Which description correctly explains how signal peptides direct proteins to mitochondria, chloroplasts, and secretory pathways?

Which description correctly explains how signal peptides direct proteins to mitochondria, chloroplasts, and secretory pathways?

• A. ER signal sequences direct nascent proteins to mitochondria.
• B. Mitochondria and chloroplasts use identical targeting signals as the secretory pathway.
• C. Signal peptides direct nascent proteins to specific destinations; ER signals direct secretory and membrane proteins; mitochondria and chloroplasts have targeting peptides recognized by translocases enabling import to these organelles.
• D. Secretory pathway proteins lack targeting signals.

Answer: (C)

Proteins are guided to their correct locations by distinct targeting signals that are recognized by specific import machinery. The signal for the secretory pathway is an ER signal sequence at the N-terminus of the nascent chain, which is recognized by the signal recognition particle (SRP). This pauses translation and directs the ribosome–nascent chain complex to the ER membrane, where the protein is threaded into the lumen or integrated into the membrane via the Sec61 translocon, and the signal peptide is typically cleaved. Mitochondria and chloroplasts use different targeting peptides, not the ER signal. These organelle-specific peptides are recognized by their respective translocases (mitochondria use TOM/TIM complexes; chloroplasts use TOC/TIC complexes) to import proteins across the outer and inner membranes. After import, the transit peptides are usually removed by processing peptidases. Thus, long-range targeting relies on signal peptides tailored to each destination: ER signals for secretory/membrane proteins, and separate targeting peptides for mitochondria and chloroplasts. The statement captures this distinction and why these pathways operate with different import machinery.

Proteins are guided to their correct locations by distinct targeting signals that are recognized by specific import machinery. The signal for the secretory pathway is an ER signal sequence at the N-terminus of the nascent chain, which is recognized by the signal recognition particle (SRP). This pauses translation and directs the ribosome–nascent chain complex to the ER membrane, where the protein is threaded into the lumen or integrated into the membrane via the Sec61 translocon, and the signal peptide is typically cleaved.

Mitochondria and chloroplasts use different targeting peptides, not the ER signal. These organelle-specific peptides are recognized by their respective translocases (mitochondria use TOM/TIM complexes; chloroplasts use TOC/TIC complexes) to import proteins across the outer and inner membranes. After import, the transit peptides are usually removed by processing peptidases.

Thus, long-range targeting relies on signal peptides tailored to each destination: ER signals for secretory/membrane proteins, and separate targeting peptides for mitochondria and chloroplasts. The statement captures this distinction and why these pathways operate with different import machinery.