The TatBC Complex of the Tat Protein Translocase in Escherichia coli and Its Transition to the Substrate-Bound TatABC Complex

The twin-arginine translocation (Tat) system serves to transport folded proteins across membranes of prokaryotes and plant plastids. In Escherichia coli, a complex consisting of multiple copies of TatB and TatC initiates the transport by binding the signal peptides of the Tat substrates. Using blue-native polyacrylamide gel electrophoresis, bands of TatBC-containing complexes can be detected at molecular masses of 440 and 580 kDa. We systematically analyzed the formation of Tat complexes with TatB or TatC variants that carried point mutations at selected positions. Several mutations resulted in specific disassembly patterns and alterations in the 440 kDa:580 kDa complex ratios. The 440 kDa complex contains only TatBC, whereas the 580 kDa complex consists of TatABC. Substrate binding results in a TatBC–Tat substrate complex at ∼500 kDa and a TatABC–Tat substrate complex at ∼600 kDa. Only the ∼600 kDa complex was detected with nonrecombinant substrate levels and thus could be the physiologically most relevant species. The results suggest that some TatA is usually associated with TatBC, regardless of substrate binding.