title: Autophagy counterbalances endoplasmic reticulum expansion during the unfolded protein response.
abstract: The protein folding capacity of the endoplasmic reticulum (ER) is regulated by the unfolded protein response (UPR). The UPR senses unfolded proteins in the ER lumen and transmits that information to the cell nucleus, where it drives a transcriptional program that is tailored to re-establish homeostasis. Using thin section electron microscopy, we found that yeast cells expand their ER volume at least 5-fold under UPR-inducing conditions. Surprisingly, we discovered that ER proliferation is accompanied by the formation of autophagosome-like structures that are densely and selectively packed with membrane stacks derived from the UPR-expanded ER. In analogy to pexophagy and mitophagy, which are autophagic processes that selectively sequester and degrade peroxisomes and mitochondria, the ER-specific autophagic process described utilizes several autophagy genes: they are induced by the UPR and are essential for the survival of cells subjected to severe ER stress. Intriguingly, cell survival does not require vacuolar proteases, indicating that ER sequestration into autophagosome-like structures, rather than their degradation, is the important step. Selective ER sequestration may help cells to maintain a new steady-state level of ER abundance even in the face of continuously accumulating unfolded proteins.
authors.affiliation: Howard Hughes Medical Institute, University of California San Francisco, San Francisco, California, United States of America. Sebastian.Bernales@ucsf.edu
authors.forename: Sebastián
authors.initials: S
authors.lastname: Bernales
authors.forename: Kent L
authors.initials: KL
authors.lastname: McDonald
authors.forename: Peter
authors.initials: P
authors.lastname: Walter
mesh_terms: D001343:Autophagy / Q000502:physiology*; D000071190:Autophagy-Related Protein 8 Family; D002470:Cell Survival; D004721:Endoplasmic Reticulum / Q000648:ultrastructure*; D049452:Green Fluorescent Proteins / Q000378:metabolism; D008869:Microtubule-Associated Proteins / Q000378:metabolism; D010588:Phagosomes / Q000648:ultrastructure; D017510:Protein Folding; D011994:Recombinant Proteins / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism; D014018:Tissue Distribution; D015003:Yeasts
chemical_list: C113757:ATG8 protein, S cerevisiae; D000071190:Autophagy-Related Protein 8 Family; D008869:Microtubule-Associated Proteins; D011994:Recombinant Proteins; D029701:Saccharomyces cerevisiae Proteins; D049452:Green Fluorescent Proteins