Ribosome Associated Factors Recruited for Protein Export and Folding

Abstract: Protein folding and export to the membrane are crucial events in the cell. Both processes may be initiated already at the ribosome, assisted by factors that bind to the polypeptide as it emerges from the ribosome. The signal recognition particle (SRP) scans the ribosome for nascent peptides destined for membrane insertion and targets these ribosomes to the site for translocation in the membrane. Trigger factor (TF) is a folding chaperone that interacts with nascent chains to promote their correct folding, prevent misfolding and aggregation. In this thesis, we first investigated membrane targeting and insertion of two heterologous membrane proteins in E. coli by using in vitro translation, membrane targeting and cross-linking. We found that these proteins are dependent on SRP for targeting and that they initially interact with translocon components in the same way as native nascent membrane proteins. Moreover we have characterised the SRP and TF interactions with the ribosome both with cross-linking experiments and with quantitative binding experiments. Both SRP and TF bind to ribosomal L23 close to the nascent peptide exit site where they are strategically placed for binding to the nascent polypeptide. Quantitative analysis of TF and SRP binding determined their respective KD values for binding to non translating ribosomes and reveals that they bind simultaneously to the ribosome, thus having separate binding sites on L23. Finally, binding studies on ribosome nascent chain adds clues as to how TF functions as a chaperone.