Probing the role of Tat proofreading chaperones in the assembly of molybdoenzymes

<br/>The twin-arginine (Tat) system is a specialised translocation machine found in prokaryotes and chloroplasts which serves to export fully folded proteins across the cytoplasmic membrane. Proteins are specifically targeted to this system by N-terminal signal peptides which bear a conserved SRRxFLK motif. Tat dependent proteins often contain redox factors that must be inserted prior to translocation. The E. coli Tat substrates trimethylamine Noxide<br/>reductase (Tor) and periplasmic nitrate reductase (Nap) are two such substrates<br/>which require the molybdenum cofactor Mo-bis-MGD in order to function as anaerobic reductases.<br/>Pre-export assembly of Tor and Nap are subject to a process called Tat proofreading, where the substrate specific chaperones, TorD and NapD, bind tightly to the signal peptides of these substrates, TorA and NapA, respectively, assisting in the assembly of the proteins.<br/>Work described in this thesis demonstrates that TorD simultaneously binds to two independent sites present on the TorA, one present in the mature domain of the substrate and one in the signal peptide. Biophysical and structural analysis shows that the pre-export complexes of TorD and TorA appear identical regardless of whether the TorA signal peptide is present or absent. However, the complex purified in the absence of the TorA signal<br/>peptide is able to interact with extrinsically-added signal peptide, confirming the presence of a separate signal peptide binding site. By contrast to TorD, NapD was shown to bind exclusively to the signal peptide of its substrate, NapA. Pre-export complexes of TorAD and NapDA showed similar low resolution structures, with both NapA and TorA exhibiting flexibility in Domain IV. It is hypothesised that this Domain acts as a gate, allowing access of<br/>the cofactor, and that closing of this gate is the final step in the folding of the substrate.

Name dc:type.qualificationname

Doctor of Philosophy

Level dc:type.qualificationlevel

Doctoral Thesis

Grantor dc:publisher.institution

University of Dundee

Year dc:date.issued

2013

Author dc:creator

• Dow, Jennifer Mhairi

Advisor dc:contributor.advisor

• Palmer, Tracy

Language dc:language

eng

Identifier

oai:discovery.dundee.ac.uk:studenttheses/8b7d7f2d-3aa8-48ce-a9eb-ec8a7d2e341f

OAI identifier oai:identifier

oai:discovery.dundee.ac.uk:studenttheses/8b7d7f2d-3aa8-48ce-a9eb-ec8a7d2e341f