ULK phosphorylation of STX17 controls autophagosome maturation via FLNA, 2023, Wang et al.

[SNT Gatchaman]

ULK phosphorylation of STX17 controls autophagosome maturation via FLNA
Wang, Yufen; Que, Huilin; Li, ChuangPeng; Wu, Zhe; Jian, Fenglei; Zhao, Yuan; Tang, Haohao; Chen, Yang; Gao, Shuaixin; Wong, Catherine C.L.; Li, Ying; Zhao, Chongchong; Rong, Yueguang

Autophagy is a conserved and tightly regulated intracellular quality control pathway. ULK is a key kinase in autophagy initiation, but whether ULK kinase activity also participates in the late stages of autophagy remains unknown.

Here, we found that the autophagosomal SNARE protein, STX17, is phosphorylated by ULK at residue S289, beyond which it localizes specifically to autophagosomes. Inhibition of STX17 phosphorylation prevents such autophagosome localization. FLNA was then identified as a linker between ATG8 family proteins (ATG8s) and STX17 with essential involvement in STX17 recruitment to autophagosomes. Phosphorylation of STX17 S289 promotes its interaction with FLNA, activating its recruitment to autophagosomes and facilitating autophagosome–lysosome fusion.

Disease-causative mutations around the ATG8s- and STX17-binding regions of FLNA disrupt its interactions with ATG8s and STX17, inhibiting STX17 recruitment and autophagosome–lysosome fusion.

Cumulatively, our study reveals an unexpected role of ULK in autophagosome maturation, uncovers its regulatory mechanism in STX17 recruitment, and highlights a potential association between autophagy and FLNA.

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[SNT Gatchaman]

Posting in relation to autophagy findings, including abnormally high levels of phosphorylated ATG-13 in serum. Also noting filamin-A as part of extracellular vesicle cargo, proposed to discriminate ME/CFS patients in 2019. Perhaps both observations might point to a common failure in autophagy?

 

[SNT Gatchaman]

ULK, the mammalian homolog of Atg1 in yeast, is a serine/threonine protein kinase essential for autophagy initiation. It includes two homologs, ULK1 and ULK2, that function in the most upstream step of autophagy.

The ULK complex, composed of ULK, ATG13, FIP200, and ATG101 in mammals, is critical for the initiation of autophagy and functions primarily through the phosphorylation of its substrates.

Filamins, including filamin A (FLNA), filamin B (FLNB), and filamin C (FLNC), are actin-binding proteins that are conserved across vertebrate species [...] Filamins carry an actin-binding domain (ABD) and 24 repeat immunoglobulin-like domains (Ig), which are interrupted by flexible hinge region 1 (H1), between Ig15 and Ig16, and flexible hinge region 2 (H2), between Ig23 and Ig24

In addition to their function as potent F-actin crosslinking proteins, filamins also participate in intracellular signal transduction, cell migration and transport

Here, we unexpectedly found that ULK also functions in autophagosome maturation by phosphorylating the autophagosomal SNARE STX17 at S289. [...] Further investigations identified FLNA as a linker between STX17 and ATG8 family proteins (collectively referred to as ATG8s) that mediates STX17 recruitment to autophagosomes.

Since abnormalities in autophagy have been implicated in neurological diseases, we further tested whether these disease-causative mutations in FLNA also affected autophagy. [...] Collectively, these results support the likelihood that disease-causing mutations in the FLNA binding regions with STX17 or ATG8s inhibit autophagy.

Here, we demonstrated that ULK1 regulates autophagosome maturation by phosphorylating STX17 to promote STX17 recruitment to autophagosomes. FLNA, which was identified as a STX17interacting protein in the current study, functions as a linker molecule to bridge STX17 and ATG8 proteins and is therefore required for STX17 recruitment to autophagosomes.

Our study thus reveals a previously unknown function of ULK during the late stages of autophagy in mammals and expands our understanding of the actin-independent functions of FLNA. In particular, this study illustrates how FLNA participates as a likely structural component in ATG8 recruitment of STX17 to autophagosomes and suggests that future investigation is warranted to explore a possible role of autophagy in FLNArelated diseases.