Twin arginine translocation (Tat)-dependent protein transport: the passenger protein participates in the initial membrane binding step.

The initial step in twin arginine translocation (Tat)-dependent thylakoid transport of the 16/23 chimera is the interaction of the protein with the lipid bilayer. It results in the formation of the early translocation intermediate Ti-1, which is represented by a protease-protected fragment of 14 kDa. Cys-scanning mutagenesis in combination with in thylakoido and liposome insertion assays was used to precisely map this membrane-interacting and protease-protected fragment within the 16/23 chimera. The fragment comprises 124 residues, which are provided both by the transit peptide (31 residues) and the mature protein (93 residues), demonstrating that the passenger protein directly participates in membrane binding. The implications of this finding on the mechanism of Tat-dependent protein transport are discussed.