calnexin

Calnexin

Degen and Williams first discovered calnexin in association with MHC class I molecules synthesized by lymphoma cell lines (Bergeron et al, 1994). Calnexin, also referred to as IP90, p88, and p90, is a 88 kDa chaperone protein found in the endoplastmic reticulum. It binds with partially folded immunoglobulin proteins and retains them in the E.R. until they are completely folded (Janeway et al, 1999). Newly synthesized MHC class I alpha chains bind to calnexin until two b microglobulins and possiby a peptide bind to the alpha chain. The heterodimer then dissociates from calnexin and continues on its pathway (Janeway et al, 1999). In addition, calnexin is believed to associate with partial complexes of T cell receptor and B cell membrane immunoglobulin, but not with receptor complexes that are complete.

Figure 1. MHC Class I do not leave the ER to go to the surface unless they bind proteins.
Courtesy of (Janeway et al, 1999).

Calnexin has a large ER luminal domain consisting of 461 amino acids, a transmembrane segment of 22 amino acids and a cytoplasmic tail (89 amino acids) (Monoclonal Anti-Calnexin Antibody, 2000).

The organization of calnexin is as follows: a signal sequence, a luminal domain with calreticulin-like segments including the repeated motifs IXDP(D/E)(A/D)XKPE (E/D)DWD(E/D) and GXWXXPXIXNPXY, a transmembrane domain, and a probable ER retention motif at the carboxyl terminus (Bergeron et al, 1994).

Figure 2. Graphical View of Calnexin Sequence. Courtesy of <<www.ncbi.nlm.nih.gov>>
Permission to Use Requested

Quality Control System- only properly assembled proteins can dissociate from calnexin and be exported from the ER. (Bergeron et al, 1994).

If nacent proteins cannot fold into their correct structures due to mutations or inability to reach the proper subunits, then they will be held in the endoplasmic reticulum and are degraded.

Calnexin binds transiently and selectively to proteins, anchoring them within the ER. This binding depends on the presence of a monoglucosylated oligosaccharide (Parham, 1996).

Calnexin's binding is mediated by a lectin site recognizing the early N-linked oligosaccharide processing intermediate, Glc1Man9GlcNAc2 (Vassilakos et al, 1998) .

Calnexin uses encounters with carbohydrates as guides to gain stronger interactions with proteins.

Figure 3. Interactions of a new human keukocyte antigen (HLA) class I heavy chain
with calnexin during its assmebly . Courtesy of (Parham, 1996)

Inhibitors of Calnexin Function:

Castanospermine, a glucose trimming inhibitor, prevents receptor monomers to bind to calnexin and calreticulin. This inhibitor also creates misfolded oligomeric proreceptors, thus decreasing their cell surface expressions by almost 30% (Bass et al, 1998).
Combined lipase deficiency (cld) may affect ER components that help with protein maturation. In cld/cld mice, the calnexin levels are severely diminished (Briquet-Laugier et al, 1999) and (Hammond et al, 1994).
Inhibited by drugs that prevent either the addition of the 14 saccharides from dolichol lipids to Asn or the trimming of glucose residues that is essential before terminal sugars can be added (Parham, 1996).
If degycosylation is inhibited by glucosidase inhibitors like deoxynojirimycin, the proteins stay achored in the ER and the proteins are then degraded (Gahmberg et al, 1996).
Treatment with tunicamycin blocks calnexin and calreticulin by preventing N-glycosylation systems (Gahmberg et al, 1996).

References

-Bass, J, G Chiu, Y Argon, DF Steiner 1998. Folding of Insulin Receptor Monomers is Facilitated by the Molecular Chaperones Calnexin and Calreticulin and Impaired by Rapid Dimerization. Journal of Cell Biology 141:637-46.
-Bergeron, JJ, MB Brenner, DY Thomas, DB Williams 1994. Calnexin: a membrane-bound chaperone of the endoplasmic reticulum. Trends in Biochemical Sciences 19:124-128.
-Briquet-Laugier, V, O Ben-Zeev, A White, MH Doolittle 1999. cld and lec23 are Disparate Mutations that Affect Maturation of Lipoprotein Lipase in the Endoplasmic Reticulum. Journal of Lipid Research 40:2044-2058.
-Gahmberg, CG, M Tolvanen. Why mammalian cell surface proteins are glycoproteins. Trends in Biochemical Sciences 21: 308-311.
-Hammond, C, A Helenius. Folding of VSV G protein: Sequential Interaction with BiP and Calnexin. Science 266: 456-8
-Janeway, CA, P Travers, M Walport, JD Capra. Immunobiology: The Immune System in Health and Disease. New York: Garland Publishers, 1999.
-Monoclonal (Mouse) Anti-Calnexin Antibody (IgG). <http://www.bioreagents.com/ma3-027h.html> Accessed
2000 Feb 27.
-Parham, P 1996. Functions for MHC Class I Carbohydrates Inside and Outside the Cell. Trends in Biochemical Sciences 21: 428-432.
-Vassilakos, A, M Michalak, MA Lehrman 1998. Oligosaccharide binding characteristics of the molecular chaperones calnexin and calreticulin. Biochemistry 37: 3480-90.

Back to my homepage

Davidson College Immunolgy Homepage

Davidson College Homepage

Questions, comments - contact: gimooney@davidson.edu