Insights into the role of collided ribosomes during the activation of the integrated stress response

Mechanisms that regulate and reprogram gene expression are particularly important under stress conditions. The integrated stress response (ISR) signaling pathway is one such pro-survival and adaptive mechanism conserved in eukaryotes. The ISR is characterized by the activation of protein kinases that phosphorylate the eukaryotic initiation factor 2α (eIF2α) in response to several stress conditions, including nutrient deprivation, viral infection, and protein misfolding. Phosphorylation of eIF2α results in global inhibition of translation, while promoting the translation of a few pro-survival genes. Here, we focus on the mechanism of activation of the eIF2α kinase general control nonderepressible 2 (Gcn2). The protein was initially discovered in yeast more than four decades ago, and it was proposed to respond to amino acid starvation through the accumulation of deacylated tRNAs. However, more recent studies have changed our understanding of its activation and suggest a direct role for ribosome stalling and collisions in the process. In this review, we discuss the classical model for the tRNA-mediated activation of GCN2 and the recent shift in this model to accommodate the observations that wide-ranging translational stresses trigger its activation.