The twin‐arginine protein transport (Tat pathway) is found in prokaryotes and plant organelles and transports folded proteins across membranes. Targeting of substrates to the Tat system is mediated by the presence of an N‐terminal signal sequence containing a highly conserved twin‐arginine motif. The Tat machinery comprises membrane proteins from the TatA and TatC families. Assembly of the Tat translocon is dynamic and is triggered by the interaction of a Tat substrate with the Tat receptor complex. This review will summarises recent advances in our understanding of Tat transport, focusing in particular on the roles played by Tat signal peptides in protein targeting and translocation.
Fig. 1. Targeting to the Sec and Tat pathways. (a) The Sec pathway transports unfolded proteins. During co‐translational targeting to Sec, the signal sequence is recognised at the translating ribosome by ribosome‐bound signal recognition particle (SRP) and the nascent chain is guided via the SRP receptor to the Sec translocon, where the energy of protein synthesis is harnessed to drive protein transport. In the post‐translational pathway, the substrate is maintained in an unfolded conformation and guided to the Sec translocon by the ATPase, SecA. ATP hydrolysis by SecA provides the driving force for Sec‐dependent post‐translational protein export (Collinson et al., 2015; Lycklama a Nijeholt & Driessen, 2012; Rapoport, Li, & Park, 2017; Tsirigotaki et al., 2017). The Tat pathway transports folded proteins without the requirement for targeting factors. (b) Signal peptides that target to Sec and Tat pathways share a similar tripartite organisation with a positively charged n‐region, hydrophobic h‐region and polar c‐region containing a signal peptidase cleavage site (AxA). Tat signal peptides have an almost invariant pair of arginines that are embedded within a SRRxFLK motif (Berks, 1996). A helix destabilising residue (#), often a glycine, serine or proline towards the C‐terminal end of the h‐region, provides flexibility at this region of the signal peptide (Hamsanathan et al., 2017). A basic residue (+) is frequently found in the Tat signal peptide c‐region and serves as a Sec avoidance motif (Bogsch et al., 1997). The arrow indicates the position of signal peptide cleavage. Amino acid sequences of two E. coli Sec signal peptides, OmpA (post‐translational Sec targeting; Fekkes et al., 1998) and DsbA (co‐translational targeting; Schierle et al., 2003)—basic residues in the n‐region and the signal peptidase cleavage site in the c‐region are underlined and shown in bold. Two well‐studied E. coli Tat signal peptides, SufI and TorA, are also shown. Residues that match the twin‐arginine consensus are in red, the Sec avoidance signal in bold typeface and the signal peptidase cleavage site in underline
A Model for the Tat transport pathway. Step1. A folded Tat substrate docks at the Tat receptor complex, the twin‐arginines in the signal peptide n‐region binding to the cytoplasmic surface of TatC. Step 2. The signal peptide transitions to bind more deeply into the receptor, inserting in a hairpin conformation. The deep insertion of the signal peptide displaces TatB from its resting state binding site on TatC to occupy the TatA binding site at TMH6. A TatA molecule is now recruited to the binding site vacated by TatB. Step 3. The positioning of TatA at the TM5 binding site allows the further recruitment and nucleation of TatA molecules to form a large oligomer. Step 4. The signal peptide hairpin unhinges and the substrate passes across the membrane facilitated by the TatA oligomer. Step 5. The signal peptide is cleaved and the mature domain is released at the periplasmic side of the membrane. Following substrate translocation, the TatA oligomer dissociates and the Tat receptor returns to the resting state
