Let’s take a look at Tom20, the protein that acts as a receptor to snag the mitochondrial targeting sequence (MTS) and shuttle it to the membrane channel.
Tom20 has been studied in fungal and mammalian systems and shown to have the a domain structure as shown in Figure 1.
Figure 1. Domain organization of Tom20 in animals and fungi (modified from Fig 5 of reference listed below). N = amino terminus (the front-end of the protein) and C = carboxyl terminus (the back end of the protein).
The gray boxes represent regions that are disordered, the blue box represents a TPR domain that binds the MTS, and the black box represents the region that spans the outer membrane of the mitochondria. The line above represents the regions exposed to the cytoplasm.
So the black box region sits in the membrane and the blue box, with TPR domains, snags the MTS.
When scientists used the sequence of this protein to query databases for homologs in plants and protozoa, nothing was found. Could it be that plants and fungi don’t have Tom20?