Question

Test yourself Many cell surface trans membrane proteins, called glycoproteins, have an attached carbohydrate group, 1. ich faces the extracellular space. This carbohydrate group is attached to the protein when it is still in the rER. Is the carbohydrate attached to the side of the protein that faces the lumen of the rERo the cytosol 2.Explain why the orientation of a transmembrane protein (i.e. which side of the protein faces the inside and which side faces the outside of the membrane) does not change when it moves from the rER to the Golgi apparatus to the plasma membrane. 3. Topology is a commonly misunderstood by students. What is meant by 'two cellular compartments bein topologically identical'?

Answer 1

1. Glycosylation or addition of carbohydrate moieties to the protein molecule, occurs at the luminal side of the Endoplasmic retriculum, after which the protein is folded correctly and transported to the cell membrane.

Cytoplasm RNA Ribosome Nascent Protein AMAAAAAnAAAAAnan Dolichol 2 x N-Acetylglucosamine Glucosidase 1 9 x Mannose Trimmed Oligosaccharide 3 x Glucose ER Lumen