title: Lectin-deficient calreticulin retains full functionality as a chaperone for class I histocompatibility molecules.
abstract: Calreticulin is a molecular chaperone of the endoplasmic reticulum that uses both a lectin site specific for Glc(1)Man(5-9)GlcNAc(2) oligosaccharides and a polypeptide binding site to interact with nascent glycoproteins. The latter mode of substrate recognition is controversial. To examine the relevance of polypeptide binding to protein folding in living cells, we prepared lectin-deficient mutants of calreticulin and examined their abilities to support the assembly and quality control of mouse class I histocompatibility molecules. In cells lacking calreticulin, class I molecules exhibit inefficient loading of peptide ligands, reduced cell surface expression and aberrantly rapid export from the endoplasmic reticulum. Remarkably, expression of calreticulin mutants that are completely devoid of lectin function fully complemented all of the class I biosynthetic defects. We conclude that calreticulin can use nonlectin-based modes of substrate interaction to effect its chaperone and quality control functions on class I molecules in living cells. Furthermore, pulse-chase coimmunoisolation experiments revealed that lectin-deficient calreticulin bound to a similar spectrum of client proteins as wild-type calreticulin and dissociated with similar kinetics, suggesting that lectin-independent interactions are commonplace in cells and that they seem to be regulated during client protein maturation.
authors.affiliation: Department of Biochemistry, University of Toronto, Toronto, Ontario M5S 1A8, Canada.
authors.forename: Breanna S
authors.initials: BS
authors.lastname: Ireland
authors.forename: Ulf
authors.initials: U
authors.lastname: Brockmeier
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Howe
authors.forename: Tim
authors.initials: T
authors.lastname: Elliott
authors.forename: David B
authors.initials: DB
authors.lastname: Williams
mesh_terms: D000818:Animals; D017951:Antigen Presentation; D037282:Calreticulin / Q000172:deficiency / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism; D005347:Fibroblasts / Q000378:metabolism; D006056:Golgi Apparatus / Q000378:metabolism; D015395:Histocompatibility Antigens Class I / Q000378:metabolism*; D042541:Intracellular Space / Q000378:metabolism; D007700:Kinetics; D051379:Mice; D018832:Molecular Chaperones / Q000378:metabolism*; D009154:Mutation / Q000235:genetics; D010047:Ovalbumin; D010455:Peptides / Q000276:immunology; D010641:Phenotype; D011485:Protein Binding; D021381:Protein Transport
chemical_list: D037282:Calreticulin; D015395:Histocompatibility Antigens Class I; D018832:Molecular Chaperones; D010455:Peptides; D010047:Ovalbumin