DIFFERENCE IN THE BINDING MODE OF 2 MANNOSE-BINDING PROTEINS - DEMONSTRATION OF A SELECTIVE MINICLUSTER EFFECT

Serum-type and liver-type mannose-binding proteins (MBP) are both pres ent in higher animals and both are composed of a carbohydrate-recognit ion domain (CRD) and a collagenous domain. Although known as mannose-b inding proteins, these proteins bind N-acetylglucosamine and other rel ated sugars quite well. An earlier specificity study using cloned CRD portions of both types of MBP from rat [Childs, R. A., Feizi, T., Yuen , C.-T., Drickamer, K., & Quesenberry, M. (1990) J. Biol. Chem. 265, 2 0770-20777] revealed that the liver MBP CRD binds the trimannosyl core structure of N-glycosides, whereas the serum MBP CRD does not. We stu died the substrate preferences of these CRDs using both solid and solu tion phase assays, testing monosaccharides, glycoproteins, and synthet ic cluster ligands. While there was no significant difference in the m onosaccharide binding specificities of the two CRDs, they displayed ve ry different affinities for natural glycoproteins and mannose-containi ng cluster glycosides. Most interestingly, synthetic cluster ligands w ith two terminal GlcNAc moieties have affinity equal to monovalent Glc NAc ligands toward both CRDs, whereas a series of structurally similar Man-terminated divalent ligand displays about 20-fold enhanced affini ty toward liver CRD only. A plausible explanation is that the liver MB P CRD has two sugar binding sites per subunit, one of which binds only mannose, and the other, both mannose and N-acetylglucosamine. In cont rast, the serum MBP CRD has only one site of the latter type. Results of isothermal titration calorimetry support this hypothesis.