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A Convenient Partnership: The Ribosome and the Nascent Chain Interact to Modulate Protein Synthesis and Folding


 Dr. Daniel Goldman
Advisor: Professor Carlos Bustamante, Department of Chemistry
During translation, the ribosome reads the genetic code of the messenger RNA, adding one amino acid at a time to the nascent polypeptide. In order to carry out it's biological function, the polypeptide must fold to the native state, and the folding process can begin before translation is complete. My thesis work has focused on ribosome-nascent chain interactions that affect both the folding process and the activity of the ribosome. Using a novel optical tweezers assay, we observed folding transitions of single ribosome-bound nascent polypeptides. We found that the ribosome can modulate the kinetics of folding, guiding the protein to the native state. 
The nascent polypeptide can also interact with the exit tunnel of the ribosome, leading to translational arrest. In bacteria, the Secretion Monitor protein stalls the ribosome upon translation of a 17 amino acid motif. Arrest release requires targeting of the stalled ribosome-nascent chain complex to the translocon; thus, it has been hypothesized that arrest is released by a mechanical pulling force generated as the polypeptide is translocated across the membrane. We demonstrated that translation in such instances can be restarted by applying a mechanical force to the polypeptide. Additionally, we found that such a force can be generated by a nascent polypeptide folding near the ribosome exit tunnel. Our results demonstrate a feedback mechanism, whereby a folding protein can modulate its synthesis through the generation of force.
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