Tom22', an 8-kDa trans-site receptor in plants and protozoans, is a conserved feature of the TOM complex that appeared early in the evolution of eukaryotes.

One of the earliest events in the evolution of mitochondria was the development a means to translocate proteins made in the cytosol into the "protomitochondrion." How this was achieved remains uncertain, and the nature of the earliest version of the protein translocation machinery is not known. Comparative sequence analysis suggests three subunits, Tom40, Tom7, and Tom22 as common elements of the protein translocase in the mitochondrial outer membrane in diverse extant eukaryotes. Tom22, the 22-kDa subunit, plays a critical role in the function of this complex in fungi and animals, and we show that an 8-kDa subunit of the plant translocase is a truncated form of Tom22. It has a single transmembrane segment conforming in sequence to the same region of Tom22 from other eukaryotic lineages and a short carboxy-terminal trans domain located in the mitochondrial intermembrane space. The trans domain from the Arabidopsis thaliana protein functions in yeast lacking their own Tom22 by complementing protein import defects and restoring cell growth. Moreover, we have identified orthologs of Tom22, Tom7, and Tom40 in diverse eukaryotes such as the diatom Phaeodactylum tricornutum, the amoebic slime Dictyostelium discoideum, and the protozoan parasite Plasmodium falciparum. This finding strongly suggests these subunits as the core of the protein translocase in the earliest mitochondria.