Tat-mediated quality control in Escherichia coli

• Main Author: Matos, Cristina Filipa Rodrigues de Oliveira
• Format: Dissertation
• Language: English
• Published: University of Warwick 2010
• Subjects: QR Microbiology

The E. coli Twin-arginine translocation (Tat) pathway transports a subset of proteins from the cytosol, across the inner membrane to the periplasmic space. One of the unique features of this pathway is its ability to transport fully folded passenger proteins. These passenger proteins often contain redox co-factors such as the iron-sulphur (FeS) proteins. This feature of the pathway suggests that any quality control of passenger proteins must occur prior to export. In this study the question of Tat pathway quality control is addressed. Initial studies (Chapter 3) addressed the degree to which the Tat pathway would tolerate the misfolding of its passenger proteins. To this end, mutant forms of the FeS proteins NrfC and NapG, were generated with incremental impairment of FeS cluster formation. Expression of these mutants in E. coli revealed that the Tat system completely blocked the export of NrfC when even one of its four FeS centres was mutagenised. Furthermore, the rejected passenger proteins were rapidly degraded in a Tat dependent manner. Dissection of the components involved in this process led to the discovery that TatA/E were essential for the degradation (Chapter 3). Furthermore, the previously neglected subunit TatD also plays a central role in Tat-mediated quality control and degradation (Chapter 4). Interestingly, the data presented here demonstrate that this quality control of Tat passengers also extends to nonmutated and non-cofactor containing proteins that are not exported in a timely manner. Investigations into the mechanism of cytosolic degradation of rejected passenger proteins led to the discovery that the ClpAPS system is involved. Interestingly, in the case of FeS proteins, ClpP is not responsible for proteolysis yet ClpS and ClpA are required. However, the degradation of the non-cofactor containing passenger protein, FhuD, is dependent on the entire ClpAPS system (Chapter 5).