Final answer:
Phospholipid scramblase PLSCR1's proline-rich termini are situated depending on membrane synthesis orientation; modifications such as pegylation can affect protein stability. Location of termini affects whether modifications increase plasma stability or rate of degradation. Peripheral proteins' final position depends on their secretory pathway.
Step-by-step explanation:
The phospholipid scramblase PLSCR1 is a transmembrane protein that can exhibit varying location of its proline-rich termini depending on how it inserts into the membrane during synthesis. The orientation of transmembrane proteins such as PLSCR1 dictates whether the N-terminal or C-terminal end will be exposed to the extracellular environment or the cytoplasmic side. Transmembrane proteins like glycophorin A, which has a hydrophobic trans-membrane alpha helix, typically expose their N-terminal end to the outside of the cell. In contrast, peripheral proteins synthesized in the lumen of the ER, such as phospholipid scramblase, will end up on the inside or outside of the plasma membrane depending on their pathway through the secretory system.
Furthermore, protein stability and function can be affected by modifications at either the N- or C-terminal ends, such as pegylation or acetylation, which can enhance plasma stability and reduce immunogenicity. Similarly, the N-end rule suggests that the identity of the amino acid at the N-terminus of a protein can influence its degradation rate. Modifications at protein termini can also be used to search for the minimum active sequence (MAS) by producing N- and/or C-terminal truncated analogues.