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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.

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