Structural insights into the evolution of mitochondrial protein import from the trypanosome ATOM complex

The translocase of the outer membrane (TOM) mediates the import of most mitochondrial preproteins. In the protozoan parasite Trypanosoma brucei , this machinery is replaced by the atypical translocase of the outer membrane (ATOM), whose composition differs substantially from the canonical TOM complexes of fungi and metazoans. Here, we report the cryoelectron microscopy structure of the T. brucei ATOM complex at 2.8 Å resolution. The structure resolves all previously identified components together with an additional, previously uncharacterized subunit. Despite its divergent composition, ATOM retains a conserved dimeric channel architecture and a protein-conducting pathway closely resembling those of opisthokont TOM complexes, indicating that the core mechanism of mitochondrial protein translocation was established early in eukaryotic evolution. In contrast, the receptor and intermembrane-space regions exhibit distinct architectures, revealing extensive evolutionary diversification and multiple examples of convergent evolution. Together, these findings provide a near-atomic view of the ATOM translocase and offer insights into the origin, conservation, and diversification of the mitochondrial protein import machinery across eukaryotes.