Question

Which of the following is not an important ER-resident enzyme that modifies proteins in the secretory pathway?

A. BiP

B. Oligosaccharyl transferase

C. Sec61

D. Protein disulfide isomerase

Answer 1

Answer. option C. Sec61. These transmembrane proteins are translocation channels through the ER membrane. As translation continues, the growing polypeptide chain is directed into the Sec61 channel into the ER. No protein modification occurs within this channel.

A. BiP : It is a molecular chaperone of the Hsp70 family that resides within the ER. It is required to pull the polypeptide chain through the Sec61 transmembrane channel into the ER and is thought to bind to the nascent polypeptide chain to drive its posttranslational translocation into the ER. BiP also binds the unfolded polypeptide chain as it crosses the ER membrane and intiates the folding process which is required to attain the native structure and also allow assembly of multisubunit proteins. Correctly folded and assembled proteins are released from BiP for transfer to the Golgi apparatus while misfolded proteins do not dissociate from BiP such that they are not able to leave the ER and subsequently degraded. Folding can be considered as a protein structure modification.

B. Oligosaccharyl transferase: Proteins are glycosylated on their Asn residues as they are translocated to the ER. An oligosaccharide of 14 sugar residues is built on a lipid carrier (dolichol) located in the ER membrane. Oligosaccharyl transferase, a membrane bound enzyme, then transfers this oligosaccharide unit to the asparagine residue situated within a consensus motif (Asn-X-Ser/Thr) in the growing polypeptide chain.

D. Protein disulfide isomerase: This enzyme is located in the ER lumen and promotes disulfide (S—S) bond formation between the cysteine side chains. Since these bonds cannot form in the cytosol due to its reducing environment, they are made in the oxidizing environment of the lumen.