Competition between Sec- and TAT-dependent protein translocation in Escherichia coli

Recently, a new protein translocation pathway, the twin-arginine translocat ion (TAT) pathway, has been identified in both bacteria and chloroplasts. T o study the possible competition between the TAT- and the well-characterize d Sec translocon-dependent pathways in Escherichia coli, we have fused the TorA TAT-targeting signal peptide to the Sec-dependent inner membrane prote in leader peptidase (Lep). We find that the soluble, periplasmic P2 domain from Lep is re-routed by the TorA signal peptide into the TAT pathway, In c ontrast, the full-length TorA-Lep fusion protein is not re-routed into the TAT pathway, suggesting that Sec-targeting signals in Lep can override TAT- targeting information in the TorA signal peptide. We also show that the Tor A signal peptide can be converted into a Sec-targeting signal peptide by in creasing the hydrophobicity of its h-region, Thus, beyond the twin-arginine motif, the overall hydrophobicity of the signal peptide plays an important role in TAT versus Sec targeting, This is consistent with statistical data showing that TAT-targeting signal peptides in general have less hydrophobi c h-regions than Sec-targeting signal peptides.