Question

The ER signal sequence on a growing polypeptide chain is recognized by a signal-recognition particle (SRP) in the cytosol. This interaction:

cleaves the ER signal sequence from the polypeptide chain.
guides the ribosome and its polypeptide to the ER membrane.
releases the polypeptide chain from the ribosome.
speeds the synthesis of the polypeptide chain.

Answer 1

The ER signal sequence on a growing polypeptide chain is recognized by an SRP, which guides the ribosome to the ER membrane. The SRP then detaches, allowing for elongation and the signal peptide's removal, ultimately leading the protein to its correct cellular destination.

Step-by-step explanation:

The interaction between the ER signal sequence on a growing polypeptide chain and a signal-recognition particle (SRP) in the cytosol guides the ribosome and its polypeptide to the ER membrane. This process involves several key steps:

1. An SRP binds to the hydrophobic signal peptide.
2. Protein synthesis (elongation) stops until the SRP-ribosome complex locates the rough endoplasmic reticulum (RER) membrane.
3. The complex binds to an SRP receptor on the RER membrane, facilitating the insertion of the nascent polypeptide.
4. The SRP detaches, allowing translation elongation to resume via a translocation channel.
5. A signal peptidase in the RER membrane catalyzes the removal of the signal peptide from the growing polypeptide chain.

Throughout this process, the elongating polypeptide chain begins folding within the RER. This sequence of events corresponds with the Signal Hypothesis, which suggests that a secretory protein bears an amino-terminal signal peptide that directs it to the RER, to be cleaved off before the mature protein reaches its final cellular destination.