eIF2Bα subcellular localisation - a potential link between translation initiation and stress granule formation?

Abstract

The eukaryotic initiation factor 2B (eIF2B) complex is a guanine nucleotide exchange factor (GEF) that regulates translation initiation and is central to the integrated stress response (ISR). eIF2B exists as a decamer composed of catalytic ({gamma}, {varepsilon}) and regulatory (, {beta}, {delta}) subunits, which localise to cytoplasmic foci termed eIF2B bodies. While eIF2B bodies exhibit dynamic changes during stress, the role of individual subunits in the formation of bodies and localisation of subunits to additional cytoplasmic foci remains unclear. Here, we investigate the contribution of eIF2B to eIF2B body integrity and investigate alterations in eIF2B localisation during cellular stress. Using immunocytochemistry in neural cell lines, we demonstrate that all endogenous eIF2B subunits form cytoplasmic foci. Whilst eIF2B co-localises with the eIF2B{gamma} catalytic subunit in large bodies (>1 m{superscript 2}) it also forms smaller foci independently. We find cell type differences in the formation of foci containing eIF2B and eIF2B{varepsilon} subunits, in the absence of changes in protein expression. Stress induction significantly increased eIF2B and eIF2B{varepsilon} foci without altering protein expression, suggesting cell-type specific and ISR-driven reorganisation. siRNA-mediated knockdown of eIF2B disrupted large eIF2B body formation, an effect reversed by ISRIB, which stabilizes eIF2B tetramers, highlighting eIF2Bs essential role in decamer assembly. Furthermore, we reveal that eIF2B, and to a lesser extent eIF2B{varepsilon}, localize to SGs under ER and oxidative stress, with colocalisation dependent on eIF2 phosphorylation. Cells harbouring an EIF2B1 p.Leu99Pro mutation, which impairs ISR sensing, exhibited reduced SG formation and abolished eIF2B-SG colocalization. These findings suggest that eIF2B not only stabilizes eIF2B bodies but also mediates ISR signalling and SG dynamics via a mechanism reliant on phosphorylated eIF2. Collectively, our study uncovers a dual role for eIF2B in maintaining eIF2B complex integrity and regulating stress-responsive mRNA triage, providing new insights into translational control and disease mechanisms linked to eIF2B dysfunction.