Final answer:
During oligosaccharide modification in the ER, after two glucose residues are removed, the remaining glucose binds to an ER chaperone for proper folding of the glycoprotein. This chaperone protein is called calnexin.
Step-by-step explanation:
During the modification of the core oligosaccharide in the endoplasmic reticulum (ER), two of the three terminal glucose residues are removed. The sequence of events that involves glycosylation begins within the rough endoplasmic reticulum (RER) where a core glycoside is created and partially attached to proteins.
These glycoproteins are important for various cellular functions, including signal recognition on the cell surface. The modification process continues as proteins make their way through the Golgi apparatus, where terminal glycosylation adds more sugar residues to the core glycoside, completing the synthesis of glycoproteins.
Vesicles budding from the trans-Golgi fuse with the plasma membrane, following which the sugars on the glycoproteins are presented on the exterior cell surface. However, before reaching this stage, the glycoproteins require proper folding—a process aided by ER chaperone proteins.
The remaining glucose residue from the core oligosaccharide binds to a specific ER chaperone protein to help the glycoprotein fold properly. This ER chaperone protein is known as calnexin (option 1). Calreticulin is a similar chaperone that can be found within the ER and also aids in the quality control of glycoprotein folding.