Which chaperone works alongside glucosidase and glycosyl-transferase to monitor protein folding in the ER?

The correct choice is indeed associated with the process of protein folding in the endoplasmic reticulum (ER). Calnexin is a calcium-dependent chaperone that plays a critical role in the quality control of glycoproteins as they are synthesized in the ER. It specifically binds to newly synthesized glycoproteins that have undergone initial glycosylation and helps to ensure that these proteins properly fold into their functional conformations.

Calnexin works in conjunction with glucosidase, which trims the glucose residues from the glycoprotein, and glycosyl-transferase, which adds glucose back to incorrectly folded proteins to re-engage the chaperones. This cycle is crucial because it ensures that only properly folded and assembled proteins are allowed to exit the ER. Misfolded proteins are retained and can be sent for degradation, preventing potentially harmful proteins from entering the secretory pathway.

While the other chaperones mentioned play important roles in protein folding and assembly, Calnexin's specific interaction with glycoproteins in partnership with the glycosylation enzymes highlights its unique role in the ER's quality control mechanism.