In many prokaryotic organisms, secretory proteins harboring a twin-arginine consensus motif are exported in a fully folded conformation via the twin-arginine translocation (Tat) pathway. In Escherichia coli, Tat involves the three structurally and functionally different membrane proteins TatA, TatB, and TatC. Whereas TatC proteins function in the specific recognition of substrate, TatA might be the major pore-forming subunit.