Robust heat shock induces eIF2α-phosphorylation-independent assembly of stress granules containing eIF3 and 40S ribosomal subunits in budding yeast, Saccharomyces cerevisiae

Environmental stresses inducing translation arrest are accompanied by the deposition of translational components into stress granules (SGs) serving as mRNA triage sites. It has recently been reported that, in Saccharomyces cerevisiae, formation of SGs occurs as a result of a prolonged glucose starvation. However, these SGs did not contain eIF3, one of hallmarks of mammalian SGs. We have analyzed the effect of robust heat shock on distribution of eIF3a/Tif32p/Rpg1p and showed that it results in the formation of eIF3a accumulations containing other eIF3 subunits, known yeast SG components and small but not large ribosomal subunits and eIF2a/Sui2p. Interestingly, under these conditions, Dcp2p and Dhh1p P-body markers also colocalized with eIF3a. Microscopic analyses of the edc Delta 3/sm4 Delta C mutant demonstrated that different scaffolding proteins are required to induce SGs upon robust heat shock as opposed to glucose deprivation. Even though eIF2 alpha became phosphorylated under these stress conditions, the decrease in polysomes and formation of SGs occurred independently of phosphorylation of eIF2 alpha. We conclude that under specific stress conditions, such as robust heat shock, yeast SGs do contain eIF3 and 40S ribosomes and utilize alternative routes for their assembly.