What is the role of the signal recognition particle (SRP) in targeting proteins to the ER?

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Multiple Choice

What is the role of the signal recognition particle (SRP) in targeting proteins to the ER?

Proteins destined for the secretory pathway start with a signal peptide that emerges from the ribosome. The signal recognition particle binds this signal sequence as soon as it appears, pausing translation so the growing chain isn’t released into the cytosol. This SRP–ribosome–nascent chain complex then docks at the ER membrane by interacting with the SRP receptor, bringing the ribosome to the Sec61 translocon. Once anchored at the translocon, translation resumes and the nascent polypeptide is threaded into the ER lumen or integrated into the ER membrane. After translocation, the signal peptide is typically cleaved by a signal peptidase in the ER. SRP thus serves to specifically recognize targeting signals and couple translation to translocation, ensuring proteins meant for secretion or membrane insertion are delivered to the ER. This is why the other options don’t fit: degradation of misfolded proteins is handled by ERAD, not SRP; signal peptides are cleaved by signal peptidase after import, not by SRP; and SRP’s role is not to transport proteins to mitochondria.

Proteins destined for the secretory pathway start with a signal peptide that emerges from the ribosome. The signal recognition particle binds this signal sequence as soon as it appears, pausing translation so the growing chain isn’t released into the cytosol. This SRP–ribosome–nascent chain complex then docks at the ER membrane by interacting with the SRP receptor, bringing the ribosome to the Sec61 translocon. Once anchored at the translocon, translation resumes and the nascent polypeptide is threaded into the ER lumen or integrated into the ER membrane. After translocation, the signal peptide is typically cleaved by a signal peptidase in the ER. SRP thus serves to specifically recognize targeting signals and couple translation to translocation, ensuring proteins meant for secretion or membrane insertion are delivered to the ER. This is why the other options don’t fit: degradation of misfolded proteins is handled by ERAD, not SRP; signal peptides are cleaved by signal peptidase after import, not by SRP; and SRP’s role is not to transport proteins to mitochondria.

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