STRUCTURAL-ANALYSIS OF MONOSACCHARIDE RECOGNITION BY RAT-LIVER MANNOSE-BINDING PROTEIN

The structural basis of carbohydrate recognition by rat liver mannose- binding protein (MBP-C) has been explored by determining the three-dim ensional structure of the C-type carbohydrate-recognition domain (CRD) of MBP-C using x-ray crystallography. The structure was solved by mol ecular replacement using rat serum mannose-binding protein (MBP-A) as a search model and was refined to maximum Bragg spacings of 1.7 Angstr om. Despite their almost identical folds, the dimeric structures forme d by the two MBP CRDs differ dramatically. Complexes of MBP-C with met hyl glycosides of mannose, N-acetylglucosamine, and fucose were prepar ed by soaking MBP-C crystals in solutions containing these sugars. Sur prisingly, the pyranose ring of mannose is rotated 180 degrees relativ e to the orientation observed previously in MBP-A, but the local inter actions between sugar and protein are preserved, For each of the bound sugars, vicinal, equatorial hydroxyl groups equivalent to the 3- and 4-OH groups of mannose directly coordinate Ca2+ and form hydrogen bond s with residues also serving as Ca2+ ligands, Few interactions are obs erved between other parts of the sugar and the protein. A complex form ed between free galactose and MBP-C reveals a similar mode of binding, with the anomeric hydroxyl group serving as one of the Ca2+ ligands, A second binding site for mannose has also been observed in one of two copies in the asymmetric unit at a sugar concentration of 1.3 M. Thes e structures explain how MBPs recognize a wide range of monosaccharide s and suggest how fine specificity differences between MBP A and MBP-C may be achieved.