The Crystal Structure of the Carbohydrate-recognition Domain of the Glycoprotein Sorting Receptor p58/ERGIC-53 Reveals an Unpredicted Metal-binding Site and Conformational Changes Associated with Calcium Ion Binding

p58/ERGIC-53 is a calcium-dependent animal lectin that acts as a cargo receptor, binding to a set of glycoproteins in the endoplasmic reticulum (ER) and transporting them to the Golgi complex. It is similar in structure to calcium-dependent leguminous lectins. We have determined the structure of the carbohydrate-recognition domain of p58/ERGIC-53 in its calcium-bound form. The structure reveals localized but large conformational changes in relation to the previously determined metal ion-free structure, mapping mostly to the ligand-binding site. It reveals the presence of two calcium ion-binding sites located 6 Å apart, one of which has no equivalent in the plant lectins. The second metal ion-binding site present in that class of lectins, binding Mn2+, is absent from p58/ERGIC-53. The absence of a short loop in the ligand-binding site in this protein suggests that it has adapted to optimally bind the high-mannose Man8(GlcNAc)2 glycan common to glycoproteins at the ER exit stage.