Uncovering the role of RNA binding proteins in the ER stress response in pancreatic beta cells TP Herbert1, A Al-Bazaz1, C Charoensuk1, R Al-Momani1, W See1, R Barnes1, J Whitehead2, D Boocock3 1School of Pharmacy, University of Lincoln, Lincoln, UK; 2School of Life and Environmental Sciences, University of Lincoln, Lincoln, UK; 3John van Geest Cancer Research Centre, Nottingham Trent University, Nottingham, UK Aim: Chronic ER stress is implicated in the development of beta cell dysfunction and death in diabetes. As RNA-binding proteins (RBPs) likely play an essential role in ER stress-induced changes in gene expression, the aims of this research were to identify global changes in the binding of RBPs to RNA in response to ER stress in beta cells, and then to determine the role of ER-stress-regulated RBPs in beta cell function. Methods: The mouse beta cell line, MIN6, were incubated in the presence or absence of thapsigargin, a pharmacological inducer of ER stress, and RNA bound RBPs identified using RNA interactome capture (RiC) and quantitative mass spectroscopy (SWATH-MS). Knock-down cells of selected stress-regulated RBPs were generated by shRNA. Changes in gene expression were determined by qPCR and Western blot analysis, whereas viability was assessed using Alamar blue. Results: Three hundred twenty-four RBPs were identified of which 49 were modulated by ER stress (i.e. p > 0.05 and 1.5× fold change) including the RNA helicase, Ddx3x, whose binding to mRNA was promoted by ER stress. Bioinformatic analysis of published CLIP data for Ddx3x revealed common Ddx3x target mRNAs associated with ER stress and the unfolded protein response (UPR). knock-down of Ddx3x in MIN6 cells reduced the expression of defined target mRNAs in response to ER stress, including ATF4 and CHoP, and delayed ER stress-induced cell death. Conclusion: Our results provide evidence that ER stress-regulated RBPs play an important role in the execution of the UPR and thus beta cell fate. Support Information: This work is supported by a Diabetes UK studentship.