UC San Diego

Secreted proteins, plasma membrane proteins, and proteins that reside within the secretory pathway must be folded in the endoplasmic reticulum (ER), which provides an environment that allows the proper folding and assembly of these nascent proteins. In response to environmental and developmental signals, the cellular requirement for the ER's protein folding function can fluctuate. When the protein folding demand exceeds the ER's capacity, this results in the accumulation of unfolded proteins in the ER, a toxic condition known as ER stress. Currently, the only pathway known to respond to ER stress is the unfolded protein response (UPR) pathway, which rapidly activates genes that help expand the ER's folding capacity. A genetic screen in yeast suggests that two mitogen- activated protein kinases (MAPKs), Slt2 and Hog1, might also become activated along with the UPR to help cells cope with ER stress. MAPKs function throughout eukaryotic cell biology to initiate cellular changes in response to environmental stimuli. In this dissertation, I show that Slt2 and Hog1 are activated by ER stress, I investigate the mechanism of activation for each kinase, and I define several downstream functions of the MAPKs during ER stress. First, I show that ER stress in budding yeast induces a cytokinesis delay that correlates with alterations in the septin complex, an important regulator of cytokinesis. This cytokinesis delay is accompanied by a delay in ER inheritance. Both may serve to prevent the propagation of ER stress. The cytokinesis delay, septin alterations, and ER inheritance delay all depend upon the MAPK SLT2. Second, I show that Hog1 becomes activated during late-stage ER stress, through a mechanism with UPR- dependent and UPR-independent components. Upon activation, Hog1 translocates from the cytoplasm to the nucleus, and then later returns to the cytoplasm. In the nucleus, Hog1 activates the transcription of at least one gene, HSP12. Hog1 also regulates the induction of autophagy during ER stress and appears to perform this function from the cytoplasm. Overall, I show that the cellular response to ER stress is much broader than the UPR, involving the activation of additional signaling modules, and affecting a wide range of cellular processes