Isolation and Characterization of Porcine Mannan-Binding Proteins of Different Size and Ultrastructure

The authors report on the purification and characterization of mannan-binding proteins (MBP) isolated from porcine serum. The MBPs were purified by use of PEG precipitation, affinity chromatography on mannan-Sepharose, protein A- and anti-porcine IgM-Sepharose followed by gel filtration. The MBP proteins were collagenase sensitive and showed γ₁-γ₂-electrophoretic mobility. The MBP designated pMBP-28 had a molecular mass of 28 kDa when analysed on SDS-PAGE under reducing conditions and eluted corresponding to a molecular mass of approximately 700 kDa on gel filtration chromatography. Electron micrographs of pMBP-28 revealed an oligomeric protein similar to rodent MBP-A and human MBP but with a predominance of penta- and hexameric molecules. Another protein designated pMBP-27 was composed of peptides of 27 kDa and had an M_r of 300–350 kDa on gel filtration chromatography. Electron microscopy of pMBP-27 showed dimer and trimer molecules; the trimers without distinct stalk regions. The N-terminal 26 (pMBP-27) and 24 (MBP-28) amino acid residues showed 54% and 58% identity with human MBP. pMBP-28 showed a higher degree of sequence similarity to rat and mouse MBP-A (60% identity) than to mouse and rat MBP-C (41–45% identity). Both pMBPs exhibited Ca²⁺-dependent binding to D-mannose immobilized on agarose but no significant binding to N-acetyl-D-glucosamine- or fucose-agarose. The results further suggested the presence of a third pMBP which copurified with pMBP-27 but this protein was not sequenced.