6. You are studying a protein which is normally located in the plasma membrane i
ID: 216675 • Letter: 6
Question
6. You are studying a protein which is normally located in the plasma membrane in a mammalian cell line, with its amino (N) terminus facing the extracellular space. The protein spans the membrane four times. Answer the questions below. A. Does the carboxy (C) terminus of this protein face the cytoplasm or the extracellular space? (3 points) ____________________________________________________ B. Does the gene for this protein encode a signal peptide that is cleaved off in the endoplasmic reticulum? (Yes/No) (3 points) ______________________ C. You isolate a cellular fraction of the vesicles that carry this protein right before its delivery to the plasma membrane, then treat these vesicles with a protease that will digest all exposed parts of the protein but not the ones in the membrane or inside the vesicles. You extract the remaining parts of the protein and run them on a gel. How many bands will you see? (3 points)
Explanation / Answer
1.Since the protein spans the membrane four times, Carboxyl terminal of the protein faces at extra-cellular side.
2.Yes,Signal peptides are usually cleaved by signal peptidase co-translationally in the ER.
3. Since the protein spans the membrane four times, only two polypeptide chain is obtained from the resulting digestion. Hence two bands can be observed.
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