The sodium-potassium pump in animal cells requires cytoplasmic ATP to pump ions across the plasma membrane. When the proteins of the pump are first synthesized in the rough ER, what side of the ER membrane will the ATP binding site be on?
A) It will be on the cytoplasmic side of the ER.
B) It will be on the side facing the interior of the ER.
C) It could be facing in either direction because proteins are properly reoriented in the Golgi apparatus.
D) It doesn't matter, because the pump is not active in the ER.

Question

Quizplus · Accepted Answer

The ATP binding site of the sodium-potassium pump will be on the cytoplasmic side of the ER membrane because proteins are synthesized in a way that orients their active sites and functional domains correctly with respect to their eventual locations in the cell. Since the pump requires cytoplasmic ATP to function, its ATP binding site must face the cytoplasm when it is embedded in any membrane, including during its initial synthesis in the rough ER.

The Sodium-Potassium Pump in Animal Cells Requires Cytoplasmic ATP to Pump