Understanding the Maturation Process of Glycosylated Plasma Membrane Proteins

Integral membrane proteins that are glycosylated undergo multiple post-translational modifications as they transit from the endoplasmic reticulum (ER) to the Golgi apparatus and finally to the plasma membrane. These steps involve:

1. Initial Translation (in the ER):

   • The nascent polypeptide is synthesized and translocated into the ER.

   • At this point, N-linked glycosylation typically begins, where carbohydrate groups are added to specific asparagine residues.

2. Processing in the ER and Golgi:

   • The protein undergoes modification and trimming of the carbohydrate groups.

   • Some sugars may be trimmed (decreasing weight), while others are added or remodeled in a complex fashion (increasing weight).

3. Final Form at Plasma Membrane:

   • The mature glycosylated protein is typically heavier than the initial unmodified protein due to complex glycosylation.

Expected Molecular Weight Pattern

• The initial increase in molecular weight is due to the addition of glycan (sugar) chains in the ER and Golgi.

• A subsequent decrease may occur as some sugar moieties are trimmed or cleaved.

• However, the final mature form usually retains more complex glycans than the nascent form, though the molecular weight may decrease from the peak seen mid-way through maturation.

Correct Answer:

3. An initial increase followed by a decrease

Conclusion:

Glycosylated integral membrane proteins gain mass early in maturation due to glycosylation and may later lose some weight through glycan trimming. This dynamic modification explains the initial increase followed by a slight decrease in molecular weight during maturation.