Purification and Characterization of Two Mannan-Binding Lectins from Mouse Serum

Abstract

Mannan-binding lectin (MBL) is a serum protein that activates the complement system after binding to glycoconjugates found on the surface of microorganisms. By molecular cloning two forms of MBL have been identified in the mouse (mMBL-A and mMBL-C), but only mMBL-A has been purified and characterized at the protein level. MBL-C has been termed the liver form of MBL. The present 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-exchange and carbohydrate-affinity chromatography. The initial identification by immunochemical technique was confirmed by N-terminal amino-acid sequencing. 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 mMBL-C in acid/urea-PAGE, in accordance with the calculated pIs. Both forms mediated activation of complement component C4 in mannan-coated microtiter wells. MBL-A showed a higher affinity for d-glucose and α-methyl-d-glucose then did MBL-C. Serum concentrations of mMBL-A in laboratory strains and wild mice were found to vary from 5 to 80 μg/ml, with wild mice tending to show higher levels than laboratory strains.