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. 2023 Jun;19(6):1893-1895.
doi: 10.1080/15548627.2022.2148900. Epub 2022 Nov 29.

Membrane Atg8ylation, stress granule formation, and MTOR regulation during lysosomal damage

Affiliations

Membrane Atg8ylation, stress granule formation, and MTOR regulation during lysosomal damage

Jingyue Jia et al. Autophagy. 2023 Jun.

Abstract

The functions of mammalian Atg8 proteins (mATG8s) expand beyond canonical autophagy and include processes collectively referred to as Atg8ylation. Global modulation of protein synthesis under stress conditions is governed by MTOR and liquid-liquid phase separated condensates containing ribonucleoprotein particles known as stress granules (SGs). We report that lysosomal damage induces SGs acting as a hitherto unappreciated inhibitor of protein translation via EIF2A/eIF2α phosphorylation while favoring an ATF4-dependent integrated stress response. SGs are induced by lysosome-damaging agents, SARS-CoV-2 open reading frame 3a protein (ORF3a) expression, Mycobacterium tuberculosis infection, and exposure to proteopathic MAPT/tau. Proteomic studies revealed recruitment to damaged lysosomes of the core SG proteins NUFIP2 and G3BP1 along with the GABARAPs of the mATG8 family. The recruitment of these proteins is independent of SG condensates or canonical autophagy. GABARAPs interact directly with NUFIP2 and G3BP1 whereas Atg8ylation is needed for their recruitment to damaged lysosomes. At the lysosome, NUFIP2 contributes to MTOR inactivation together with LGALS8 (galectin 8) via the Ragulator-RRAGA-RRAGB complex. The separable functions of NUFIP2 and G3BP1 in SG formation vis-a-vis their role in MTOR inactivation are governed by GABARAP and Atg8ylation. Thus, cells employ membrane Atg8ylation to control and coordinate SG and MTOR responses to lysosomal damage.Abbreviations: Atg8: autophagy related 8; ATG: autophagy related; ATF4: activating transcription factor 4; EIF2A/eIF2α: eukaryotic translation initiation factor 2A; GABARAP: GABA type A receptor-associated protein; G3BP1: G3BP stress granule assembly factor 1; LLOMe: L-leucyl-L-leucine methyl ester; LysoIP: lysosome immunopurification; mRNA: messenger ribonucleic acid; MTOR: mechanistic target of rapamycin kinase; NUFIP2: nuclear FMR1 interacting protein 2; ORF3a: open reading frame 3a protein; SARS-CoV-2: severe acute respiratory syndrome coronavirus 2; SG: stress granule; TIA1: TIA1 cytotoxic granule associated RNA binding protein.

Keywords: Atg8ylation; MTOR; Mycobacterium tuberculosis; NUFIP2; PKR; SARS-CoV-2 ORF3a; integrated stress response; lysosomal damage; proteopathic tau; stress granules.

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Conflict of interest statement

No potential conflict of interest was reported by the author(s).

Figures

Figure 1.
Figure 1.
Lysosomal damage results in generalized protein synthesis inhibition by induction of stress granule (SG) formation and MTOR inactivation. The signal from damaged lysosomes is transduced to the SG machinery via a pool of EIF2AK2/PKR that resides on lysosomes. Upon damage, PKR phosphorylates EIF2A/eIF2α, a key regulator of translational arrest and SG formation. Inhibition of capped mRNA translation due to EIF2A phosphorylation and SG formation elicits an ATF4-driven integrated stress response in response to lysosomal damage. Complementary to SGs, lysosomal damage also inactivates MTOR. Under normal conditions, MTOR phosphorylates its substrates RPS6KB1/S6K1 and EIF4EBP1 to promote protein translation. Damaged lysosomes are Atg8ylated by GABARAPs which directly bind and recruit SG proteins NUFIP2 and G3BP1 to the lysosomal surface where they act to inactivate MTOR. The pools of NUFIP2 and G3BP1 are split between SG formation and the Ragulator-RRAG complex to control MTOR activity together with LGALS8 on the lysosomes. This results in both a competition (limiting SG formation) and a synergy (inhibiting translation by both sequestering mRNAs in SGs and inhibiting MTOR) thus coordinating these two roles of NUFIP2 and G3BP1 for the optimal contribution of both aspects of protein translation.

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References

    1. Jia J, Wang F, Bhujabal Z, et al. Stress granules and mTOR are regulated by membrane atg8ylation during lysosomal damage. J Cell Biol. 2022 Sep 30;221(11):e202207091. - PMC - PubMed

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