G3BPs tether the TSC complex to lysosomes and suppress mTORC1 signaling

Abstract

Summary Ras GTPase-activating protein-binding proteins 1 and 2 (G3BP1 and G3BP2, respectively) are widely recognized as core components of stress granules (SGs). We report that G3BPs reside at the cytoplasmic surface of lysosomes. They act in a non-redundant manner to anchor the tuberous sclerosis complex (TSC) protein complex to lysosomes and suppress activation of the metabolic master regulator mechanistic target of rapamycin complex 1 (mTORC1) by amino acids and insulin. Like the TSC complex, G3BP1 deficiency elicits phenotypes related to mTORC1 hyperactivity. In the context of tumors, low G3BP1 levels enhance mTORC1-driven breast cancer cell motility and correlate with adverse outcomes in patients. Furthermore, G3bp1 inhibition in zebrafish disturbs neuronal development and function, leading to white matter heterotopia and neuronal hyperactivity. Thus, G3BPs are not only core components of SGs but also a key element of lysosomal TSC-mTORC1 signaling.

TSC1 binding to lysosomal PIPs is required for TSC complex translocation and mTORC1 regulation

“…Supernatant (S) and pellet (P) fractions containing the liposomes were analyzed using SDS-PAGE and Coomassie staining. Menon et al, 2014;Prentzell et al, 2021), we suspected that PIP binding of TSC1 could be the underlying molecular mechanism of TSC1-dependent translocation of the TSC complex to the lysosomal surface. To test this hypothesis, we stably reconstituted TSC1 À/À MEFs with either WT hTSC1 or the PIP binding-deficient mutant M1 (Figure 6C) and investigated the resulting effects on TSC complex localization upon starvation.…”

“…These data support a model in which lysosomal PIP levels contribute to membrane recruitment of the TSC complex via TSC1. A number of protein factors have been implicated as recruiting factors, including the inactive GDP-bound form of Rheb (Menon et al, 2014), the Rag heterodimer in its inactive form (Demetriades et al, 2014), phosphorylated RhoB (Liu et al, 2018), and the G3BP1/2 proteins (Prentzell et al, 2021). Collective membrane recruitment of protein complexes via multiple regulated interactors, frequently involving PIPs, through coincidence detection is a common motif in modulating the localization of signaling modules (Carlton and Cullen, 2005) and adds to the complexity of controlling TSC complex activity (Figure 7K).…”

“…It has been reported that TSC1 and TSC2 form oligomeric assemblies in cells, but the stoichiometry of these complexes and the potential significance of oligomerization have not been explored (Hoogeveen-Westerveld et al, 2012b;Menon et al, 2014). Several proteins, including the Rag GTPase heterodimers, Rheb, G3BP1, and RhoB, have been implicated in regulating localization of the TSC complex via binding to TSC2 (Carroll et al, 2016;Demetriades et al, 2014Demetriades et al, , 2016Liu et al, 2018;Menon et al, 2014;Prentzell et al, 2021). However, the precise molecular mechanism of TSC lysosomal recruitment, and whether TSC1 also plays a role in this process, remained elusive.…”

Cancer cell adaptability: turning ribonucleoprotein granules into targets

“…Beyond SGs, G3BP1 and G3BP2 form heterocomplexes that can localize at the surface of lysosomes [146]. This interaction is required for the lysosomal localization of the TSC (tuberous sclerosis complex) which subsequently inhibits mTORC1.…”

“…This interaction is required for the lysosomal localization of the TSC (tuberous sclerosis complex) which subsequently inhibits mTORC1. This activity of G3BP1 and G3BP2, independently of SGs, controls mTORC1 according to fluctuations in metabolic state, and variations in G3BP protein levels directly impact on mTOR-mediated cancer processes such as cell growth and motility [146]. Interestingly, SGs also interact with lysosomes through annexin A11 for transport along microtubules [147], highlighting the intricate connection between organelles and regulatory complexes to fine-tune intracellular processes.…”