Translocation of proteins is an important process in essentially all living organisms and there are two main pathways for the export of proteins in most free-living bacteria: the secretory (Sec) pathway and the twin-Arg translocation (Tat) pathway. This chapter focuses on the twin-Arg translocation (Tat) pathway with the aim of describing the structure, function, and mechanism of this unusual system. The two pathways differ in fundamental respects, particularly in the folding state of the substrate; Sec substrates are delivered into the Sec pathway in an unfolded state and are maintained in this state for the duration of the targeting pathway, while the Tat system is highly unusual in transporting its substrates in a folded state. The Tat translocase is present in many, but not all bacteria, and is widely distributed in archaea; however, is not present in animals or yeasts. Many prokaryotes use the Tat pathway predominantly for the secretion of redox proteins, but an analysis of the predicted substrates suggests that certain bacteria and archaea secrete mainly nonredox proteins via the Tat system, suggesting that the Tat system is predominantly used for the export of two types of protein: those that are obliged to fold prior to export and those that cannot be transported by the Sec pathway for other reasons. Most lumenal proteins in plants are transported by the Tat pathway in chloroplasts, indicating a critical role for this system in chloroplast biogenesis. It is widely accepted, though perhaps not formally proven, that the Tat system transports large proteins in a folded form, but the actual translocation mechanism is still poorly understood.
ASJC Scopus subject areas
- Molecular Biology