Crystal structures reveal transient <scp>PERK</scp> luminal domain tetramerization in endoplasmic reticulum stress signaling

Stress caused by accumulation of misfolded proteins within the endoplasmic reticulum (ER) elicits a cellular unfolded protein response (UPR) aimed at maintaining protein-folding capacity. PERK, a key upstream component, recognizes ER stress via its luminal sensor/transducer domain, but the molecular events that lead to UPR activation remain unclear. Here, we describe the crystal structures of mammalian PERK luminal domains captured in dimeric state as well as in a novel tetrameric state. Small angle X-ray scattering analysis (SAXS) supports the existence of both crystal structures also in solution. The salient feature of the tetramer interface, a helix swapped between dimers, implies transient association. Moreover, interface mutations that disrupt tetramer formation in vitro reduce phosphorylation of PERK and its target eIF2α in cells. These results suggest that transient conversion from dimeric to tetrameric state may be a key regulatory step in UPR activation. Synopsis Activation of unfolded protein response (UPR) upon ER stress involves key regulatory roles of ER-luminal sensor/transducer domains in UPR signaling factors. Structural and functional analyses of the PERK luminal domain reveal a novel tetrameric arrangement, whose transient formation may be an important step in UPR activation. Crystal structure of human PERK luminal domain shows a novel tetramer arrangement. Crystal structure of mouse PERK luminal domain is captured in dimeric form. Biophysical analysis confirm that both mouse and human proteins exist as dimers as well as tetramers in solution. Mutations that disrupt tetramerization in solution reduce phosphorylation of PERK and its target eIF2α in cells. Structural and functional analyses of the PERK luminal domain reveal a novel tetrameric arrangement, whose transient formation may be an important step in activation of the unfolded protein response.