If Protein X has its N-terminus facing the lumen of the ER, where will this part face after exocytosis?

When Protein X has its N-terminus facing the lumen of the endoplasmic reticulum (ER), it is positioned such that the part of the protein that is attached to the lumen will eventually face the outside of the cell after the process of exocytosis.

During exocytosis, vesicles containing proteins and other materials bud off from the ER and move to the Golgi apparatus for further processing. From the Golgi, these vesicles then transport the proteins to the cell membrane. When the vesicle fuses with the plasma membrane during exocytosis, the lumen of the vesicle merges with the extracellular space. Therefore, the N-terminus, which was initially in the ER lumen, is then exposed to the extracellular environment.

This mechanism is crucial for proteins that need to be secreted outside the cell or that perform their functions in the extracellular space, such as hormones and enzymes. Hence, the N-terminus of Protein X would face the extracellular space following exocytosis, confirming that the correct answer is indeed the one indicating that the N-terminus would be positioned in the extracellular space.