Protein targeting to the eukaryotic translocation apparatus is mediated by the signal recognition particle (SRP). In eukaryotes, this particle consists os six polypeptides complexed with a 7S RNA. Binding of the SRP to the nascent chain as it emerges from the ribosome creates a cytosolic targeting complex containing the nascent polypeptide chain, the translating ribosome and the SRP. This complex is directed to the endoplasmic reticulum membrane as a result of its interaction with the SRP receptor. In the presence of GTP, SRP receptor binding to the SRP causes the SRP to dissociate from both the signal sequence and the ribosome. GTP is then hydrolyzed, and the SRP is released from the SRP receptor and returned to the cytosol. At this stage, the ribosome resumes translation, and the nascent chain is directed into or across the endoplasmic reticulum membrane by the translation machinery of the cell. The 54 kD subunit of the SRP (SRP54) is a multi-domain enzyme which binds to GTP/GDP, 7S RNA and to signal sequences as they emerge from the translating ribosome. The mechanism of protein translocation across the endoplasmic reticulum membrane of eukaryotic cells and the plasma membrane or prokaryotic cells appear to be evolutionary related. We seek to understand the mechanism by which SRP54 coordinates binding and release of signal sequences during targeting of the nascent chain to the translocation apparatus of the cell. Therefore, we have initiated structural studies on the SRP54 homologue from Thermus acquaticus.
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