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Elife. 2014 Oct 30;3. doi: 10.7554/eLife.04418.

Real-time observation of signal recognition particle binding to actively translating ribosomes.

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Howard Hughes Medical Institute, University of California, San Francisco, San Francisco, United States.
Department of Structural Biology, Stanford University School of Medicine, Stanford, United States.


The signal recognition particle (SRP) directs translating ribosome-nascent chain complexes (RNCs) that display a signal sequence to protein translocation channels in target membranes. All previous work on the initial step of the targeting reaction, when SRP binds to RNCs, used stalled and non-translating RNCs. This meant that an important dimension of the co-translational process remained unstudied. We apply single-molecule fluorescence measurements to observe directly and in real-time E. coli SRP binding to actively translating RNCs. We show at physiologically relevant SRP concentrations that SRP-RNC association and dissociation rates depend on nascent chain length and the exposure of a functional signal sequence outside the ribosome. Our results resolve a long-standing question: how can a limited, sub-stoichiometric pool of cellular SRP effectively distinguish RNCs displaying a signal sequence from those that are not? The answer is strikingly simple: as originally proposed, SRP only stably engages translating RNCs exposing a functional signal sequence.


E. coli; biophysics; cell biology; protein targeting; ribosome translation; signal recognition particle; single molecule fluorescence; structural biology

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