Mannan-binding protein (MBP) is
Mannan-binding protein (MBP),1 also called mannose-binding protein (MBP) or mannan-binding lectin, is a Ca 2ϩ -dependent (C-type) serum lectin and an important serum component associated with innate immunity (1-4). Human MBP is a homo-oligomer of an ϳ31-kDa subunit, each subunit containing a carbohydrate recognition domain followed by a short neck region on the COOH-terminal side and a collagen-like domain followed by a short cysteine-rich region on the NH 2 -terminal side. Three subunits form a structural unit, and MBP normally consists of two to six structural units joined through disulfide bonds at the amino termini, the whole molecular mass being ϳ200 -600 kDa (5, 6).The carbohydrate specificity of MBP is rather broad (it binds to mannose, N-acetylglucosamine, and fucose) in accordance with the fact that MBP binds to the cell surfaces of a wide variety of pathogens (7,8). MBP exhibits complement-dependent bactericidal activity, i.e. the Escherichia coli K12 and B strains, which have exposed N-acetylglucosamine and L-glycero-D-mannoheptose residues, respectively, are killed by MBP with the aid of complement (8). In addition, 1-deoxymannojirimycin (an ␣-mannosidase inhibitor)-treated baby hamster kidney cells, which have high mannose type oligosaccharides exposed on their surfaces, are also killed by MBP with the aid of complement (9). This complement activation pathway is called the lectin pathway (10). MBP has also been shown to function as a direct opsonin (11) and to prevent virus infection (12, 13). 1 The abbreviations used are: MBP, mannan-binding protein; AAL, A. aurantia lectin; CID, collision-induced dissociation; ConA, concanavalin A; FACS, fluorescence-activated cell sorter; FCS, fetal calf serum; FITC, fluorescein 4-isothiocyanate; Fuc, fucose; Hex, hexose; HexNAc, N-acetylhexosamine; HPLC, high performance liquid chromatography; LA, lectin affinity; LacNAc, N-acetyllactosamine; LCA, Lens culinaris lectin; Le, Lewis antigen; LTA, Lotus tetragonolobus lectin; mAb, monoclonal antibody; MALDI, matrix-assisted laser desorption/ ionization; MLO, MBP ligand oligosaccharides; MLO-A1, MBP ligand oligosaccharide-acidic fraction (monosialylated); MLO-N, MBP ligand oligosaccharide-neutral fraction; MS, mass spectrometry; MS/MS, tandem mass spectrometry; nano-ESI, nanoelectrospray ionization; PA, 2-aminopyridine; PBS, phosphate-buffered saline; PHA-E4, P. vulgaris erythroagglutinating lectin; PHA-L4, P. vulgaris leukoagglutinating lectin; PNA, peanut lectin; TOF, time-of-flight. All of the sugar residues have the D configuration except fucose, which has the L configuration.