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) targets and temporarily stops protein synthesis by binding to the signal peptide, guides the ribosome to the RER, and resumes translation once interacting with the RER membrane where the signal peptide is cleaved and the SRP is recycled.

Step-by-step explanation:

The signal recognition particle (SRP) plays a crucial role in the targeting and secretion of proteins in eukaryotic cells. Its main function is to recognize and bind to the signal peptide that emerges from a ribosome during protein synthesis. Once bound, it stops translation by preventing the elongation of the polypeptide chain. This is a temporary inhibition, after which the SRP-ribosome complex is guided to the rough endoplasmic reticulum (RER) membrane, where it interacts with an SRP receptor.

Upon reaching the RER membrane, the SRP is released, allowing translation to resume through a translocation channel. As translation continues, a signal peptidase in the RER membrane catalyzes the hydrolysis of the signal peptide, which then remains in the membrane. The elongation process goes on, and the growing polypeptide begins to fold within the RER. Furthermore, SRPs are not permanently attached to the ER membrane; they can detach and be recycled. SRPs also bind GTP, which is necessary for their function.