Role of the unfolded protein response and phosphatidylinositol phosphate signaling in regulating the mucin-dependent MAPK pathways
MetadataShow full item record
Signaling mucins are evolutionarily conserved regulators of signal transduction pathways that regulate fungal pathogenesis and oncogenesis. Msb2p is a signaling mucin that regulates a cell-differentiation MAPK pathway in yeast. In my thesis work, I have focused on the mechanisms that regulate the processing, trafficking, and activation of Msb2p that will impact the activity of the MAPK pathway. I have uncovered that the unfolded protein response (UPR) pathway regulates the activation of the filamentous growth MAPK pathway. Changes in the glycosylation of the extracellular domain of Msb2 resulted in its enhanced processing that was dependent on its protease, Yps1p. The underglycosylated form of Msb2p associated with the UPR regulator, Ire1p. The UPR regulated the activity of the MAPK pathway by inducing the expression of the protease, YPS1. Therefore, this study has uncovered an important intracellular activation mechanism of a mucin-type protein regulating a MAPK pathway. This regulatory mechanism could also be extended onto other signaling glycoproteins (MUC1, dystroglycan) that cause various cancers and neurodegenerative diseases in higher eukaryotes. My work on Msb2p also led me to study a related MAPK pathway, the High Osmolarity Glycerol (HOG) response pathway. Two separate stimuli are known to activate the two MAPK pathways. The HOG pathway is a p38-type MAPK pathway that is activated in response to changes in osmolarity whereas nutrient limitation activates the filamentous growth pathway. The filamentous growth pathway shares components with the HOG pathway. I sought to determine the extent of functional overlap between the two MAPK pathways. RNA sequencing under nutrient limitation conditions revealed an unexpected role for the HOG pathway in regulating the differentiation response. The HOG pathway required the nutrient regulatory AMP-dependent protein kinase (AMPK) Snf1p, the unfolded protein response (UPR) regulator, Ire1p in this context. Paradoxically, both the MAPK pathways inhibited each other's activity to mount an appropriate differentiation response. This feature was also observed in the human fungal pathogen Candida albicans. Thus, this study identifies the existence of a modulatory circuit that could play an important regulatory role in fungal pathogenesis. From my previous study on Msb2p, I have uncovered that the trafficking of Msb2p in the cell is an important aspect in regulating the MAPK pathway. Reversible phosphorylation of the phospholipid phosphatidylinositol (PI) is a key event in the determination of organelle identity that regulates many biological processes like trafficking, cytoskeleton re- organization and secretion. I have also examined the impact of PI (phosphatidylinositol) signaling on the regulation of the filamentous growth MAPK pathway. PI signaling was required for the trafficking of the components of the filamentous growth pathway that in turn regulated the MAPK activity. Disruption of the actin cytoskeleton by a pharmacological inhibitor, Latrunculin A dampened the filamentous growth pathway activity that was dependent on Msb2p. Thus, this study highlights the role of PI signaling in regulating a cell-differentiation MAPK pathway. Along with Msb2p, two other transmembrane proteins, Sho1p and Opy2p regulate the filamentous growth pathway. A study on the comparative analysis of the trafficking of the transmembrane regulators was explored. This study sheds light on the unique turnover and localization patterns of the three transmembrane regulators. As a result of this compartmentalization, each of the three proteins imparts a unique function in regulating the differentiation response.