTOR ubiquitination has a non-proteolytic function with respect to mechanistic target of rapamycin complex 2 (mTORC2) activity
We have identified a mechanism by which mTORC2 assembly and consequent activity are regulated upstream of mechanistic target of rapamycin (mTOR) by USP9X activity

Summary

Introduction

The mechanistic target of rapamycin (mTOR) coordinates cellular metabolism and growth in response to environmental and intracellular stimuli, including nutrients and growth factors (Kim and Guan, 2019; Saxton and Sabatini, 2017). MTORC1 plays a central role in regulating cell growth through protein synthesis, biosynthesis of nucleotides, and autophagy, whereas mTORC2 primarily controls proliferation and survival. MTORC2 directly phosphorylates AKT, serum- and glucocorticoidinduced protein kinase 1 (SGK1), and protein kinase C (PKC), thereby regulating cell proliferation, survival, and metabolism (Facchinetti et al, 2008; Garcıa-Martınez and Alessi, 2008; Ikenoue et al, 2008; Sarbassov et al, 2004, 2005). Growth-factor-mediated phosphorylation of different mTORC2 subunits (Oh and Jacinto, 2011), association with ribosomes (Zinzalla et al, 2011), or localization at the plasma membrane (Gan et al, 2011; Liu et al, 2015; Yang et al, 2006) have been reported to influence mTORC2 activity, while not affecting complex assembly. While mTORC2 complex assembly is a critical determinant of its activity, the factors regulating this event are not understood, and it is unclear whether this process is regulated by nutrients and signals that stimulate mTORC2 (Jain et al, 2014; Wang et al, 2017)

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