Final answer:
Regarding whether proteins end up inside or outside the plasma membrane when synthesized in the lumen of the ER, they are positioned on the outside of the plasma membrane. This is due to the process of the endomembrane system with receptor-mediated endocytosis playing a role in the transport and presentation of antigens.
Step-by-step explanation:
The process of determining whether a substance is endogenous or exogenous involves understanding how it interacts with membrane proteins. In the case of β-2 microglobulin, it is a component of the MHC class I molecules, which are involved in presenting endogenous antigens (those produced inside the cell) to the immune system. However, MHC class II molecules, which associate with the β-2 microglobulin-like invariant chain, are involved in presenting exogenous antigens (those produced outside the cell).
To answer the question regarding a peripheral membrane protein synthesized in the lumen of the ER: If it were synthesized inside the ER lumen, it would end up on the outside of the plasma membrane. This is because the ER is part of the endomembrane system, which sorts proteins based on their destination signals.
During the process of receptor-mediated endocytosis, substances bind to receptors and are brought inside the cell via vesicles, which then fuse with lysosomes or, in the case of MHC class II molecules, with vesicles containing antigens processed for presentation outside the cell.