Adaptation to ER Stress by Slt2, Counterpart of Human MAP Kinase ERK1/2, via Enhancing Splicing and Translation of HAC1 mRNA in Saccharomyces cerevisiae

Unfolded protein response (UPR) is a cellular strategy to increase the protein folding capacity of cells in response to stress within the endoplasmic reticulum (ER). In metazoan cells, three major UPR sensors Ire1, PERK and ATF6 work in concert by simultaneously activating intracellular signaling pathways and modulating a series of physiological processes such as attenuation of the general protein synthesis and expression of protein chaperones. In yeast Saccharomyces cerevisiae, Ire1 is known to be the only UPR sensor, which mediates splicing of HAC1 mRNA in the cytoplasm and derepresses its translation. Hac1 is a transcription factor that increases the expression of protein folding enzymes and chaperones, thus enhancing the protein folding capacity of cells. In this study, we provide compelling evidence that kinase Slt2 plays a significant role in facilitating both the splicing and translation of HAC1 mRNA, while also serving as a key mediator in the activation of UPR genes through an alternative route. We also provide evidence that human extracellular signal-regulated kinase 1 (ERK1) or ERK2 served as a functional substitute for yeast Slt2 in the context of UPR. Furthermore, ERK1 exhibits an enhanced activation in human primary cells when grown in the presence of ER stressor. These findings collectively suggest that Slt2 responds to ER stress by activating the Ire1 pathway as well as initiating a parallel signaling pathway.