Final answer:
A peripheral membrane protein synthesized in the ER lumen would end up on the outside of the plasma membrane, following the secretory pathway and maintaining orientation from ER to plasma membrane.
Step-by-step explanation:
If a peripheral membrane protein were synthesized in the lumen of the endoplasmic reticulum (ER), it would likely end up on the outside of the plasma membrane. This is because, during the process of protein synthesis and transport, a protein synthesized in the ER lumen is typically routed through the secretory pathway, moving from the ER to the Golgi apparatus, and then to the plasma membrane, maintaining the same orientation. As such, the side of the protein that faces the lumen in the ER will face the extracellular space once the protein is integrated into the plasma membrane.
Moreover, the orientation of transmembrane proteins, which can have multiple alpha-helical domains, is determined during their insertion into the membrane. The N-terminal end of a plasma membrane polypeptide is generally exposed to the outside of the cell. Therefore, when considering the synthesis and insertion of membrane proteins, it is crucial to understand the role of hydrophobic alpha-helical domains and how they interact with the lipid bilayer of the plasma membrane.