6. A single-pass transmembrane protein is destined to be transported to the plas

Question

6. A single-pass transmembrane protein is destined to be transported to the plasma membrane. In the wild-type protein, a lysine flanks the transmembrane domain on the N- terminal side, and a glutamic acid flanks on the C-terminal side. Both the N- & C-terminal soluble domains carry sequences that can by glycosylated by oligosaccharyltransferase. A. (1pt) After the protein is transported to the plasma membrane, will the N- or the C- terminal soluble domain be on the extracellular side? B.(1p) Is the glycosylation adddthe ERonto the N or the C-terminal soluble domain?

Explanation / Answer

Answer 6.A) Since, overall negative charge dominates on the inner leaflet towards the cytoplasm and positive charge on the outer leaflet on the extracellular side, so N-terminal with lysine will be towards the extracellular side and C-terminal will be towards the cytoplasm.

Answer 6.B) Since, the N-terminal side is in the lumen of the ER where the glycosylation is added, hence after reaching at the Plasma membrane the N-terminal side will have the glycosylation towards extracellular side.

Answer 6.C) if in the mutant protein the flanking amino acid lysine (N-terminal) and glutamic acid (C-terminal) are reversed then, then the positive charge portion i.e. C-terminal with lysine will be towards extracellular side.

Answer 6.D) Disagree, the glycosylation will be towards the extracellular side. This is because the glycosylation occur in ER lumen which mimic the extracellular side and since the c-terminal is positive charged so the C-terminal will have glycosylation and when transported to plasma membrane will face extracellular side.

Answer 6.E) when protein is synthesized in the ER lumen and glycosylated, then the molecular chaperons of ER recognize the specific glycosylation and promote proper protein folding. They also prevent premature exit from the ER and prevent aggregation, thus helping in proper folding. Example includes calnexin and calreticulin. If a protein is misfolded it is recognized by ER-associated degradation pathway (ERAD) which then send the misfolded protein to cytoplasm with the help of retrotranslocation, where it is degraded by ubiquitin–proteasome machinery thus ensuring quality control in ER.

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