The multistep quality control of mitochondrial β-barrel protein import

Mitochondrial β-barrel proteins fulfill essential functions for cell viability including transport of proteins, ions and metabolites across the outer membrane. The translocase of the outer membrane (TOM complex) and the sorting and assembly machinery (SAM complex) transport these proteins into the outer membrane. Although β-barrel proteins are essential for cell viability, the quality control mechanisms that monitor their import into the outer membrane remain unknown. We uncovered that Msp1 and Ubx2-recruited Cdc48 cooperate to remove a SAM-arrested Tom40 variant. Prior to degradation, the β-barrel protein is deposited in cytosolic protein storages, termed MitoStores, to minimize accumulation of aberrant proteins in the cell. The cytosolic AAA ATPase Hsp104 disaggregates these proteins to facilitate their proteasomal degradation. Altogether, mitochondrial and cytosolic factors cooperate to degrade translocation-arrested mitochondrial β-barrel proteins.