Stress granules: sites of mRNA triage that regulate mRNA stability and translatability

Abstract

Mammalian stress granules (SGs) are cytoplasmic domains into which mRNAs are sorted dynamically in response to phosphorylation of eukaryotic initiation factor (eIF) 2α, a key regulatory step in translational initiation. The activation of one or more of the eIF2α kinases leads to SG assembly by decreasing the levels of eIF2-GTP-tRNAMet, the ternary complex that is normally required for loading the initiator methionine onto the 48 S preinitiation complex to begin translation. This stress-induced scarcity of eIF2-GTP-tRNAMet allows the RNA-binding proteins TIA-1 (T-cell internal antigen-1) and TIAR (TIA-1-related protein) to bind the 48 S complex in lieu of the ternary complex, thereby promoting polysome disassembly and the concurrent routing of the mRNA into a SG. The actual formation of SGs occurs upon auto-aggregation of the prion-like C-termini of TIA-1 proteins; this aggregation is reversed in vivo by overexpression of the heat-shock protein (HSP) chaperone HSP70. Remarkably, HSP70 mRNA is excluded from SGs and is preferentially translated during stress, indicating that the RNA composition of the SG is selective. Moreover, the effects of HSP70 on TIA aggregation suggest a feedback loop whereby HSP70 synthesis is auto-regulated. Proteins that promote mRNA stability [e.g. HuR (Hu protein R)] and destabilize mRNA [i.e. tristetraprolin (TTP)] are also recruited to SGs, suggesting that SGs effect a process of mRNA triage, by promoting polysome disassembly and routing mRNAs to cytoplasmic domains enriched for HuR and TTP. This model reveals connections between the eIF2α kinase system, mRNA stability and cellular chaperone levels.