Microscopic dissection of the process of stress granule assembly.

Stress granules (SGs) are mRNA triage sites that are formed in response to a variety of cellular stress. To study how SGs bring about the massive spatial compartmentalization, we monitored the localization of various RNA-binding proteins (RBPs) targeted to SGs upon exposure to stress. We discovered that concomitant with the onset of eIF2alpha phosphorylation, RBPs accumulate locally in the cytoplasm, which leads to increased inter-molecular interactions and the formation of robustly detergent-resistant foci. Subsequently, microtubules (MTs) mediate 1) the ordered spatial organization of SGs and 2) the recruitment of a set of nuclear-localized SG components to the cytoplasm. Meanwhile, MTs did not appear to be required for the maintenance of SG distribution after its assembly. Our data suggest that the process of SG formation is composed of MT-independent and -dependent pathways, which take place sequentially during stress response.