C-terminal truncation of a Tat passenger protein affects its membrane translocation by interfering with receptor binding

During thylakoid transport of the chimeric model twin-arginine translocation (Tat) substrate 16/23, two consecutive translocation intermediates with different membrane topology are observed. The early translocation intermediate Ti-1 is bound to the membrane such that almost half of the protein is protected against proteolysis and it was concluded that not only the signal peptide but also part of the passenger protein participates in membrane binding. However, topology studies using a membrane-impermeable thiol-reactive reagent show that most of the passenger remains accessible from the stromal side in Ti-1 conformation. Establishment of such Ti-1 topology at the membrane apparently requires the fully folded passenger protein, as it was not observed with 16/23 truncation derivatives lacking the C-terminal 20, 40, 60, or 88 residues. Thylakoid transport of these mutants, which depends on a fully functional Tat machinery, is progressively reduced with increasing size of the truncated passenger polypeptide. The same holds true also for the interaction with the thylakoidal TatBC complexes, suggesting that in this case receptor binding, which is apparently impaired by extended unfolded or malfolded passenger polypeptides, is the rate-limiting step of Tat-dependent membrane transport.