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

Recently, a new protein translocation pathway, the twin‐arginine translocation (TAT) pathway, has been identified in both bacteria and chloroplasts. To study the possible competition between the TAT‐ and the well‐characterized Sec translocon‐dependent pathways in Escherichia coli, we have fused the TorA TAT‐targeting signal peptide to the Sec‐dependent inner membrane protein 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 contrast, 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 TorA signal peptide can be converted into a Sec‐targeting signal peptide by increasing 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 hydrophobic h‐regions than Sec‐targeting signal peptides.