Decreased accumulation of superoxide dismutase 2 within mitochondria in the yeast model of Shwachman-Diamond syndrome

Mutations in the human SBDS gene is the most common cause of Shwachman-Diamond syndrome (SDS). The SBDS protein participates in ribosome biogenesis; however, effects beyond reduced translation efficiency are thought to be involved in SDS progression. Impaired mitochondrial function has been reported for cells lacking either SBDS or Sdo1p, the Saccharomyces cerevisiae SBDS ortholog. To better understand how the loss of SBDS/Sdo1p leads to mitochondria damage, we utilized the S. cerevisiae model of SDS. Yeast deleted for SDO1 show increased oxidative damage to mitochondrial proteins and a marked decrease in protein levels and activity of mitochondrial superoxide dismutase 2 (Sod2p), a key enzyme involved in defense against oxidants. Immature forms of Sod2p are observed in sdo1 increment cells suggesting a defect in proteolysis of the presequence. Yeast deleted for CYM1, encoding a presequence protease, display a similar reduction in Sod2p activity as sdo1 increment cells, as well as elevated oxidative damage, to mitochondrial proteins. Sod2p protein levels and activity are largely restored in a por1 increment sdo1 increment strain, lacking the major mitochondrial voltage-dependent anion channel. Together these results indicate that mitochondrial insufficiency in sdo1 increment cells may be linked to the accumulation of immature presequence containing proteins and this effect is a consequence, at least in part, from loss of counter-regulation of Por1p by Sdo1p.