40. The Na⁺/K⁺ pump is a critical electrogenic transport protein that uses ATP to actively move ions against their concentration gradients, contributing to the membrane potential of animal cells. This integral protein is synthesized in the Endoplasmic Reticulum (ER) and modified through the Golgi apparatus before insertion into the plasma membrane. The completed pump moves 3 Nat ions out of the cell and 2 K+ ions into the cell. Considering the required sidedness of the plasma membrane, which component of the Na⁺/K⁺ pump is exposed to the ER lumen during the initial stages of synthesis?
(A) The segment of the protein that releases the terminal phosphate group derived from ATР.
(B) The region of the protein that initially binds the three Nat ions.
(C) The binding domain that triggers the release of the two K⁺ ion into the cytoplasm.
(D) The nonpolar a-helical stretches that span the lipid bilayer.
(E) The binding site that has a high affinity for K⁺ ions when the pump has been phosphorylated.