ULK1 phosphorylation of striatin activates protein phosphatase 2A and autophagy

Zehan Hu,Alexandre Leytens,Bich Vu,Christine Vionnet,Devanarayanan Siva Sankar,Wenxian Wu,Björn Stork,Jörn Dengjel,Michael Stumpe,Esther Martínez-Martínez

doi:10.1016/j.celrep.2021.109762

Zehan Hu, Alexandre Leytens + Show 8 more

Open Access

https://doi.org/10.1016/j.celrep.2021.109762

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Abstract

The evolutionarily conserved ULK1 kinase complex acts as gatekeeper of canonical autophagy and regulates induction of autophagosome biogenesis. To better understand control of ULK1 and analyze whether ULK1 has broader functions that are also linked to the later steps of autophagy, we perform comprehensive phosphoproteomic analyses. Combining invivo with invitro data, we identify numerous direct ULK1 target sites within autophagy-relevant proteins that are critical for autophagosome maturation and turnover. In addition, we highlight an intimate crosstalk between ULK1 and several phosphatase complexes. ULK1 is not only a PP2A target but also directly phosphorylates the regulatory PP2A subunit striatin, activating PP2A and serving as positive feedback to promote autophagy-dependent protein turnover. Thus, ULK1 and phosphatase activities are tightly coordinated to robustly regulate protein degradation by autophagy.

Highlights

Macroautophagy, hereafter referred to as autophagy, summarizes constitutive and stimulus-dependent lysosomal degradation pathways that are critical for cell homeostasis (Dikic and Elazar, 2018; Mizushima and Levine, 2020)
Autophagosome initiation is regulated by two evolutionary conserved kinase complexes: (1) the protein kinase ULK1/2 complex (Chan et al, 2007), which activates, among others, (2) the VPS34 lipid kinase complex, a class III phosphatidylinositol 3-kinase complex (PtdIns3K), which generates phosphatidylinositol3-phosphate that serves as docking site for further protein recruitment (Nakatogawa, 2020; Siva Sankar and Dengjel, 2020)
We identified numerous direct ULK1 target sites on proteins being important in later stages of autophagy, i.e., in autophagosome maturation and autophagosome-lysosome fusion

Summary

Introduction

Macroautophagy, hereafter referred to as autophagy, summarizes constitutive and stimulus-dependent lysosomal degradation pathways that are critical for cell homeostasis (Dikic and Elazar, 2018; Mizushima and Levine, 2020). ULK1 is activated, phosphorylating itself and its complex members. This leads to endoplasmic reticulum (ER) recruitment of the complex and the formation of active phagophore initiation sites through direct interactions with ER membrane proteins VAPA/VAPB (vesicle-associated proteins A/B) (Zhao et al, 2018). Its yeast homolog Atg has recently been shown to phosphorylate proteins that are important in autophagosome maturation and autophagosome-lysosome fusion, indicating that the function of ULK1 in autophagy regulation might be more complex than anticipated (Barz et al, 2020; Gao et al, 2020; Hu et al, 2019)

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