Purification and characterization of Mannan‐binding protein from mouse serum

Mouse mannan‐binding protein (MBP) was identified in serum by its Ca²⁺ ‐dependent binding to mannan. On gel permeation chromatography, the protein eluted corresponding to a molecular weight of approximately 750 kDa. Analysed on SDS‐PAGE under reducing conditions, the polypeptide showed an apparent molecular weight of 28 kDa, while several high molecular weight bands were seen under non‐reducing conditions. The presence of collagen‐like domains within the molecule was indicated by a high glycine content (14.9%) and substantiated by sensitivity to collagenase. Rabbit anti‐mouse MBP antisera were raised. The concentration of MBP in serum from normal mice was measured by rocket immunoelectrophoresis and found to be from below 1 μg/ml to 100 μg/ml (average 50 μg/ml, n= 60). The binding of mouse MBP to mannan could be inhibited by mono‐ and disaccharides in the following order of potency: L‐fucose > D‐mannose > N‐acetyl‐D‐glucosamine > maltose > D‐mannoheptulose > D‐glucose > N‐acetyl‐D‐mannosamine ≫ lactose > D‐galactose ≫ N‐acetyl‐D‐galactosamine. Mouse MBP was shown to activate the classical complement cascade after binding to mannan. The sequence of 14 NH_2‐ terminal amino acid residues of the molecule showed 93% identity to rat MBP‐A and complete identity to the translated cDNA sequences for mouse MBP‐A and mouse Ra‐reactive factor component P28b (RaRF P28b) published previously. The amino acid composition of mouse MBP showed a high degree of homology to MBPs from other species and mouse RaRF P28b.