Rapid diffusion of large TatA complexes detected using single particle tracking microscopy

The twin-arginine translocation (Tat) system transports folded proteins across the cytoplasmic membrane of most bacteria and archaea. TatA, which contains a single membrane-spanning helix, is believed to be responsible for the actual translocation. According to the prevalent model, multiple TatA subunits form a transient protein-conducting pore, which disassembles after each translocation event. An alternative model exists, in which TatA proteins locally weaken the lipid bilayer to translocate folded proteins. Here, we imaged eGFP-fused TatA expressed from the genome in live E. coli cells. Images showed TatA occuring both in highly mobile monomers or small oligomers and in large, stable complexes that do not dissociate. Single-particle tracking revealed that large TatA complexes switch between fast and slow diffusion. The fast diffusion is too fast for a transmembrane protein complex consisting of multiple TatA monomers. In line with recent data on rhomboid proteases, we propose that TatA complexes switch between a slowly diffusing transmembrane conformation and a rapidly diffusing membrane-disrupting state that enables folded proteins to cross the membrane, in accordance with the membrane-weakening model.