Assembly mechanism of the Twin Arginine Transporter

The twin-arginine protein translocation (Tat) system transports folded proteins across the bacterial cytoplasmic membrane and the thylakoid membrane of chloroplasts. The Escherichia coli Tat system comprises TatC and two additional sequence-related proteins, TatA and TatB. The Escherichia coli Tat system comprises TatC and two additional sequence-related proteins, TatA and TatB. Tat-directed proteins are distinguished by a conserved twin-arginine (RR-) motif in their signal sequences. Substrate binding promotes a conformation rearrangement of the TatABC receptor complex triggering recruitment and assembly of multiple additional copies of TatA. The central challenge in understanding Tat transport is to determine how the Tat components are assembled to form the active translocon allowing folded substrates to be transferred across the membrane bilayer. Unlike conventional protein translocation pathways that contain an aqueous transmembrane channel through it the substrate can migrate, Tat transporter seems to work by a nonconventional mechanism in which a translocation conduit is assembled on demand. Understanding how the Tat components are assembled to form the active translocon can be a challenging task due the conformational and compositional heterogeneity of the translocation complex. In this work, we used molecular dynamics simulations to describe the TatA oligomerization mechanism and how the translocation pathway is formed. Our results show that the Tat complex becomes more dynamic after substrate binding. The presence of the substrate induces a “breathing” movement, increasing the separation between two adjacent TatC subunits. Moreover, our simulations suggest that substrate binding induces an asymmetry in the Tat Complex, favouring TatA aggregation around the interface opposite to the TatC subunit where the signal peptide is bound. The TatA oligomer anchors to the Tat complex through contacts with the TM1 and the TM6 of the adjacent TatC subunits, promoting a local weakening of the membrane bilayer. Our extensive coarse-grained MD simulations provide a molecular description of the Tat translocon assembly and new insights into the Tat transport cycle.