The twin-arginine translocation (Tat) system catalyzes the transport of folded proteins across the bacterial plasma membrane or the chloroplast thylakoid membrane. In Escherichia coli and most other species, three important tat genes have been identified but the structure and mechanism of this system are poorly understood; the role and location of TatA are particularly unclear. In this report we have used site-specific mutagenesis to probe the significance of conserved features of the related TatA/B subunits. We find that an apparent ‘hinge’ region between the transmembrane (TM) span and an adjacent amphipathic region is important in both proteins, in that substitution of turn-inducing residues inhibits the export of a natural Tat substrate. Surprisingly, large-scale mutagenesis of the conserved amphipathic regions of TatA and TatB leads only to minor effects on Tat-dependent export suggesting that this particular feature is not central to the translocation mechanism. This domain is, however, critical for the translocation process and we identify Gly/Pro residues in these regions of TatA/B that are essential for efficient export.
