Purification and characterization of two mannan-binding lectins from mouseserum

Mannan-binding lectin (MBL) is a serum protein that activates the complemen t system after binding to glycoconjugates found on the surface of microorga nisms, By molecular cloning two forms of MBL have been identified in the mo use (mMBL-A and mMBL-C), but only mMBL-A has been purified and characterize d at the protein level, MBL-C has been termed the liver form of MBL. The pr esent report describes the purification and characterization of mMBL-A and mMBL-C from serum. The two forms of mMBL could be separated both by ion-exc hange and carbohydrate-affinity chromatography. The initial identification by immunochemical technique was confirmed by N-terminal amino-acid sequenci ng. Both proteins give bands corresponding to polypeptide chains of 28 kDa on SDS-PAGE in the reduced state, but mMBL-A migrated more rapidly than mMB L-C in acid/urea-PAGE, in accordance with the calculated pIs, Both forms me diated activation of complement component C4 in mannan-coated microtiter we bs, MBL-A showed a higher affinity for D-glucose and alpha-methyl-D-glucose then did MBL-C, Serum concentrations of mMBL-A in laboratory strains and w ild mice were found to vary from 5 to 80 mu g/ml, with wild mice tending to show higher levels than laboratory strains.