Cellular eIF2B subunit localisation: implications for the integrated stress response and its control by small molecule drugs.

Eukaryotic initiation factor 2 (eIF2) is a G protein critical for translation. It is tightly regulated in the integrated stress response (ISR) via phosphorylation of eIF2 alpha and the subsequent control of eukaryotic initiation factor 2B (eIF2B), a multisubunit guanine nucleotide exchange factor. Through studying the localization of eIF2B subunits, we identified cytoplasmic eIF2B bodies in mammalian cells. We highlight a relationship between body size and the eIF2B subunits localizing to them; larger bodies contain all subunits and smaller bodies contain predominantly catalytic subunits. eIF2 localizes to eIF2B bodies and shuttles within these bodies in a manner that correlates with eIF2B activity. On stress, eIF2 alpha-P localizes predominately to larger bodies and results in a decreased shuttling of eIF2. Interestingly, drugs that inhibit the ISR can rescue eIF2 shuttling in a manner correlating to levels of eIF2 alpha-P. In contrast, smaller bodies show increased eIF2 shuttling in response to stress, which is accompanied by the localization of eIF2B delta to these bodies, suggesting the formation of a novel trimeric complex of eIF2B. This response is mimicked by ISR-inhibiting drugs, providing insight into their potential mechanism of action. This study provides evidence that the composition and function of mammalian eIF2B bodies are regulated by the ISR and the drugs that control it.