Question

The signal-recognition particle (SRP) ...

1) is a heterodimeric protein.
2) transiently inhibits translation and polypeptide elongation by binding to and inhibiting the elongation factors.
3) accompanies the nascent polypeptide all the way into the ER lumen.
4) binds GTP.
5) is permanently attached to the cytosolic face of the ER membrane, thus bringing the ribosomes into close proximity of the translocon.

Answer 1

The signal-recognition particle (SRP) binds to the signal peptide of a nascent protein, stops translation, then targets the complex to the ER where it resumes synthesis and enters the ER for folding and processing.

Step-by-step explanation:

The signal-recognition particle (SRP) is an important component in the translation and targeting process of proteins to the endoplasmic reticulum. SRPs recognize and bind to the hydrophobic signal peptide emerging from the ribosome during protein synthesis. Once bound, translation is temporarily halted until the SRP-ribosome-nascent chain complex is targeted to the SRP receptor on the rough ER membrane. At the membrane, SRP detaches, allowing translation to resume and the growing protein to enter the ER. This machinery ensures that proteins destined for secretion or membrane localization are properly directed to the ER.

Furthermore, SRPs are not permanently attached to the ER membrane; they recycle and bind to GTP, which is necessary for their function. Elongation continues and the growing polypeptide begins to fold within the ER lumen, sometimes assisted by chaperones, while a signal peptidase catalyzes the cleavage of the signal peptide.