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

Question

7 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 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 oligosaccharyltransterase 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. (1pt) Is the glycosylation added in the ER onto the N-or the C-terminal soluble domain?

Explanation / Answer

A. Lysine carries positive charge and is present on the N-terminal, conventionally the rule is positive charge is present inside the cytosol i.e. the N-terminal will be on the cytoplasmic side and C-terminal will be on the side of extracellular domain.

B. In ER glycosylation is added to N-terminal formin N-linked glycosylation

C. Now, it will be reversed, C-terminal bearing the positively charged lysine will be inside on the cytoplasmic side where the N terminal with glutamin acid will be on extracellular domain.

D. Disagree. ER only adds oligosaccharides to the N-terminal, now that the N-terminal is on the extracellular domain, the N linked glycosylation will now be on the exraacellular domain.

E. Two chaperons present in the ER calnexin and calreticulin bind only the oligosaccharides linked to the native proteins. In this way they help in retaining the proteins in the ER where other chaperons like BiP help it to hold properly.

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