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= 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.

[[Category:Chaperone proteins]]
[[Category:Endoplasmic reticulum]]
[[Category:Glycoproteins]]

Calnexin - Revision history

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