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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.

Author information

1
Howard Hughes Medical Institute, University of California, San Francisco, San Francisco, United States.
2
Department of Structural Biology, Stanford University School of Medicine, Stanford, United States.

Abstract

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.

KEYWORDS:

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

PMID:
25358118
PMCID:
PMC4213662
DOI:
10.7554/eLife.04418
[Indexed for MEDLINE]
Free PMC Article

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