Tat-dependent protein translocation and its evolution.

The Tat system is a general translocation machinery responsible for the transport of folded proteins across biological membranes that are energized by a proton or sodium motive force at these membranes. Protein substrates are characterized by usually amino-terminal signal peptides harboring a conserved twin-arginine sequence motif. Phylogenetic analyses suggest that a Tat system was already present in the last universal common ancestor (LUCA). Today, Tat systems are found in bacteria, archaea, and bacteria-derived organelles. Minimally, Tat systems are composed of one TatC-family component and one TatA/B-family component, with multiple copies of these components forming the functional translocon, while more complex Tat systems exist that require additional homologs of TatA/B or TatC. Due to its ability to translocate folded proteins with bound cofactors, the Tat system is essential for a wide range of important physiological pathways, such as many anaerobic respirations or photosynthesis. With a focus on evolutionary aspects, and by including recent structural and mechanistic insights, we herein review our current knowledge on this mechanistically unique translocation pathway.