REVIEW The UPRosome – decoding novel biological outputs of IRE1α function Hery Urra 1,2,3, *, Philippe Piha ́ n 1,2,3 and Claudio Hetz 1,2,3,4, * ABSTRACT Different perturbations alter the function of the endoplasmic reticulum (ER), resulting in the accumulation of misfolded proteins in its lumen, a condition termed ER stress. To restore ER proteostasis, a highly conserved pathway is engaged, known as the unfolded protein response (UPR), triggering adaptive programs or apoptosis of terminally damaged cells. IRE1α (also known as ERN1), the most conserved UPR sensor, mediates the activation of responses to determine cell fate under ER stress. The complexity of IRE1α regulation and its signaling outputs is mediated in part by the assembly of a dynamic multi-protein complex, named the UPRosome, that regulates IRE1α activity and the crosstalk with other pathways. We discuss several studies identifying components of the UPRosome that have illuminated novel functions in cell death, autophagy, DNA damage, energy metabolism and cytoskeleton dynamics. Here, we provide a theoretical analysis to assess the biological significance of the UPRosome and present the results of a systematic bioinformatics analysis of the available IRE1α interactome data sets followed by functional enrichment clustering. This in silico approach decoded that IRE1α also interacts with proteins involved in the cell cycle, transport, differentiation, response to viral infection and immune response. Thus, defining the spectrum of IRE1α-binding partners will reveal novel signaling outputs and the relevance of the pathway to human diseases. KEY WORDS: IRE1α, UPRosome, ER stress, Cell fate Introduction The endoplasmic reticulum (ER) is a highly dynamic and complex membranous network, responsible for a variety of crucial cellular functions, including protein synthesis and folding, and intracellular Ca 2+ storage (Schwarz and Blower, 2016). A complex network of chaperones, foldases and cofactors, in addition to specific ionic and redox requirements, tightly control protein folding and quality within the ER lumen (Dubnikov et al., 2017). However, a significant amount of newly synthetized proteins do not reach their proper folding state and are delivered to the proteasome by the ER- associated degradation (ERAD) machinery (Hwang and Qi, 2018). Altered ER function can lead to the abnormal accumulation of unfolded or misfolded proteins, a condition known as ‘ER stress’ (Walter and Ron, 2011). ER stress triggers a series of adaptive mechanisms collectively known as the unfolded protein response (UPR) (Hetz, 2012; Walter and Ron, 2011). UPR signaling results in transcriptional and translational responses in order to increase the protein folding capacity of the cell and restore proteostasis (Oakes and Papa, 2015). If the UPR is unable to cope with protein misfolding stress, the pathway activates self-destruction programs to eliminate damaged cells by apoptosis (Urra et al., 2013). Abnormal levels of ER stress are implicated in a variety of human diseases, including cancer, metabolic disorders, inflammation and neurodegenerative diseases (Wang and Kaufman, 2016). However, novel biological functions of the UPR beyond its traditional role in protein homeostasis are currently emerging. The UPR consists of three arms and the most conserved branch is initiated by the stress sensor inositol-requiring enzyme 1α (IRE1α; also known as ERN1) (Walter and Ron, 2011). The activation status of IRE1α signaling is regulated by the assembly of a multiprotein platform at the ER membrane, which we have previously termed the UPRosome (Hetz and Glimcher, 2009). The UPRosome also controls the crosstalk between the UPR and other stress pathways through the binding of adapter and signaling proteins and, in addition, might mediate non- canonical functions of the UPR (Hetz et al., 2020). Here, we review all available protein–protein interaction studies to discuss emerging roles of the UPR in the control of cell function in addition to highlight novel regulatory aspects of IRE1α. We also present a new global analysis of available interactome data sets to speculate about possible novel functions of IRE1α in normal physiology and disease. ER stress and the UPR Under normal conditions, specialized secretory cells (i.e. pancreatic β-cells, B cell lymphocytes and salivary glands) require an active UPR to cope with the high demand for folded proteins, which generates abnormal levels of misfolded or unfolded intermediates. In addition, a number of conditions, such as hypoxia, nutrient deprivation, mutations in secretory cargoes and loss of Ca 2+ , redox or lipid homeostasis, can also result in altered ER protein homeostasis or ‘proteostasis’ (Walter and Ron, 2011). In the past 20 years, chronic ER stress and overactivation of the UPR have been proposed as a relevant contributor to the development of several diseases, including cancer, diabetes, neurodegeneration and inflammatory disorders, among others (Wang and Kaufman, 2016). Activation of the UPR reprograms the transcription of hundreds of genes involved in different aspects of the secretory pathway including the translocation of proteins into the ER, protein folding, glycosylation, redox metabolism, protein quality control, translation, ERAD and lipid biogenesis, among others (Hetz, 2012). If chronic ER stress results in irreversible cellular damage, UPR signaling switches from adaptive to pro-apoptotic programs through the engagement of several cell death mechanisms (Tabas and Ron, 2011; Urra et al., 2013). The UPR is initiated by three types of ER transmembrane proteins that act as ER stress sensors and transducers, including IRE1α and IRE1β (ERN2), activating transcription factor 6 (ATF6; also known as ATF6α) and ATF6β (also known as 1 Biomedical Neuroscience Institute (BNI), Faculty of Medicine, University of Chile, Santiago 8380453, Chile. 2 Center for Geroscience, Brain Health and Metabolism (GERO), Santiago 7800003, Chile. 3 Program of Cellular and Molecular Biology, Institute of Biomedical Sciences (ICBM), University of Chile, Santiago 8380453, Chile. 4 The Buck Institute for Research in Aging, Novato, CA 94945, USA. *Authors for correspondence (chetz@med.uchile.cl; hery.urra@ug.uchile.cl) H.U., 0000-0002-6846-558X; P.P., 0000-0002-1109-858X; C.H., 0000-0001- 7724-1767 1 © 2020. Published by The Company of Biologists Ltd | Journal of Cell Science (2020) 133, jcs218107. doi:10.1242/jcs.218107 Journal of Cell Science