The ER, consisting of tubules and sheets, forms extensive membrane contact sites (MCSs) with other membrane-bound structures, including endosomes, mitochondria, the Golgi apparatus, lipid droplets, and the plasma membrane ( Phillips and Voeltz, 2016 Prinz, 2014 Shibata et al., 2006 Zhang and Hu, 2016). Our study provides insights into the dynamic assembly of the ULK1 complex on the ER for autophagosome formation. ATL2/3 also participate in forming ER–IM tethering complexes.
ATL2/3 directly interact with ULK1 and ATG13 and facilitate the ATG13-mediated recruitment/stabilization of ULK1 and ATG101. Consequently, IM initiation is compromised and slowed.
In ATL2/3 KO cells, formation of FIP200 and ATG13 puncta is unaffected, while targeting of ULK1 and ATG101 is severely impaired. Here, we demonstrate that the ER-localized transmembrane proteins Atlastin 2 and 3 (ATL2/3) contribute to recruitment and stabilization of ULK1 and ATG101 at the FIP200-ATG13–specified autophagosome formation sites on the ER. Little is known about how the ULK1 complex, which comprises FIP200, ULK1, ATG13, and ATG101 and does not exist as a constitutively coassembled complex, is recruited and stabilized on the ER. Dynamic targeting of the ULK1 complex to the ER is crucial for initiating autophagosome formation and for subsequent formation of ER–isolation membrane (IM autophagosomal precursor) contact during IM expansion.
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