Characterization of ligand binding to a carbohydrate-recognition domain of the macrophage mannose receptor.

Abstract

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.