FIGURE 1.

The ubiquitin proteasome system (UPS) regulates mitochondrial protein turnover. (A) The UPS monitors mitochondrial precursor proteins before they are imported into mitochondria. After import via the TOM/TIM complex in an unfolded state, the mitochondrial targeting sequence (MTS) is cleaved off by a protease and chaperones ensure proposer folding of the protein. (B) Inhibition of UPS results in the accumulation of polyubiquinated mitochondrial precursor proteins in the cytosol. Some of the ubiquitinated proteins are still imported into the mitochondrion which can contribute to mitochondrial dysfunction and activation of cell death.