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Translational regulation in prokaryotes ensures efficient protein synthesis by controlling ribosome access to mRNA. This regulation is mediated by secondary RNA structures, including translational riboswitches, RNA thermometers, and small RNAs (sRNAs), which respond to intracellular and environmental signals to modulate gene expression.Translational RiboswitchesRiboswitches in the leader region of mRNAs can regulate translation by altering the accessibility of the Shine-Dalgarno (SD) sequence,...
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Related Experiment Video

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Quantitative Immunofluorescence to Measure Global Localized Translation
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Temporal Regulation of Signal Recognition Particle During Translation.

Ruilin Qian1, Radoslaw J Gora1, Sowmya Chandrasekar1

  • 1Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA.

Journal of Molecular Biology
|October 11, 2025
PubMed
Summary
This summary is machine-generated.

Signal recognition particle (SRP) loses its ability to target proteins to the ER as nascent chains lengthen. The nascent polypeptide associated complex (NAC) further hinders SRP function, limiting cotranslational targeting.

Keywords:
protein targetingribosomesignal recognition particlesignal sequencesingle molecule fluorescence

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Area of Science:

  • Molecular Biology
  • Cell Biology
  • Biochemistry

Background:

  • Signal recognition particle (SRP) is essential for directing proteins to the endoplasmic reticulum (ER).
  • SRP binds signal sequences on nascent proteins emerging from ribosomes.
  • The mechanism by which SRP targeting competence is lost with increasing nascent chain length is not fully understood.

Purpose of the Study:

  • To investigate the molecular mechanisms underlying the loss of SRP targeting competence as nascent polypeptide chains elongate.
  • To elucidate the role of the nascent polypeptide associated complex (NAC) in regulating SRP function during protein synthesis.

Main Methods:

  • Utilized steady-state and single-molecule fluorescence spectroscopy.
  • Employed a Förster resonance energy transfer (FRET) assay to monitor signal sequence dynamics.
  • Studied SRP interaction with ribosomes bearing nascent chains of varying lengths.

Main Results:

  • Longer nascent chains increase dynamic excursions of the signal sequence from SRP, leading to a suboptimal SRP conformation.
  • Impaired interaction kinetics between SRP and the SRP receptor (SR) were observed with longer nascent chains.
  • NAC amplifies the inhibitory effects of longer nascent chains on SRP, further excluding it from ER targeting.

Conclusions:

  • Elongating nascent polypeptides significantly impact SRP conformation and activity.
  • NAC plays a crucial role in the temporal regulation of SRP function.
  • These factors together create a limited window for cotranslational ER protein targeting.