Question

Which eukaryotic structure is normally the last one to make modifications to a polypeptide (like glycosylation of a protein), before it is secreted from the cell?

A. the trans face of the Golgi apparatus

B. the smooth endoplasmic reticulum

C. the cis face of the Golgi apparatus

D. the rough endoplasmic reticulum

E. the nuclear envelope

Answer 1

Protein synthesis or the translation process occurs in the cytosol of a cell or on the ribosomes in present on the rough endoplasmic reticulum. The nascent polypeptide chain that is synthesized is properly folded and modified (process is called post- translational modification, PTM) before being secreted out of a cell or sent to the target organelle within the cell.

When the polypeptide is still in the stage of synthesis, the initial amino acid sequence, that is unfolded, is stabilized by the molecular chaperone Hsp70 (heat shock protein 70) and then the complete polypeptide chain is properly folded by another chaperone of the same protein family known as Hsp60. The polypeptide chain is then transported to the specific cell organelle. If the polypeptide has extracellular function, it is targeted towards the endoplasmic reticulum, for folding (if not yet folded), and modifications. The polypeptide chain enters the endoplasmic reticulum, after a signal sequence of about 20 amino acids at the N-terminal of the polypeptide chain (which are hydrophobic in nature), is removed and the remaining chain enters the lumen of the endoplasmic reticulum. The cleaved polypeptide chain is now mature and requires modifications, for its complete functioning. Addition of carbohydrate moieties to the polypeptide chain called glycosylation occurs in the endoplasmic reticulum. It may be N-linked or O-linked.

N-linked glycosylation involves the nitrogen atom of the side chain of asparagine amino acid is the site of attachment of the carbohydrate, whereas O-linked glycosylation, the carbohydrate attaches to the oxygen atom of the side chain of the amino acid serine or threonine.

N- linked and O- linked carbohydrate moieties are further modified in the Golgi apparatus, by addition of carbohydrate molecule to the existing oligosaccharide. (the carbohydrate containing protein is called glycoprotein).

Glycosylation occurs in both rough and smooth endoplasmic reticulum.

Post translational modification of the secretory proteins (that are secreted out of the cell) does not occur in the nuclear envelope.

In the Golgi apparatus, the post translational modification such as glycosylation occurs in the lumen of the Golgi apparatus, in cis and trans, both cisternae. The face of Golgi apparatus towards the endoplasmic reticulum is cis while towards the plasma membrane is trans.

Therefore, the polypeptide chain undergoes modification in the trans face of the Golgi apparatus, before it is packaged in a vesicle. The vesicle fuses into the plasma membrane, to secrete the polypeptide outside of the cell.

So, the correct answer is option A.