ABSTRACT
The non-hydrolysing bacterial UDP-N-acetylglucosamine 2-epimerase (UDP-GlcNAc 2-epimerase) catalyses the conversion of UDP-GlcNAc into UDP-N-acetylmannosamine, an intermediate in the biosynthesis of several cell-surface polysaccharides. This enzyme is allosterically regulated by its substrate UDP-GlcNAc. The structure of the ternary complex between the Bacillus anthracis UDP-GlcNAc 2-epimerase, its substrate UDP-GlcNAc and the reaction intermediate UDP, showed direct interactions between UDP and its substrate, and between the complex and highly conserved enzyme residues, identifying the allosteric site of the enzyme. The binding of UDP-GlcNAc is associated with conformational changes in the active site of the enzyme. Kinetic data and mutagenesis of the highly conserved UDP-GlcNAc-interacting residues confirm their importance in the substrate binding and catalysis of the enzyme. This constitutes the first example to our knowledge, of an enzymatic allosteric activation by direct interaction between the substrate and the allosteric activator.
Subject(s)
Bacillus anthracis/enzymology , Allosteric Regulation , Bacillus anthracis/growth & development , Binding Sites , Carbohydrate Conformation , Carbohydrate Epimerases/chemistry , Carbohydrate Epimerases/metabolism , Catalysis , Crystallography, X-Ray , Hydrolysis , Kinetics , Models, Molecular , Protein Conformation , Structure-Activity Relationship , Substrate Specificity , Uridine Diphosphate N-Acetylglucosamine/chemistry , Uridine Diphosphate N-Acetylglucosamine/metabolismABSTRACT
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 6A apart, one of which has no equivalent in the plant lectins. The second metal ion-binding site present in that class of lectins, binding Mn(2+), 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 Man(8)(GlcNAc)(2) glycan common to glycoproteins at the ER exit stage.
Subject(s)
Calcium/metabolism , Carbohydrate Metabolism , Mannose-Binding Lectins/chemistry , Membrane Proteins/chemistry , Metals/metabolism , Binding Sites , Crystallography, X-Ray , Mannose-Binding Lectins/metabolism , Membrane Proteins/metabolism , Models, Molecular , Protein ConformationABSTRACT
p58/ERGIC-53 is a mammalian calcium-dependent lectin that serves as a glycoprotein-sorting receptor between the endoplasmic reticulum (ER) and the Golgi complex. It is a type I transmembrane protein with two lumenal domains, one of which is a carbohydrate-recognition domain (CRD) and homologous to leguminous lectins. The CRD of p58, the rat homologue of human ERGIC-53, was overexpressed in insect cells and Escherichia coli, purified and crystallized using Li(2)SO(4) as a precipitant. The crystals belong to space group I222, with unit-cell parameters a = 49.6, b = 86.1, c = 128.1 A, and contain one molecule per asymmetric unit, corresponding to a packing density of 2.4 A(3)Da(-1). Knowledge of the structure of p58/ERGIC-53 will provide a starting model for understanding receptor-mediated glycoprotein sorting between the ER and the Golgi.
Subject(s)
Endoplasmic Reticulum/metabolism , Glycoproteins/metabolism , Lectins/chemistry , Mannose-Binding Lectins , Membrane Proteins/chemistry , Animals , Carbohydrates/chemistry , Crystallization , Crystallography, X-Ray , Humans , Membrane Proteins/biosynthesis , Protein Conformation , Protein Folding , Protein Structure, Tertiary , RatsABSTRACT
p58/ERGIC-53 is an animal calcium-dependent lectin that cycles between the endoplasmic reticulum (ER) and the Golgi complex and appears to act as a cargo receptor for a subset of soluble glycoproteins exported from the ER. We have determined the crystal structure of the carbohydrate recognition domain (CRD) of p58, the rat homologue of human ERGIC-53, to 1.46 A resolution. The fold and ligand binding site are most similar to those of leguminous lectins. The structure also resembles that of the CRD of the ER folding chaperone calnexin and the neurexins, a family of non-lectin proteins expressed on neurons. The CRD comprises one concave and one convex beta-sheet packed into a beta-sandwich. The ligand binding site resides in a negatively charged cleft formed by conserved residues. A large surface patch of conserved residues with a putative role in protein-protein interactions and oligomerization lies on the opposite side of the ligand binding site. Together with previous functional data, the structure defines a new and expanding class of calcium-dependent animal lectins and provides a starting point for the understanding of glycoprotein sorting between the ER and the Golgi.
