Roles of the Unfolded Protein Response in the Mammalian Cell Cycle

Throughout the cell cycle, genome duplication is coordinated with the multiplication and growth of organelles, which requires membrane biosynthesis at the endoplasmic reticulum (ER). By this reasoning, ER growth and increased ER function would be a pre-requisite for cell division. Because the unfolded protein response (UPR)—a fundamental homeostatic mechanism that maintains ER integrity—increases the size and protein-processing capacity of the ER, I reasoned that it may oversee ER physiology during the cell cycle. To investigate ER growth and activation of the UPR during the cell cycle, I optimized and characterized a well-described fluorescent reporter of cell cycle progression, known as the FAST-FUCCI system. This live-cell reporter enabled me to separate G1 and S/G2 cell populations by fluorescence activated cell sorting (FACS). My data show that mammalian cells increased in size and granularity during interphase. These hallmarks were correlated with an increase in ER-resident protein content, suggesting that the ER enlarges in preparation for cell division. Moreover, I found that inhibition of IRE1 via pharmacological agents delayed progression through the G1/S boundary. While investigating a plausible mechanism that could regulate UPR activity during the cell cycle, I identified PKMYT1, an ER- and Golgi apparatus-associated G2/M cell cycle checkpoint kinase, in a candidate-based approach. I found that IRE1 activity is suppressed by PKMYT1, suggesting that PKMYT1 exerts regulatory control over IRE1 prior to cell division. Preliminary data suggests that PKMYT1 and IRE1 do not physically interact, which suggests regulation via an unidentified intermediate or a transcriptomic regulation by downstream transcription factors, such as XBP1s. Taken together, my results provide evidence to suggest that mammalian cells engage a physiological UPR involving IRE1 signaling during cell cycle progression