Question

2. (3pts) You are studying a single-pass transmembrane protein which is destined to the plasma membrane. The protein has a lysine amino acid which flanks the N-terminal side of the transmembrane domain and a glutamic acid amino acid which flanks the C-terminal side of the transmembrane domain. Both the N-terminus and the Cterminus of the protein carries amino acid sequences that can by glycosylated.

A) (0.5pt) When the protein reaches the plasma membrane, will the N-terminus or the C-terminus point to the extracellular side?

B) (0.5pt) When the protein reaches the plasma membrane, will the N-terminus or the C-terminus be glycosylated?

C) (0.5pt) In a mutant protein, the flanking “K” and “E” amino acids are switched with each other. After the protein is transported to the plasma membrane, will the Nterminus or the C-terminus point to the cytosolic side?

D) (0.5pt) When the protein is glycosylated in the ER, will the glycosylation occur in the lumen side or cytosolic side of the protein?

E) (1pt) How does glycosylation contribute to proper protein folding and quality control in the ER? (5 sentence max)

Answer 1

Ans- A) C-terminus will be on extracellular side because this follows positive-inside rule i.e the positively charged amino acid will be in cytosol hence, N-terminus will be in cytosol and C-terminus will be out.

B) The C-terminus of the protein will be glycosylated.

C) After switching lysine and glutamic acid, the C-terminus will be inside and N-terminus will be out following the positive-inside rule.

D) In ER, the glycosylation will occur on the polypeptide terminus present in ER lumen i.e N-terminus. This involves addition of oligosaccharide dolichol at arginine residue.

E) The quality control and protein folding in ER by glycosylation is regulated by two chaperones, calnexin and calreticulin. These bind to N-linked oligosaccharide with one terminal glucose. When the protein is completely folded, this terminal glucose is removed, causing chaperones to release protein which can then leave ER. Hence, glycosylation helps chaperons to keep unfolded protein in ER.