Domain stealing by receptors in a protein transport complex.

The mitochondrion is an essential cellular compartment in eukaryotes. The mitochondrial proteins Tom20 and Tom22 are receptors that ensure recognition and binding of proteins imported for mitochondrial biogenesis. Comparison of the sequence for the Tom20 and Tom22 subunits in the yeasts Saccharomyces cerevisiae and Saccharomyces castellii, show a rare case of domain stealing, where in Saccharomyces castellii Tom22 has lost an acidic domain, and Tom20 has gained one. This example of domain stealing is a snapshot of evolution in action and provides excellent evidence that Tom20 and Tom22 are subunits of a single, composite receptor that binds precursor proteins for import into mitochondria.