Discussion

Mitochondrial protein import is essential for all eukaryotes. Here we show that the early diverging eukaryote Trypanosoma brucei has a non-canonical inner membrane protein translocase (TIM) which unlike in other systems appears to mediate import of both presequence-containing and mitochondrial carrier proteins. We have analyzed the composition of the trypanosomal TIM complex. Besides TbTim17, which is the single member of the Tim17/22/23 family in trypanosomes, it contains nine subunits that are co-purified in reciprocal immunoprecipitations and with an import substrate that is trapped in the translocation channel. Two of the newly discovered TIM subunits are rhomboid-like proteins, which are essential for growth and mitochondrial protein import. Rhomboid-like proteins were proposed to form the protein translocation pore of the ER-associated degradation system (ERAD). No further candidates for the translocation channel are found among the subunits of the trypanosomal TIM complex, suggesting that the rhomboid-like proteins may contribute to pore formation.