Structure and Function of Serum Mannan-binding Protein.

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Serum mannan-binding protein (MBP) is a calcium dependent lectin specific for mannose and N-acetylglucosamine and was first isolated by our group from rabbits and subsequently from human and rats. Analysis of the human serum-MBP and its cDNA clones reveals that the protein is divided into four domains: a cyctein-rich NH2-terminal domain that cross-link the subunits each other, the second collagen like domain which consisted of twenty repeats of a Gly-X-Y sequence, and a third neck domain located between the second collagen like domain and a fourth COOH-terminal carbohydrate-recognition domain, the sequence homology of which is markedly high among calcium dependent mammalian lectins. The lectin has an apparent molecular mass of 600, 000 daltons consisting of multimers of a single subunit of around 31, 000 daltons. MBP has a role in host defense; this is suggested first by our finding that the lectin can activate the complement system through the classical pathway, and by the demonstration by Ezekowitz's group that the lectin inhibits in vitro infection by the human immunodeficiency virus and stimulates phagocytosis of bacterium with mannose residues on their surfaces by neutrophiles and macrophages. The physiological importance of MBP is confirmed by the finding that low serum MBP concentrations are associated with a common opsonic defect, which is found in 5-7% of the general population.
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