Impairment of twin-arginine-dependent export by seemingly small alterations of substrate conformation

The twin-arginine translocation (Tat) machinery is able to transport fully folded proteins across bacterial and thylakoidal membranes. Previous in vivo and in vitro studies indicated that the model Tat substrate TorA–PhoA acquires Tat-competence only if its four cysteines form disulfide bonds. We now show that removal of the last 33 amino acids of PhoA, although not affecting the formation of disulfide bonds, converts TorA–PhoA into a poor Tat substrate. This finding suggests that even incomplete folding of a substrate can interfere with transport by the Tat translocase of Escherichia coli.