The Use of Calnexin and Calreticulin by Cellular and Viral Glycoproteins
Institute for Research in Biomedicine, 6500 Bellinzona, Switzerland. Journal of Biological Chemistry
(Impact Factor: 4.57).
09/2005; 280(31):28265-71. DOI: 10.1074/jbc.M501020200
Calnexin and calreticulin are homologous lectin chaperones that assist maturation of cellular and viral glycoproteins in the mammalian endoplasmic reticulum. Calnexin and calreticulin share the same specificity for monoglucosylated protein-bound N-glycans but associate with a distinct set of newly synthesized polypeptides. We report here that most calnexin substrates do not associate with calreticulin even upon selective calnexin inactivation, while BiP associates more abundantly with nascent polypeptides under these conditions. Calreticulin associated more abundantly with orphan calnexin substrates only in infected cells and preferentially with polypeptides of viral origin, showing stronger dependence of model viral glycoproteins on endoplasmic reticulum lectins. This may explain why inactivation of the calnexin cycle affects viral replication and infectivity but not viability of mammalian cells.
Available from: Peter Højrup
- "However, they differ with respect to several features: Cnx is a transmembrane protein and has a short cytoplasmic C-terminus and ER luminal N and P domains , whereas Crt is a soluble ER luminal protein with a large capacity for Ca 2+ binding in the C-terminus. Despite their similar structures and overlapping properties they seem to complement each other in the folding of other proteins, in particular the folding of MHC I    . Moreover, they seem to be able to substitute for each other in many respects  . "
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ABSTRACT: Calreticulin (Crt) and calnexin (Cnx) are homologous endoplasmic reticulum (ER) chaperones involved in protein folding and quality control. Crt is a soluble ER luminal Mr 46 kDa protein and Cnx is a Mr 67 kDa ER membrane protein. During purification of Crt from human placenta a soluble form of Cnx (sCnx) was consistently identified in a separate ion exchange chromatography peak. The sCnx was further purified and characterised. This showed that the protein had been cleaved after residue 472 (between Gln and Met), thus liberating it from the transmembrane and cytoplasmic parts of Cnx. The extraction and initial purification steps were carried out in the presence of protease inhibitors, thus ruling out that the cleavage was an artefact of the isolation procedure. This indicates that sCnx may have a physiological chaperone function similar to that of Crt.
Available from: Thawornchai Limjindaporn
- "BiP is one of multiple chaperone systems important for ER quality control . Calnexin and calreticulin are lectin-like chaperones that interact with glycosylated proteins and are important for viral glycoprotein processing and maturation . Since DENV E is a glycoprotein, we examined its interaction with calnexin and calreticulin. "
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ABSTRACT: Dengue virus infection is an important mosquito-borne disease and a public health problem worldwide. A better understanding of interactions between human cellular host and dengue virus proteins will provide insight into dengue virus replication and cellular pathogenesis. The glycosylated envelope protein of dengue virus, DENV E, is processed in the endoplasmic reticulum of host cells and therefore reliant on host processing functions. The complement of host ER functions involved and nature of the interactions with DENV E has not been thoroughly investigated. By employing a yeast two-hybrid assay, we found that domain III of DENV E interacts with human immunoglobulin heavy chain binding protein (BiP). The relevance of this interaction was demonstrated by co-immunoprecipitation and co-localization of BiP and DENV E in dengue virus-infected cells. Using the same approach, association of DENV E with two other chaperones, calnexin and calreticulin was also observed. Knocking-down expression of BiP, calnexin, or calreticulin by siRNA significantly decreased the production of infectious dengue virions. These results indicate that the interaction of these three chaperones with DENV E plays an important role in virion production, likely facilitating proper folding and assembly of dengue proteins.
Available from: Paolo Paganetti
- "Rather, it substantially inhibits the glucosidase II-operated de-glucosylation of labeled glycoproteins , which is required for their efficient release from calnexin (Hebert et al., 1995). We anticipated that this treatment would substantially decrease secretion of those mutants that associate with calnexin during maturation (please note that soluble variants of BACE do not associate with calreticulin; Pieren et al., 2005 and unpublished data). Cells were pulsed with radioactivity as described above and were chased for 5 min to allow BACEs association with calnexin. "
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ABSTRACT: BACE is an aspartic protease involved in the production of a toxic peptide accumulating in the brain of Alzheimer's disease patients. After attainment of the native structure in the endoplasmic reticulum (ER), BACE is released into the secretory pathway. To better understand the mechanisms regulating protein biogenesis in the mammalian ER, we determined the fate of five variants of soluble BACE with 4, 3, 2, 1, or 0 N-linked glycans. The number of N-glycans displayed on BACE correlated directly with folding and secretion rates and with the yield of active BACE harvested from the cell culture media. Addition of a single N-glycan was sufficient to recruit the calnexin chaperone system and/or for oligosaccharide de-glucosylation by the ER-resident alpha-glucosidase II. Addition of 1-4 N-glycans progressively enhanced the dissociation rate from BiP and reduced the propensity of newly synthesized BACE to enter aberrant soluble and insoluble aggregates. Finally, inhibition of the proteasome increased the yield of active BACE. This shows that active protein normally targeted for destruction can be diverted for secretion, as if for BACE the quality control system would be acting too stringently in the ER lumen, thus causing loss of functional polypeptides.
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