[Campbell Biology P.403] A polypeptide destined for secretion contains a signal peptide at i... | Practice Question
A polypeptide destined for secretion contains a signal peptide at its N-terminus. Which of the following describes the correct order of events for its synthesis and targeting to the ER?
A: 1. Polypeptide synthesis begins on a free ribosome in the cytosol. 2. An SRP binds to the signal peptide, halting synthesis. 3. The SRP binds to a receptor protein in the ER membrane. 4. Synthesis resumes, with translocation across the membrane. 5. The signal peptide is cleaved. 6. The completed polypeptide folds in the ER lumen.
B: 1. An SRP binds to the signal peptide, halting synthesis. 2. Polypeptide synthesis begins on a free ribosome in the cytosol. 3. The SRP binds to a receptor protein in the ER membrane. 4. Synthesis resumes, with translocation across the membrane. 5. The signal peptide is cleaved. 6. The completed polypeptide folds in the ER lumen.
C: 1. Polypeptide synthesis begins on a free ribosome in the cytosol. 2. The SRP binds to a receptor protein in the ER membrane. 3. An SRP binds to the signal peptide, halting synthesis. 4. Synthesis resumes, with translocation across the membrane. 5. The signal peptide is cleaved. 6. The completed polypeptide folds in the ER lumen.
D: 1. Polypeptide synthesis begins on a free ribosome in the cytosol. 2. An SRP binds to the signal peptide, halting synthesis. 3. The signal peptide is cleaved. 4. The SRP binds to a receptor protein in the ER membrane. 5. Synthesis resumes, with translocation across the membrane. 6. The completed polypeptide folds in the ER lumen.
Explanation
The described steps align perfectly with the 'signal mechanism for targeting proteins to the ER' presented in Figure 17.22 and its accompanying text. Polypeptide synthesis starts, SRP binds to the signal peptide and halts synthesis, SRP escorts the ribosome to the ER receptor, synthesis resumes with translocation, the signal peptide is cleaved, and finally the polypeptide folds.