Final answer:
Calnexin is a chaperone protein that assists in the proper folding of glycoproteins in the endoplasmic reticulum, binding to those with a single glucose residue as a sign of incorrect folding. This single glucose acts as a quality control mechanism to prevent premature export of unfolded proteins.
Step-by-step explanation:
Role of Calnexin
Calnexin is a chaperone protein located in the endoplasmic reticulum of cells. Its primary role is to ensure the proper folding of newly synthesized glycoproteins. Calnexin specifically binds to glycoproteins that have a single glucose residue left in their oligosaccharide chains, which is a signal that the protein is not yet correctly folded. The binding of calnexin to these glycoproteins prevents them from being exported prematurely and allows for proper folding and quality control.
Significance of a Single Glucose Residue
The presence of a single glucose molecule attached to a glycoprotein is a quality control check within the endoplasmic reticulum. The addition of glucose residues aids in the folding process by interacting with calnexin and its homologue, calreticulin. Upon achieving correct folding, these glucose residues are trimmed off, and the protein can then proceed to the Golgi apparatus for further processing and eventual secretion or membrane localization. If the protein does not fold properly, the single glucose residue is a signal for further cycles of binding with calnexin or calreticulin for additional attempts at folding, or it can mark the protein for degradation if proper folding is not achieved.