Question

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.)

Options:
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

Protein Targeting Pathway

PFK _______________
Insulin ____________

Answer 1

PFK (protein): cytoplasm only

Insulin (protein): ER->Golgi->outside cell

Step-by-step explanation:

The protein Phophofructokinase (PFK) has to function inside the cytoplasm of the cell where it is made. Hence, it does not have to go through the modification process needed for transport. Such kind of proteins which are made and function in the same cell are translated on free cytoplasmic ribosomes.

The proteins which have to be transported to other areas like insulin are translated in the rough endoplasmic reticulum. From there, they travel to the Golgi complex where they are modified and packaged so that they can travel outside the cell.