Abstract
The evolution of the mitochondrion has involved the remodelling of the two membranes that enclose this organelle. During the transformation of the endosymbiotic bacterium into a genetically dependent organelle, the flow of proteins across the membranes reversed.
This change is reflected by the distinct sets of protein transport machinery that operate in bacterial and mitochondrial membranes [1]. One of the exceptions is a β-barrel assembly machine, Sam50, a member of the Omp85 superfamily of proteins, which has been retained in the mitochondrial membranes.
Other core components of mitochondrial translocases, such as Tom40 in the outer membrane and the Tim17 family of proteins in the inner membrane, cannot be directly related to any bacterial proteins. Two studies by Pusnik et al. recently showed that the mitochondrion of Trypanosoma brucei was found to be devoid of the essential Tom40 channel [2]; instead, it was found to contain an essential protein called the archaic translocase of the outer mitochondrial membrane (ATOM) that was directly linked to bacterial YtfM proteins, which are members of the Omp85 superfamily [3].
Thus, it was suggested by Pusnik et al. that ATOM and Tom40 represent mutually exclusive functional analogues of distinct origins [3]. We analysed the ATOM amino acid sequences to identify homology to known protein families and to determine the phylogenetic distribution of the closest relatives of ATOM.
Surprisingly, our results clearly refute the link between ATOM and bacterial Omp85-like proteins. Moreover, we propose that ATOM is, in fact, a divergent form of the 'classical' Tom40.