The mitochondrial ratchet: Examples from non-bilaterian animals

The twin-arginine translocation (Tat) pathway is a protein transport system that moves completely folded proteins across lipid membranes. Genes encoding components of the pathway have been found in the genomes of many Bacteria, Archaea, and eukaryotic organelles including chloroplasts, plant mitochondria and the mitochondria of many protists. However, with a single exception, Tat genes are absent from the mitochondrial genomes of all animals. The only exception comes from the homoscleromorph sponges in the family Oscarellidae, whose mitochondrial genomes encode a gene for tatC, the largest subunit of the complex. Here we explore the origin and evolution of the mitochondrial tatC gene in Oscarellidae, and use bioinformatic approaches to evaluate its functional significance. We conclude that tatC in homoscleromorph sponges was likely inherited from the ancestral proto-mitochondrial genome, implying multiple independent losses of the mitochondrial Tat pathway during the evolution of opisthokonts. In addition, bioinformatic evidence suggests that tatC comprises the entire Tat pathway in Oscarellidae, and that the Rieske Fe/S protein of mitochondrial complex III is its likely substrate. Introduction The twin-arginine translocase (Tat) pathway is a widespread protein export system, found throughout all domains of life, and is responsible for the task of transporting completely folded