The DNA in a cell's nucleus encodes proteins that are eventually targeted to every membrane and compartment in the cell, as well
as proteins that are targeted for secretion from the cell. For example, consider these two proteins:
1. Phosphofructokinase (PFK) is an enzyme that functions in the cytoplasm during
glycolysis.
2. Insulin, a protein that regulates blood sugar levels, is secreted from specialized
pancreatic cells.
Assume that you can track the cellular locations of these two proteins from the time that translation is complete until the proteins reach their final destinations.
For each protein, identify its targeting pathway: the sequence of cellular locations in which the protein is found from when translation is complete until it reaches its final (functional) destination. (Note that if an organelle is listed in a pathway, the location implied is inside the organelle, not in the membrane that surrounds the organelle.)
a. Cytoplasm only, ER --> cytoplasm, ER --> Golgi --> outside cell, cytoplasm --> ER --> outside cell, Golgi --> ER --> outside cell, cytoplasm --> Golgi --> outside cell, nucleus --> cytoplasm, ER --> Golgi --> cytoplasm
b. Protein Targeting Pathway
c. PFK _______________
d. Insulin _______________
Translation can be described as the mechanism by which a protein is produced through a mRNA.
If a particular protein is to function inside the same cell, then it doesn't need to get any modification so that it can travel outside the cell. Such type of proteins are translated on free cytoplasmic ribosomes and are released directly into the cytoplasm to perform their function. For example, PFK protein.
If a protein has to function outside the cell where it is produced, then it has to be modified. Such type or proteins are translated on the rough ER. They are then modified in the Golgi complex and then transported outside the cell to the area where they have to function. For example, insulin.
