GFP is fused to a signal peptide, and no other sequence, and transfected into a cell. Where is the exact fluorescence when the chimeric protein has reached its final destination, and the cell is examined by fluorescence microscopy?
a) in the lysosome lumen
b) in the ER lumen
c) on the plasma membrane on the cytosolic side
d) on the plasma membrane on the extracellular side
e) in the extracellular space/ ECM
2. Where is the exact fluorescence when a stop transfer sequence is added at the most carboxy terminus to the protein above (Question 1)
Update:In textbooks, the signal sequence does not remain in the protein after it is synthesized, it is cleaved by signal peptidase right? So how can it travel to the extracellular space, if it is not in the protein, wouldn't it remain in the ER lumen?
Answers & Comments
Verified answer
1. e) in the extracellular space/ ECM The signal sequence causes the protein to be synthesized on the rough ER and to enter the default constitutive secretory pathway. Thus, it will be secreted from the cell.
2. d) on the plasma membrane on the extracellular side. The stop transfer signal causes the protein to be inserted into the membrane at that point. Since most of the protein has been passed into the ER lumen during synthesis, it will be exposed to the extracellular side of the plasma membrane.