Calnexin, Calreticulin and Glycoprotein Folding Within the Endoplasmic Reticulum

Abstract

Proteins destined for secretion or for various locations along the secretory pathway are initially translocated wholly or in part into the endoplasmic reticulum (ER). Within this compartment the vast majority of proteins are glycosylated at Asn residues with a preassembled oligosaccharide of structure Glc3MangGlcNAc2. These Asn-linked glycoproteins subsequently fold and assemble with the aid of a diverse array of protein folding catalysts and molecular chaperones. The protein folding catalysts include the thiol oxidoreductases protein disulfide isomerase (PDI), ERp57, and ERp72, that catalyze the formation and reshuffling of disulfide bonds, as well as peptidyl prolyl isomerases that catalyze the interconversion between cis and trans forms of peptide bonds preceding proline residues. By contrast, molecular chaperones enhance the efficiency of protein folding by binding transiently to hydrophobic segments of folding intermediates thereby preventing the formation of irreversible aggregates. Molecular chaperones of the ER also participate in a process termed quality control in which their interactions cause non-native proteins to be retained in this organelle either until they are degraded or until a native conformation is attained. Multiple molecular chaperones reside within the ER, including Grp94, a member of the HSP90 family of heat shock proteins, and BiP, an HSP70 family member. In addition, two other molecular chaperones, calnexin and calreticulin, appear to have evolved specifically to participate in the biogenesis of Asnlinked glycoproteins. In this review, we discuss the molecular basis for the glycoprotein specificity of these unique chaperones, their structural and functional relationship, current models for their mechanism of action, and their interplay with other ER chaperones. Structure and Properties of Calnexin and CalreticulinCalnexin, calreticulin, and the testis-specific homolog calmegin constitute a family of abundant ER proteins. Calnexin is a type I transmembrane protein that is pres-

Calreticulin functions in vitro as a molecular chaperone for both glycosylated and non-glycosylated proteins

“…Although we have focused largely upon CRT‘s capacity to bind to polypeptide segments of unfolded proteins, it is clear that its lectin site plays an important role in its functions. This is exemplified by a host of in vivo studies demonstrating that the formation of immunoprecipitable complexes of CRT and most of its glycoprotein substrates can be prevented by blocking N‐linked glycosylation or by preventing the formation of monoglucosylated oligosaccharides (Helenius et al ., 1997; Leach and Williams, 1999). The in vitro experiments presented here also underscore the contribution of lectin–oligosaccharide interactions in CRT’s aggregation‐suppressing function.…”

“…It is widely believed that CRT functions as a molecular chaperone for Asn‐linked glycoproteins, since it associates predominantly with folding or assembly intermediates in vivo but not with fully folded glycoproteins. Furthermore, when the binding of CRT and CNX are simultaneously prevented by interfering with the formation of their oligosaccharide ligand, the efficiency of folding of most glycoproteins is diminished and the formation of misfolded aggregates is frequently observed (reviewed in Helenius et al ., 1997; Leach and Williams, 1999). However, these in vivo experiments are indirect and it remains an open question whether CRT functions as a true molecular chaperone, i.e.…”

“…The ‘dual‐binding model’ includes the main tenets of the lectin‐only model but proposes that CRT (and CNX) possesses a second binding site that is able to recognize polypeptide segments of unfolded glycoproteins, i.e. it functions as a classical chaperone (Ware et al ., 1995; Leach and Williams, 1999). This model requires that the dissociation of CRT–glycoprotein complexes occurs through the action of glucosidase II as well as a conformational change in the polypeptide binding site.…”