Multiple precursor proteins bind individual Tat receptor complexes and are collectively transported.

The thylakoid twin arginine protein translocation (Tat) system is thought to have a multivalent receptor complex with each cpTatC-Hcf106 pair constituting a signal peptide-binding unit. Conceptual models suggest that translocation of individual precursor proteins occurs upon assembly of a Tha4 oligomer with a precursor-occupied cpTatC-Hcf106. However, results reported here reveal that multiple precursor proteins bound to a single receptor complex can be transported together. Precursor proteins that contain one or two cysteine residues readily formed intermolecular disulphide bonds upon binding to the receptor complex, resulting in dimeric and tetrameric precursor proteins. Three lines of evidence indicate that all members of precursor oligomers were specifically bound to a receptor unit. Blue native-polyacrylamide gel electrophoresis analysis showed that oligomers were present on individual receptor complexes rather than bridging two or more receptor complexes. Upon energizing the membrane, the dimeric and tetrameric precursors were transported across the membrane with efficiencies comparable with that of monomeric precursors. These results imply a novel aspect of Tat systems, whereby multiple precursor-binding sites can act in concert to transport an interlinked oligo-precursor protein.