Characterization of ligand binding to a carbohydrate-recognition domain of the macrophage mannose receptor.
The extracellular portion of the macrophage mannose receptor, an endocytic receptor involved in clearance of glycoconjugates, contains eight domains related to the Ca(2+)-dependent carbohydrate-recognition domains (CRDs) of other C-type animal lectins. The characteristics of ligand binding to an expressed form of one of these CRDs (CRD-4) have been investigated. The expressed domain was found to be a monomer in solution. Results of a solid phase binding assay and a protease resistance assay show that CRD-4 of the mannose receptor undergoes a conformational rearrangement upon binding of Ca2+, correlating with its ability to bind sugar. CRD-4 requires two Ca2+ for sugar binding, even though sequence comparisons with other C-type CRDs suggested that it might bind only one Ca2+. The results are consistent with a ternary complex being formed between CRD-4, sugar, and Ca2+ as is seen in the crystal structure of the CRD of rat mannose-binding protein in complex with an oligosaccharide. The stability of Ca2+ binding is shown to be pH-dependent, a result that is pertinent to release of ligand by the receptor in the endosome. However, CRD-4 retains sugar binding activity at a lower pH than does the whole receptor, suggesting that the conformational change in this CRD alone may not be sufficient to allow release of ligand in the endosomes.
Mullin
N P
NP
Department of Biochemistry, University of Oxford, United Kingdom.
Hall
K T
KT
Taylor
M E
ME
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism; D002122:Calcium Chloride / Q000494:pharmacology; D050260:Carbohydrate Metabolism; D002352:Carrier Proteins / Q000737:chemistry; D002846:Chromatography, Affinity; D003001:Cloning, Molecular; D017124:Conserved Sequence; D018360:Crystallography, X-Ray; D004926:Escherichia coli; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D037102:Lectins; D037181:Lectins, C-Type; D008024:Ligands; D008264:Macrophages / Q000378:metabolism*; D008358:Mannose / Q000378:metabolism; D000090323:Mannose Receptor; D037241:Mannose-Binding Lectins; D008958:Models, Molecular; D008969:Molecular Sequence Data; D008970:Molecular Weight; D017433:Protein Structure, Secondary; D051381:Rats; D011956:Receptors, Cell Surface / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism*; D011994:Recombinant Proteins / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism; D017386:Sequence Homology, Amino Acid
D002352:Carrier Proteins; D037102:Lectins; D037181:Lectins, C-Type; D008024:Ligands; D000090323:Mannose Receptor; D037241:Mannose-Binding Lectins; D011956:Receptors, Cell Surface; D011994:Recombinant Proteins; D002122:Calcium Chloride; D008358:Mannose; D002118:Calcium