Proteasome regulation of petite-negativity in fission yeast.

Background: Mitochondria carry out essential functions in eukaryotic cells. The mitochondrial genome encodes factors critical to support oxidative phosphorylation and mitochondrial protein import necessary for these functions. However, organisms like budding yeast can readily lose their mitochondrial genome, yielding respiration-deficient petite mutants. The fission yeast Schizosaccharomyces pombe is petite-negative, but some nuclear mutations enable the loss of its mitochondrial genome.

Results: Here, we characterize the classical petite-positive mutation ptp1-1 as a loss of function allele of the proteasome 19S regulatory subunit component mts4/rpn1, involved in the ubiquitin-dependent degradation pathway. By comparison with another petite-enabling mutation in the g-subunit of the F₁-ATPase, we show that ptp1-1 does not rescue mitochondrial membrane potential. Instead, the mutation results in increased levels of mitochondrial and cytoplasmic chaperones and an altered oxidative stress response.

Conclusions: ptp1-1 is a partial loss of function mutation of the proteasome that enables growth of cells devoid of mitochondrial DNA through a mechanism that is independent of mitochondrial membrane potential rescue and associated with proteasome-dependent regulation of mitochondrial protein import precursors and the oxidative stress response.