title: Localization of the lectin, ERp57 binding, and polypeptide binding sites of calnexin and calreticulin.
abstract: Calnexin and calreticulin are membrane-bound and soluble chaperones, respectively, of the endoplasmic reticulum (ER) which interact transiently with a broad spectrum of newly synthesized glycoproteins. In addition to sharing substantial sequence identity, both calnexin and calreticulin bind to monoglucosylated oligosaccharides of the form Glc(1)Man(5-9)GlcNAc(2), interact with the thiol oxidoreductase, ERp57, and are capable of acting as chaperones in vitro to suppress the aggregation of non-native proteins. To understand how these diverse functions are coordinated, we have localized the lectin, ERp57 binding, and polypeptide binding sites of calnexin and calreticulin. Recent structural studies suggest that both proteins consist of a globular domain and an extended arm domain comprised of two sequence motifs repeated in tandem. Our results indicate that the primary lectin site of calnexin and calreticulin resides within the globular domain, but the results also point to a much weaker secondary site within the arm domain which lacks specificity for monoglucosylated oligosaccharides. For both proteins, a site of interaction with ERp57 is centered on the arm domain, which retains approximately 50% of binding compared with full-length controls. This site is in addition to a Zn(2+)-dependent site located within the globular domain of both proteins. Finally, calnexin and calreticulin suppress the aggregation of unfolded proteins via a polypeptide binding site located within their globular domains but require the arm domain for full chaperone function. These findings are integrated into a model that describes the interaction of glycoprotein folding intermediates with calnexin and calreticulin.
authors.affiliation: Department of Biochemistry, University of Toronto, Toronto, Ontario M5S 1A8, Canada.
authors.forename: Michael R
authors.initials: MR
authors.lastname: Leach
authors.forename: Myrna F
authors.initials: MF
authors.lastname: Cohen-Doyle
authors.forename: David Y
authors.initials: DY
authors.lastname: Thomas
authors.forename: David B
authors.initials: DB
authors.lastname: Williams
mesh_terms: D001483:Base Sequence; D001665:Binding Sites; D002135:Calcium-Binding Proteins / Q000737:chemistry / Q000378:metabolism*; D037281:Calnexin; D037282:Calreticulin; D017931:DNA Primers; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D037102:Lectins / Q000378:metabolism*; D010455:Peptides / Q000378:metabolism*; D011487:Protein Conformation; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D012261:Ribonucleoproteins / Q000737:chemistry / Q000378:metabolism*; D015032:Zinc / Q000378:metabolism*
chemical_list: D002135:Calcium-Binding Proteins; D037282:Calreticulin; D017931:DNA Primers; D037102:Lectins; D010455:Peptides; D011993:Recombinant Fusion Proteins; D012261:Ribonucleoproteins; D037281:Calnexin; D015032:Zinc