A UPR-Induced Soluble ER-Phagy Receptor Acts with VAPs to Confer ER Stress Resistance.

Autophagic degradation of the endoplasmic reticulum (ER-phagy) is triggered by ER stress in diverse organisms. However, molecular mechanisms governing ER stress-induced ER-phagy remain insufficiently understood. Here we report that ER stress-induced ER-phagy in the fission yeast Schizosaccharomyces pombe requires Epr1, a soluble Atg8-interacting ER-phagy receptor. Epr1 localizes to the ER through interacting with integral ER membrane proteins VAPs. Bridging an Atg8-VAP association is the main ER-phagy role of Epr1, as it can be bypassed by an artificial Atg8-VAP tether. VAPs contribute to ER-phagy not only by tethering Atg8 to the ER membrane, but also by maintaining the ER-plasma membrane contact. Epr1 is upregulated during ER stress by the unfolded protein response (UPR) regulator Ire1. Loss of Epr1 reduces survival against ER stress. Conversely, increasing Epr1 expression suppresses the ER-phagy defect and ER stress sensitivity of cells lacking Ire1. Our findings expand and deepen the molecular understanding of ER-phagy.

The ortholog of human REEP1-4 is required for autophagosomal enclosure of ER-phagy/nucleophagy cargos in fission yeast.

Selective macroautophagy of the endoplasmic reticulum (ER) and the nucleus, known as ER-phagy and nucleophagy, respectively, are processes whose mechanisms remain inadequately understood. Through an imaging-based screen, we find that in the fission yeast Schizosaccharomyces pombe, Yep1 (also known as Hva22 or Rop1), the ortholog of human REEP1-4, is essential for ER-phagy and nucleophagy but not for bulk autophagy. In the absence of Yep1, the initial phase of ER-phagy and nucleophagy proceeds normally, with the ER-phagy/nucleophagy receptor Epr1 coassembling with Atg8. However, ER-phagy/nucleophagy cargos fail to reach the vacuole. Instead, nucleus- and cortical-ER-derived membrane structures not enclosed within autophagosomes accumulate in the cytoplasm. Intriguingly, the outer membranes of nucleus-derived structures remain continuous with the nuclear envelope-ER network, suggesting a possible outer membrane fission defect during cargo separation from source compartments. We find that the ER-phagy role of Yep1 relies on its abilities to self-interact and shape membranes and requires its C-terminal amphipathic helices. Moreover, we show that human REEP1-4 and budding yeast Atg40 can functionally substitute for Yep1 in ER-phagy, and Atg40 is a divergent ortholog of Yep1 and REEP1-4. Our findings uncover an unexpected mechanism governing the autophagosomal enclosure of ER-phagy/nucleophagy cargos and shed new light on the functions and evolution of REEP family proteins.

REEPing the harvest of reticulophagy and nucleophagy.

Under stress conditions, the endoplasmic reticulum and nucleus undergo turnover through selective macroautophagy/autophagy processes termed reticulophagy and nucleophagy, respectively. Our recent study has identified the protein Hva22/Rop1/Yep1, a member of the REEP1-REEP4 subfamily of the REEP protein family, as an essential factor for both processes in the fission yeast Schizosaccharomyces pombe. In the absence of Hva22/Yep1, reticulophagy and nucleophagy cargos without surrounding autophagic membranes accumulate in the cytoplasm. Interestingly, human proteins in the REEP1-REEP4 subfamily can functionally substitute for Hva22/Yep1 to facilitate reticulophagy. Phylogenetic and synteny analyses further reveal that the budding yeast reticulophagy receptor Atg40 is also a REEP1-REEP4 subfamily member. Similar to human REEP1-REEP4 subfamily proteins, Atg40 can functionally replace Hva22/Yep1. Based on our findings, we propose that promoting reticulophagy is a conserved function of REEP1-REEP4 subfamily proteins.