Med13p Prevents Stress-Independent Mitochondrial Hyperfragmentation and Aberrant Apoptosis Activation in Saccharomyces cerevisiae by Controlling Cyclin C Nuclear Localization

During aging, and as a result of environmental changes, cells are exposed to elevated levels of reactive oxygen species (ROS). High ROS levels induce lipid oxidation, protein aggregation, mitochondrial hyperfragmentation, DNA damage and programmed cell death (PCD), also called apoptosis. PCD is a highly regulated process and its misregulation has been linked to neurodegenerative diseases and cancer development. Our hypothesis is that cyclin C plays a role in the initiation of apoptosis. During normal conditions, cyclin C represses the transcription of stress response genes (SRG). In response to stress, cyclin C translocates to the cytoplasm where it facilitates mitochondrial hyperfragmentation and PCD. Our laboratory demonstrated that cyclin C loss increases viability of hydrogen peroxide exposed cells. Furthermore, cyclin C overexpression allows stress-independent relocalization into the cytoplasm which triggers mitochondria hyperfragmentation and increases cell sensitivity to oxidative stress. These results indicate that cyclin C localization is critical for proper regulation of PCD. Stress-induced cyclin C cytoplasmic translocalization and mitochondrial hyperfragmentation insures timely activation of apoptosis in stressed cells. Therefore, understanding the molecular switch controlling cyclin C stress induced relocalization may provide ways to protect cells from dangerous ROS or promote the death of transformed cells. In this work, I describe the role of Med13p as the gatekeeper that maintains cyclin C nuclear localization in unstressed cells. Med13p, together with cyclin C, cyclin dependent kinase (Cdk8p) and Med12 form a complex termed the Cdk8 module. This module associates with the RNA polymerase II holoenzyme complex and mediates gene repression. Using a combination of genetic and biochemical approaches, I demonstrated that loss of Med13p (med13Δ) permitted cyclin C relocalization into the cytoplasm in the absence of stress. Aberrant cyclin C localization to the cytoplasm has three consequences. First, more than 90% of cells exhibited hyperfragmented mitochondria. Second, the lack of mitochondrial fusion in unstressed cells induces mtDNA instability resulting in respiration deficiency. Third, cells become hyper-sensitive to oxidative stress. These data support a model that Med13p plays an important role in protecting the cell from aberrant ROS-induced apoptosis by retaining cyclin C in the nucleus