Final answer:
ER resident proteins are retained in the ER lumen through a mechanism that involves retention signals recognized by the ER. Peripheral membrane proteins synthesized in the ER lumen are usually transported to the outside of the plasma membrane through vesicular transport after processing in the Golgi apparatus.
Step-by-step explanation:
How Do ER Resident Proteins Stay in the ER Lumen?
Endoplasmic reticulum (ER) resident proteins stay in the ER lumen despite the continuous flow of materials through the system. This is achieved through a retention mechanism involving specific signals or retention tags that these proteins contain, which are recognized by receptors in the ER. Consequently, despite the ongoing movement and transport of substances, these resident proteins are continually retrieved back to the ER if they happen to be transported out. For example, the KDEL sequence is a well-known ER retention signal for soluble proteins.
Peripheral Membrane Proteins and the Plasma Membrane
If a peripheral membrane protein were synthesized inside the luminal space of the RER, its eventual location in relation to the plasma membrane would be determined by the vesicular transport systems within the cell. After synthesis, the protein would move from the ER to the Golgi apparatus, where it is further modified. It would then be packaged into vesicles that will fuse with the plasma membrane, releasing the protein to the cell's exterior unless it contains signals that target it to other specific locations within the cell. Thus, typically, a peripheral protein synthesized in the ER lumen would end up on the outside of the plasma membrane.