【多选题】How are ER proteins modified by glycosylation?
A.
Many of the proteins that enter the ER lumen or ER membrane are converted to glycoproteins in the ER by the covalent attachment of short, branched oligosaccharide side chains composed of multiple sugars.
B.
This process of glycosylation is carried out by glycosylating enzymes present in the ER but not in the cytosol. Very few proteins in the cytosol are glycosylated, and those that are have only a single sugar attached to them.
C.
The oligosaccharides on proteins can serve various functions. They can protect a protein from degradation, hold it in the ER until it is properly folded, or help guide it to the appropriate organelle by serving as a transport signal for packaging the protein into appropriate transport vesicles.
D.
When displayed on the cell surface, oligosaccharides form part of the cell’s outer carbohydrate layer or glycocalyx and can function in the recognition of one cell by another.