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Chapter 2 : The Tat Protein Export Pathway
Category: Bacterial Pathogenesis; Microbial Genetics and Molecular Biology
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Protein transport by the Tat pathway is powered solely by the proton electrochemical gradient. This paper describes what is known about the targeting and transport of proteins to the periplasm by the Tat pathway, focusing particularly on the model organism Escherichia coli. Prediction programs such as TatFind and TatP use the salient features of the Tat signal peptide to identify candidate Tat substrates. It is not known which of the Tat components is responsible for transducing the proton motive force nor whether the proton motive force is required for stages beyond formation of the TatABC complex. Proteins are targeted to the Tat transport machinery by means of N-terminal signal peptides. Tat signal peptides often have very extended n-regions prior to the twin-arginine motif. It has been reported that the Tat system of both thylakoids and E. coli has an intrinsic “quality control” activity that allows it to distinguish between folded and unfolded substrates. Whether and how this quality control interrelates with the chaperone-mediated proofreading process remains to be established.
(A) Chromosomal location and organization of genes encoding components of the E. coli Tat pathway. Genes that are known or likely to be found in the same transcriptional unit ( Jack et al., 2001 ) have the same fill. The spacing between each gene is shown in base pairs (bp) above the figure. (B) Schematic representation of the topological organization of the E. coli Tat components.
The Tat protein transport cycle. In the resting state, TatA and TatBC form separate, oligomeric complexes (top). A substrate docks at the TatBC complex, with the twin-arginine motif of the signal peptide interacting with TatC (right). This activated complex now interacts with TatA in an energy-dependent step (bottom). The substrate is transported through a channel made up of TatA. The energetic requirements for this are unknown (left). At some stage during the transport cycle the signal peptide is cleaved and the exported protein is released at the periplasmic face of the membrane (or integrated into the lipid bilayer for the membrane-anchored Tat substrates). The TatA and TatBC components dissociate and the system returns to the initial state (top). (Figure adapted from Ben C. Berks, Tracy Palmer, and Frank Sargent, Protein targeting by the bacterial twin-arginine translocation (Tat) pathway. Curr. Opin. Microbiol. 8:174–181. Copyright , with permission from Elsevier.)
A model for chaperone-mediated assembly of a cofactor-containing Tat substrate based on studies with the paradigm TorD/TorA binary system. A de novo-synthesized, unfolded, precursor protein is initially recognized and bound by a specific chaperone. In some cases, the chaperone could bind to at least two sites on the protein, one being the signal peptide itself and the second site being elsewhere on the mature portion of the protein ( Pommier et al., 1998 ; Jack et al., 2004 ). This is probably achieved by the independent binding of two chaperone molecules, but an alternative model would involve the binding of a single chaperone that contains two separate interaction sites. Association of the chaperone with the precursor leads to conformational changes in both. A binding site for GTP would be exposed on the chaperone ( Hatzixanthis et al., 2005 ) (left), and the precursor would bind its cofactor(s). Once cofactor loading is complete, the mature portion of precursor would attain its final fully folded conformation and this would result in the release of at least one of the bound chaperones. Chaperone release may be triggered by GTP hydrolysis (bottom). If the Tat substrate precursor forms a cotranslocation complex with other proteins, they are probably associated at this stage. At this juncture, if the signal-bound chaperone also acts as an “escortase,” the folded substrate will be targeted to the Tat machinery by the chaperone (left). Alternatively, release of the signal peptide-bound chaperone would allow the substrate to follow a generic route to the Tat machinery.
The known or likely E. coli Tat-signal peptide-bearing substrate proteins a