A. Does the carboxy (C) terminus of this protein face the cytoplasm or the extra

Question

A. Does the carboxy (C) terminus of this protein face the cytoplasm or the extracellular space? B. Does the gene for this protein encode a signal peptide that is cleaved off in the endoplasmic reticulum? (Yes/No) 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?

Explanation / Answer

A. The carboxy terminus of this seven transmembrane helices will be on the extracellular space. The C terminus will jut out from the membrane into the extracellular space.

B. yes, The signal peptide gets cleaved off once it enters the membrane of endoplasmic reticulum by a signal peptidase.

C. The protease would not be able to cleave the transmembrane region (hydrophobic region ). Hence the transmembrane region will remain intact. Reamining parts that are exposed to cytosol gets cleaved by the protease (depends on what protease it is and what level of specificity does this protease have for the cleavage site). Once you run a non reducing SDS PAGE, we can see minimum two bands (one corresponding to membrane region, other correpsonding to cleaved cytosolic fraction provided the protease cleaves only once at the juxtacrine side of the protein. If the protease is a non-specific protease, we may find one intact membrane part band on the gel along with several cleaved fragments in SDS PAGE.

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