Question

The fourth stage of cotranslational import is when:_______

Answer 1

The fourth stage of cotranslational import is the release and recycling of SRP from the polypeptide, allowing translation to resume and co-translational modifications to occur.

Step-by-step explanation:

The fourth stage of cotranslational import involves the detachment of the signal recognition particle (SRP) from the growing polypeptide chain, allowing the translation process to continue unhindered. Once the SRP is released, it can be recycled for future use.

This step is critical because it allows the elongation of the protein to proceed within the protected environment of the rough endoplasmic reticulum (RER), where the signal peptidase eventually cleaves the signal peptide, allowing co-translational modifications of the newly synthesized protein.

Answer 2

The fourth stage of cotranslational import occurs when the nascent protein chain is translocated across or integrated into the membrane of the endoplasmic reticulum (ER).

Step-by-step explanation:

Cotranslational import is a process crucial in protein synthesis where the nascent polypeptide chain is simultaneously translated and directed to specific cellular compartments, such as the endoplasmic reticulum (ER). The fourth stage of cotranslational import involves the translocation or integration of the nascent protein chain into the ER membrane. As the ribosome synthesizes the protein, the signal peptide, a specific sequence within the growing polypeptide, guides the ribosome to the ER membrane. Here, the signal recognition particle (SRP) binds to the signal peptide, pausing translation and facilitating the docking of the ribosome-SRP complex to the ER membrane, initiating the translocation process.

At this stage, the nascent protein chain undergoes translocation across the ER membrane through a protein channel called the translocon. This translocation process involves the opening of the translocon channel and the movement of the polypeptide chain into the ER lumen or integration into the membrane, depending on the protein's final destination. The translocation machinery within the ER membrane helps ensure the correct localization and proper folding of the nascent protein, vital for its functionality within the cell.

The translocation into the ER during cotranslational import ensures that certain proteins are appropriately processed, modified, and directed to their designated cellular locations, contributing to cellular function and structure. This stage of cotranslational import plays a pivotal role in the quality control and sorting of proteins, ensuring their correct localization and functionality within the cell.