Abstract
The conserved Blm10/PA200 activators bind to the proteasome core particle gate and facilitate turnover of peptides and unfolded proteins in vitro. We report here that Blm10 is required for the maintenance of functional mitochondria. BLM10 expression is induced 25-fold upon a switch from fermentation to oxidative metabolism. In the absence of BLM10, Saccharomyces cerevisiae cells exhibit a temperature-sensitive growth defect under oxidative growth conditions and produce colonies with dysfunctional mitochondria at high frequency. Loss of BLM10 leads to reduced respiratory capacity, increased mitochondrial oxidative damage, and reduced viability in the presence of oxidative stress or death stimuli. In the absence of BLM10, increased fragmentation of the mitochondrial network under oxidative stress is observed indicative of elevated activity of the mitochondrial fission machinery. The degradation of Dnm1, the main factor mediating mitochondrial fission, is impaired in the absence of BLM10 in vitro and in vivo. These data suggest that the mitochondrial functional and morphological changes observed are related to elevated Dnm1 levels. This hypothesis is supported by the finding that cells that constitutively overexpress DNM1 display the same mitochondrial defects as blm10Δ cells. The data are consistent with a model in which Blm10 proteasome-mediated turnover of Dnm1 is required for the maintenance of mitochondrial function and provides cytoprotection under conditions that induce increased mitochondrial damage and programmed cell death.
Highlights
Blm10 binds to the proteasome core particle and stimulates its proteolytic activity
Loss of BLM10 Results in Reduced Fitness during Respiratory Growth and under Oxidative Stress—We reported previously that BLM10 overexpression decreases the viability of yeast cells under conditions that induce oxidative metabolism at elevated temperatures, such as growth on nonfermentable carbon sources [14]
Increased frequency of petite colonies, indicative of impaired mitochondrial function [42] upon loss of BLM10 in different strain backgrounds was observed (Fig. 1, A–C) [22]. Because of their ability to ferment carbon sources, yeast cells can dispense with functional mitochondria, and petite strains are viable, yet growth is abolished if nonfermentable carbon sources are
Summary
Blm binds to the proteasome core particle and stimulates its proteolytic activity. Results: Loss of BLM10 results in impaired respiration, elevated oxidative stress sensitivity, increased mitochondrial fission, and stabilization of the fission protein Dnm. Overexpression of BLM10 in yeast causes growth defects on nonfermentable carbon sources at elevated temperatures [14]; recently, increased frequency of cells with dysfunctional mitochondria in cells lacking BLM10 was reported [22] These observations indicate that the cellular functions ofBlm10/PA200 proteasomes might be related to the regulation of metabolic adaptation and stress response. Loss of BLM10 leads to Dnm stabilization, and overexpressing DNM1 phenocopied the mitochondrial functional and structural changes of blm10⌬ cells These data argue for a model in which the regulated turnover of Dnm mediated by Blm proteasomes is required for normal mitochondrial function and under oxidative stress. Our data indicate that Blm proteasome-mediated degradation of Dnm is a regulatory mechanism to counteract mitochondrial fission and provides a cytoprotective function under conditions that induce mitochondrial stress
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