Eukaryotic translation initiation factor 3d regulates stress granule assembly via its RNA binding domain

Stress granules are cytoplasmic mRNA-protein complexes that form by liquid-liquid phase separation in response to a variety of stresses. Their assembly is contingent upon the inhibition of mRNA translation. Depending on the type of stress and severity, they can promote stress resistance or act to reduce cellular fitness. As such, stress granules are implicated in ageing and a range of related pathologies. Many translation factors are components of stress granules, but it is unclear how they contribute to granule assembly. Here we show that the eIF3d component of the eIF3 translation initiation complex is recruited to stress granules in human cells and is required for stress granule assembly in response to specific stresses. The RNA-binding domain of eIF3d mediates its recruitment to stress granules and deletion of this domain blocks granule formation and decreases cell viability. Furthermore, the exogenous expression of just the eIF3d RNA-binding domain can rescue stress granule assembly in eIF3d-depleted cells. We confirmed the importance of eIF3d for robust stress granule assembly in vivo using the nematode worm C. elegans. This study demonstrates that eIF3d is a critical evolutionary conserved stress granule assembly factor, rather than simply coalescing passively into stress granules following the inhibition of translation initiation.