The endoplasmic reticulum (ER) unfolded protein response (UPR) is comprised of several intracellular signaling pathways that alleviate ER stress. The ER-localized transmembrane kinase PERK is one of three major ER stress transducers. Oligomerization of PERK's N-terminal ER luminal domain by ER stress promotes PERK trans-autophosphorylation of the C-terminal cytoplasmic kinase domain at multiple residues including Thr980 on the kinase activation loop. Activated PERK phosphorylates Ser51 of the Ξ±-subunit of translation initiation factor 2 (eIF2Ξ±), which inhibits initiation of protein synthesis and reduces the load of unfolded proteins entering the ER. The crystal structure of PERK's kinase domain has been determined to 2.8β
Γ
resolution. The structure resembles the back-to-back dimer observed in the related eIF2Ξ± kinase PKR. Phosphorylation of Thr980 stabilizes both the activation loop and helix Ξ±G in the C-terminal lobe, preparing the latter for eIF2Ξ± binding. The structure suggests conservation in the mode of activation of eIF2Ξ± kinases and is consistent with a `line-up' model for PERK activation triggered by oligomerization of its luminal domain.