Dissection of the mitochondrial import and assembly pathway for human Tom40

Tom40 is the channel-forming subunit of the translocase of the mitochondrial outer membrane (TOM complex), essential for protein import into mitochondria. Tom40 is synthesized in the cytosol and contains information for its mitochondrial targeting and assembly. A number of stable import intermediates have been identified for Tom40 precursors in fungi, the first being an association with the sorting and assembly machinery (SAM) of the outer membrane. By examining the import pathway of human Tom40, we have been able to elucidate additional features in its import. We identify that Hsp90 is involved in delivery of the Tom40 precursor to mitochondria in an ATP-dependent manner. The precursor then forms its first stable intermediate with the outer face of the TOM complex before its membrane integration and assembly. Deletion of an evolutionary conserved region within Tom40 disrupts the TOM complex intermediate and causes it to stall at a new complex in the intermembrane space that we identify to be the mammalian SAM. Unlike its fungal counterparts, the human Tom40 precursor is not found stably arrested at a SAM intermediate. Nevertheless, we show that Tom40 assembly is reduced in mitochondria depleted of human Sam50. These findings are discussed in context with current models from fungal studies.