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First published online February 8, 2006
doi: 10.1242/10.1242/jcs.02856

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Beyond lectins: the calnexin/calreticulin chaperone system of the endoplasmic reticulum

Departments of Biochemistry and Immunology, University of Toronto, Toronto, Ontario, Canada, M5S 1A8

View larger version (20K): [in a new window]   Fig. 1. Composition of the Glc3Man9GlcNAc2 oligosaccharide initially transferred to nascent glycoproteins. The outer box surrounds the monoglucosylated Glc1Man9GlcNAc2 oligosaccharide recognized by calnexin and calreticulin. It is formed by the initial action of -glucosidase I followed by cleavage of a second glucose by -glucosidase II. The lectin sites of Cnx and Crt bind to the terminal glucose residue of the Glc1Man9GlcNAc2 oligosaccharide as well as to three underlying mannose residues (dashed box). Also indicated is the terminal mannose residue cleaved by -mannosidase I to create a signal associated with rapid degradation of misfolded glycoproteins. Glc, glucose; Man, mannose; GlcNAc, N-acetylglucosamine.

View larger version (23K): [in a new window]   Fig. 2. (A) Crystal structure of the ER luminal domain of calnexin. The globular domain contains the oligosaccharide-binding site with amino acids that contact the terminal glucose residue shown in red. A bound Ca2+ is indicated by the black sphere, residues coordinating the ion depicted in cyan. Disulfide bonds are shown in yellow. The extended arm domain consists of two strands, one containing four repeats of motif 1 [I-DP(D/E)A-KPEDWD(D/E)] and the other containing four copies of motif 2 [G-W-P-IN-P-Y]. Each motif-1-repeat is paired with a motif-2-repeat on the opposite strand. The four motif pairs are shown. (B) Model for the interaction of a folding glycoprotein with calnexin or calreticulin. Calnexin (green) is shown associated with a hypothetical model of ERp57 (blue) drawn on the basis of the NMR structure of the PDI `a' domain (Kemmink et al., 1996). The four domains of ERp57 are indicated: a, b, b', a'. A folding glycoprotein (thin blue line) may enter the cavity between the arm and globular domains interacting both with the lectin site as well as a polypeptide-binding site. This may sequester it from other folding glycoproteins thereby minimizing aggregation. Aggregation is further suppressed by the ability of the polypeptide-binding site to shield exposed hydrophobic segments. The two CGHC active sites of ERp57 (red) are well-placed to catalyze disulfide-bond formation, reduction or isomerization.

View larger version (26K): [in a new window]   Fig. 3. Proposed mechanisms of action for calnexin and calreticulin. As a nascent polypeptide enters the ER lumen via the translocon pore, Asn residues within Asn-X-Ser(Thr) sequences may be recognized by oligosaccharyl transferase (OST) and glycosylated with the preassembled Glc3Man9GlcNAc2 oligosaccharide. The outer two glucoses are then rapidly removed by glucosidases I and II to generate the Glc1Man9GlcNAc2 oligosaccharide recognized by Cnx and Crt. In the lectin-only model (red arrows), cycles of glycoprotein release and re-binding are controlled solely by the removal and re-addition of the terminal glucose residue by glucosidase II and UDP-glucose:glycoprotein glucosyltransferase (UGGT), respectively. UGGT is the folding sensor because it only reglucosylates non-native glycoprotein conformers. Chaperone binding serves to retain non-native glycoproteins within the ER and also recruits ERp57 to promote disulfide-bond formation and isomerization. It is unclear whether binding to glycoproteins only through the lectin site is sufficient to suppress aggregation. In the dual-binding model (green arrows), Cnx and Crt recognize non-native glycoproteins through their lectin sites as well as through a polypeptide-binding site specific for non-native conformers. This allows them to prevent off-pathway aggregation reactions similarly to other molecular chaperones. Binding via the polypeptide binding site is influenced by ATP and by the free Ca2+ concentration, either of which may regulate the interaction. In both models, folding takes place upon release from the chaperone, followed by further oligosaccharide trimming and export to the Golgi apparatus. For misfolded glycoproteins that remain for prolonged periods in the Cnx/Crt cycle, trimming by -mannosidase I generates a Man8GlcNAc2 structure that may be recognized by a putative lectin termed EDEM as part of a signal leading to retrotranslocation and proteasomal degradation [ER-associated degradation (ERAD)]. Ub, ubiquitin.