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Abstract
The signal recognition particle (SRP) is an RNA-protein complex that directs ribosomes to the rough endoplasmic reticulum membrane by binding to targeting signals found on the nascent chain of proteins destined for export to the endoplasmic reticulum. We found evidence from studies with fragments of the protein component of the Escherichia coli SRP that a long hydrophobic loop (the so-called "finger loop") is detrimental to the stability of its signal peptide-binding domain, the M domain. This hydrophobic loop is highly conserved and thus may have a critical role in the function of the SRP. Given our previously reported evidence that 4.5 S RNA stabilizes the tertiary fold of the M domain (Zheng, N., and Gierasch, L. M. (1997) Mol. Cell 1, 79-87), we now propose that the functional requirement for 4.5 S RNA resides in its ability to counteract the destabilizing influence of the finger loop.
Highlights
In eukaryotic cells the signal recognition particle (SRP)1 targets ribosomes synthesizing secretory and membrane proteins to the rough endoplasmic reticulum membrane
We report that the metastability of the E. coli Ffh M domain in the absence of RNA can be attributed to the presence of the long, hydrophobic finger loop
We found evidence that this disordered, exposed loop is retained as a feature of the E. coli Ffh M domain in solution
Summary
In eukaryotic cells the signal recognition particle (SRP)1 targets ribosomes synthesizing secretory and membrane proteins to the rough endoplasmic reticulum membrane. We report that the metastability of the E. coli Ffh M domain in the absence of RNA can be attributed to the presence of the long, hydrophobic finger loop.
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