Characterization and applications of the twin-arginine transporter pathway
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The twin-arginine translocase allows the translocation of folded protein substrates across the cytoplasmic membrane of bacteria and archaea or the thylakoid membrane of plants. In Escherichia coli, its protein components TatA, TatB and TatC assemble dynamically upon interaction with protein substrates. Prior to export, the machinery performs a quality control check so that only correctly folded proteins are translocated. The first objective of this work was to derive and apply new methodologies based on the inherent qualities of the pathway. We developed a new bacterial two-hybrid system that capitalizes on the folding quality control mechanism of the Tat pathway. One protein (prey) is fused to Tat-specific signal peptide. A second (bait) protein is produced as a fusion to a reporter that produces a "signal" (growth or enzymatic activity) only when the bait-reporter fusion binds to the prey and the resulting complex is exported into the periplasm via the Tat pathway. As a second biotechnological application of the Tat pathway, we developed a phage display system that allows the protein of interest to fold within the cytoplasm prior export and display onto phage particles. This is in contrast to the conventional phage display system, in which displayed protein folds in the periplasm. We took advantage of this new system to screen a library of 2 x 10⁶ of fluorescent GFP variants containing a hexameric peptide insertion for ligand binding. Despite the diversity of the hexamer, we were not able to isolate single GFP variants that would bind with specificity to various ligands. This highlights the difficulty in engineering GFP variants that can bind to other proteins while retaining the ability to fluoresce. The second aspect of this research was to examine mechanistic aspects of the Tat pathway. TatB and TatC are responsible for the recognition of Tat signal peptides. Here, we established the importance of TatC as the crucial component of the Tat pathway for the interaction with the hallmark twin-arginine motif within Tat signal peptide. Substitution of the RR dipeptide with a KK sequence completely abolishes export. In a genetic screen using a ssTorA(KK)-GFP-SsrA as a reporter. We identified several amino acid substitutions within TatC that allowed the alteration of the substrate specificity of the pathway as indicated by the impairment of indigenous Tat substrates. Finally, we analyzed the conformational dynamics of TatA using GFP fusions and by incorporation of the chemically reactive, non-canonical amino acid azidohomoalanine.