Mammalian stress granules represent sites of accumulation of stalled translation initiation complexes

Area of Science:

• Cell Biology
• Molecular Biology
• Stress Granule Formation

Background:

• Eukaryotic cells form stress granules during environmental stress, accumulating untranslated mRNA.
• Stress granules contain mRNA, 40S ribosomal subunits, and translation factors eIF4E/eIF4G.
• eIF2, crucial for initiator tRNA binding to 40S subunits, was previously undetected in stress granules.

Purpose of the Study:

• To investigate the presence of eIF2 and its guanine nucleotide exchange factor, eIF2B, in stress granules.
• To determine the role of eIF2 alpha-subunit phosphorylation in stress granule formation during the unfolded protein response.
• To characterize the composition of stress granules concerning translation initiation complexes.

Main Methods:

• Immunofluorescence microscopy in NIH-3T3 cells and mouse embryo fibroblasts.
• Analysis of stress granule composition under conditions of unfolded protein response.
• Investigation of the necessity and sufficiency of eIF2 alpha-subunit phosphorylation for stress granule formation.

Main Results:

• Stress granules were found to contain both eIF2 and eIF2B.
• Phosphorylation of eIF2 alpha-subunit was demonstrated to be necessary and sufficient for stress granule formation during the unfolded protein response.
• Stress granules were shown to contain components of the 48S preinitiation complex but lack 60S ribosomal subunits.

Conclusions:

• Stress granules contain eIF2 and eIF2B, key components of translation initiation.
• eIF2 alpha-subunit phosphorylation is a critical trigger for stress granule formation during the unfolded protein response.
• Stress granules represent stalled translation initiation complexes, not sites of mRNA degradation or storage.