Np. Mullin et al., CHARACTERIZATION OF LIGAND-BINDING TO A CARBOHYDRATE-RECOGNITION DOMAIN OF THE MACROPHAGE MANNOSE RECEPTOR, The Journal of biological chemistry, 269(45), 1994, pp. 28405-28413
The extracellular portion of the macrophage mannose receptor, an endoc
ytic receptor involved in clearance of glycoconjugates, contains eight
domains related to the Ca2+-dependent carbohydrate-recognition domain
s (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 i
nvestigated. The expressed domain was found to be a monomer in solutio
n. Results of a solid phase binding assay and a protease resistance as
say show that CRD-4 of the mannose recep tor undergoes a conformationa
l rearrangement upon binding of Ca2+, correlating with its ability to
bind sugar. CRD-4 requires two Ca2+ for sugar binding, even though seq
uence 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 struc
ture of the CRD of rat mannose-binding protein in complex with an olig
osaccharide. The stability of Ca2+ binding is shown to be pH-dependent
, a result that is pertinent to release of ligand by the receptor in t
he endosome. However, CRD-4 retains sugar binding activity at a lower
pH than does the whole receptor, suggesting that the conformational ch
ange in this CRD alone may not be sufficient to allow release of ligan
d in the endosomes.