The core TatABC complex of the twin-arginine translocase in Escherichia coli: TatC drives assembly whereas TatA is essential for stability

Current models for the action of the twin-arginine translocation (Tat) system propose that substrates bind initially to the TatBC subunits, after which a separate TatA complex is recruited to form an active translocon. Here, we have studied the roles of individual subunits in the assembly and stability of the core TatBC-containing substrate-binding complex. Previous studies have shown that TatB and TatC are active when fused together; we show here that deletion of the entire TatB transmembrane span from this Tat(BC) fusion inactivates the Tat system but does not affect assembly of the core complex. In this mutated complex, TatA is present but more loosely bound, indicating a role for TatB in the correct binding of TatA. In the absence of TatA, the truncated TatBC fusion protein still assembles into a complex of the correct magnitude, demonstrating that the transmembrane spans of TatC are the only determinants within the membrane bilayer that specify assembly of this complex. Further studies on both the Tat(BC) construct and the wild-type TatBC subunits show that the TatBC complex is unstable in the absence of TatA, and we show that TatA stabilises the TatB subunit specifically within this complex. The results demonstrate a dual role and location for TatA: in the functioning/maintenance of the core complex, and as a separate homo-oligomeric complex.