Abstract
The twin-arginine protein translocation (Tat) system has been characterized in bacteria, archaea and the chloroplast thylakoidal membrane. This system is distinct from other protein transport systems with respect to two key features. Firstly, it accepts cargo proteins with an N-terminal signal peptide that carries the canonical twin-arginine motif, which is essential for transport. Second, the Tat system only accepts and translocates fully folded cargo proteins across the respective membrane. Here, we review the core essential features of folded protein transport via the bacterial Tat system, using the three-component TatABC system of Escherichia coli and the two-component TatAC systems of Bacillus subtilis as the main examples. In particular, we address features of twin-arginine signal peptides, the essential Tat components and how they assemble into different complexes, mechanistic features and energetics of Tat-dependent protein translocation, cytoplasmic chaperoning of Tat cargo proteins, and the remarkable proofreading capabilities of the Tat system. In doing so, we present the current state of our understanding of Tat-dependent protein translocation across biological membranes, which may serve as a lead for future investigations.
Highlights
Two major transport systems exist for protein translocation across the bacterial cytoplasmic membrane, namely the general secretory (Sec) pathway and the twin-arginine translocation (Tat) pathway (Fig. 1)
To ensure proteins are appropriately directed into the Sec or Tat pathways and to initiate the translocation process, specific signal peptides are present on the N-terminus of each protein
B. subtilis TatAd can replace TatA and TatB in E. coli [76], whereas TatAc expressed in E. coli can functionally replace TatA and TatE and form active translocases with TatCd and TatCy [77, 78]
Summary
To function correctly and efficiently, every cell needs to be highly organised, tightly regulated and compartmentalised. Proteins are essential macromolecules synthesised by ribosomes in the cytoplasm that often require localisation to a particular subcellular compartment before they can carry out their respective functions. Their proper formation, targeting and activity are imperative to the survival of the cell. The Sec pathway facilitates export of the majority of bacterial proteins, whereas the Tat pathway is quite restricted. It transports ~ 30 proteins in Escherichia coli and only four in Bacillus subtilis [2].
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