Calnexin

Calnexin

Calnexin is a type of chaperone protein that plays a crucial role in the proper folding and quality control of newly synthesized glycoproteins within the endoplasmic reticulum (ER) of eukaryotic cells. It is a member of the calnexin/calreticulin family of molecular chaperones.

Structure

Calnexin is an integral membrane protein of the ER, characterized by a large luminal domain, a single transmembrane helix, and a short cytoplasmic tail. The luminal domain contains a lectin site that specifically binds to monoglucosylated N-linked oligosaccharides on nascent glycoproteins.

Function

Calnexin functions as part of the calnexin cycle, a quality control system that ensures only properly folded glycoproteins proceed through the secretory pathway. It binds to glycoproteins that have a single glucose residue on their N-linked oligosaccharides, a modification performed by the enzyme glucosidase II.

Calnexin Cycle

1. Binding: Calnexin binds to monoglucosylated glycoproteins, stabilizing them and preventing aggregation. 2. Folding: While bound to calnexin, glycoproteins undergo folding with the assistance of other chaperones and folding enzymes, such as protein disulfide isomerase (PDI). 3. Release: Once properly folded, the glucose residue is removed by glucosidase II, leading to the release of the glycoprotein from calnexin. 4. Quality Control: If the glycoprotein is not correctly folded, it is reglucosylated by the enzyme UDP-glucose:glycoprotein glucosyltransferase (UGGT), allowing it to rebind to calnexin for another attempt at folding.

Clinical Significance

Mutations or dysregulation of calnexin can lead to improper protein folding and are associated with various diseases, including certain congenital disorders of glycosylation and neurodegenerative diseases.

Research Applications

Calnexin is often used as a marker for the ER in cell biology research. Its role in protein folding makes it a target for studies on protein misfolding diseases and the development of therapeutic interventions.

See Also

• Calreticulin
• Endoplasmic reticulum stress
• Unfolded protein response

References

• Helenius, A., & Aebi, M. (2004). Roles of N-linked glycans in the endoplasmic reticulum. Annual Review of Biochemistry, 73, 1019-1049.
• Williams, D. B. (2006). Beyond lectins: the calnexin/calreticulin chaperone system of the endoplasmic reticulum. Journal of Cell Science, 119(4), 615-623.