Translocation of protein from the ribosome into the lipid bilayer of a membrane occurs most frequently at the level of which of the following? A) cis Golgi lamellae OB) Nucleolus OC) Plasma membrane D) Rough endoplasmic reticulum E) Secretory vesicles
The correct answer and explanation is:
The correct answer is: D) Rough endoplasmic reticulum
Explanation (300 words):
Protein translocation into a lipid bilayer occurs most frequently at the rough endoplasmic reticulum (RER). The RER is characterized by the presence of ribosomes attached to its cytoplasmic surface, which gives it a “rough” appearance under a microscope. This organelle plays a central role in the synthesis of membrane-bound and secretory proteins.
During translation, ribosomes synthesizing proteins that are destined for secretion, insertion into membranes, or residence in organelles of the endomembrane system are directed to the RER. This targeting is mediated by a signal sequence on the nascent polypeptide chain. As this signal emerges from the ribosome, it is recognized by the signal recognition particle (SRP), which halts translation temporarily and directs the ribosome to the SRP receptor on the RER membrane.
Once docked, the ribosome binds to a translocon (a protein-conducting channel) embedded in the RER membrane. Translation resumes, and the growing polypeptide chain is threaded into the lumen of the RER or inserted into the membrane itself. If the protein is membrane-bound, hydrophobic transmembrane domains are integrated directly into the lipid bilayer as the polypeptide elongates.
This cotranslational translocation is efficient and ensures that proteins destined for the membrane or secretion are properly processed. Inside the RER, proteins often undergo post-translational modifications such as glycosylation and folding with the assistance of chaperones.
In contrast:
- The cis Golgi lamellae (A) further modifies proteins but does not initiate their membrane insertion.
- The nucleolus (B) is involved in ribosome assembly, not protein translocation.
- The plasma membrane (C) receives proteins via vesicular trafficking but is not the primary site of insertion during translation.
- Secretory vesicles (E) transport proteins but do not participate in their synthesis or membrane integration.
Thus, the rough endoplasmic reticulum is the primary site of protein translocation into membranes.