UC San Diego

All living organisms must adapt to their ever-changing environment in order to maintain homeostasis and viability. The folding, processing, and assembly of secreted proteins or proteins residing within the secretory pathway begins in the endoplasmic reticulum (ER). When the equilibrium between the client protein load and the ERs capacity to process that load is off balance, the ER must quickly respond to prevent toxic accumulation of improperly folded proteins within the ER. In mammalian cells ER homeostasis is maintained by three signaling pathways initiated by ER transmembrane proteins, IRE1, PERK, and ATF6, and are collectively referred to as the unfolded proteins response (UPR). The work in Chapter 1 demonstrates that UPR components display distinct sensitivities towards different forms of ER stress. Disruption of ER calcium in particular revealed fundamental differences in the properties of UPR signaling branches. Depletion of ER calcium by thapsigargin, an inhibitor of the ER calcium ATPase, lead to the rapid activation of both IRE1 and PERK while the response of ATF6 was markedly delayed. This study was the first side- by-side comparison of UPR signaling branch activation revealing intrinsic properties of UPR stress sensors in response to alternate forms of ER stress. Chapter 2 focuses on the coordinate regulation of ribosomal RNA (rRNA) transcription and translation inhibition by the PERK signaling branch during ER stress. Here we show that phosphorylation of eukaryotic translation initiation factor alpha (eIF2[alpha]) by PERK is necessary for disrupting the rRNA preinitiation complex leading inactivation of at least one rRNA transcription factor and dissociation of RNA polymerase I, thus downregulating rRNA transcription. This study is the first to link phosphorylation of eIF2[alpha] with regulation rRNA synthesis, and provides an initial framework for understanding how the UPR communicates with the nucleolus in order to maintain ER homeostasis