The bacterial twin-arginine translocation pathway

Abstract

The twin-arginine translocation (Tat) pathway is responsible for the export of folded proteins across the cytoplasmic membrane of bacteria. Substrates for the Tat pathway include redox enzymes requiring cofactor insertion in the cytoplasm, multimeric proteins that have to assemble into a complex prior to export, certain membrane proteins, and proteins whose folding is incompatible with Sec export. These proteins are involved in a diverse range of cellular activities including anaerobic metabolism, cell envelope biogenesis, metal acquisition and detoxification, and virulence. The Escherichia coli translocase consists of the TatA, TatB, and TatC proteins, but little is known about the precise sequence of events that leads to protein translocation, the energetic requirements, or the mechanism that prevents the export of misfolded proteins. Owing to the unique characteristics of the pathway, it holds promise for biotechnological applications.

Figure 1

Features of a typical Tat leader peptide, ssTorA, from E. coli. The n-region is shown in pink, the h-region in blue, and the c-region in yellow. The consensus Tat motif is boxed. The vertical dashed line indicates the site of cleavage by signal peptidase I. Charged residues in the c-region and mature proteins are marked.

Figure 2

The predicted structure and topology of the E. coli Tat components. Predicted helical regions are shown as boxes.

Figure 3

Model of Tat targeting and transport. In the membrane TatA is depicted in red, TatB in blue, and TatC in yellow. (a) Upon emerging from the ribosome the preprotein must avoid targeting to other pathways such as Sec, which is aided by the characteristics of the signal peptide and mature protein (; D. Tullman-Ercek, M. P. DeLisa, Y. Kawarasaki, P. Iranpour, B. Ribnicky, et al., unpublished data) and/or the binding of Tat-specific chaperones (red circles) (70, 129). (b) After folding, any cofactors and/or additional subunits are added prior to targeting to the TatBC receptor complex (c) (1, 27, 34). (d) The proton motive force drives the formation of an active translocase and the substrate is transported through a pore consisting mainly of TatA (1, 52, 85). (e) Upon removal of the signal peptide the mature protein is released on the periplasmic side of the membrane.

Figure 4

Three-dimensional density maps of TatA complexes. (a) TatA complexes viewed from the closed end of the channel proposed to be at the cytoplasmic side of the membrane (C-face). (b) TatA complexes viewed from the open end of the channel proposed to be at the periplasmic side of the membrane (P-face). (c) Side views of TatA. The front half of each molecule has been cut away to reveal internal features. (d) Views of TatA parallel to the membrane plane. The proposed position of the lipid bilayer is indicated in blue. (Scale bar, 100 Å.) Reproduced with permission from Reference © 1993−2005 by The National Academy of Sciences of the United States of America, all rights reserved.