Identification of the mannan-binding protein from rat livers as a hepatocyte protein distinct from the mannan receptor on sinusoidal cells.

Cellular distribution in rat livers of the mannan-binding protein, a liver lectin specific for mannose and N-acetylglucosamine was investigated. Estimation of mannan binding activity of isolated cells demonstrated that more than 99% of the mannan binding activity recovered from whole livers was derived from hepatocytes. Intracellular localization of the mannan-binding protein disproved its function as a receptor of endocytosis. A small quantity of mannan binding activity detectable in sinusoidal cells seems to represent a receptor for mannan endocytosis. The receptor resembled the mannan-binding protein in its high affinity for mannan, requirement of Ca2+ for binding, and specificity for mannose and N-acetylglucosamine. However, the receptor was clearly distinct from the mannan-binding protein as indicated by the lack of response to antimannan-binding protein antiserum, by the potent inhibition by L-fucose and by the differences of specificity for various glycoproteins. The results in this study confirm the recent report of Maynard and Baenziger (Maynard, Y., and Baenziger, J. (1982) J. Biol. Chem. 257, 3788-3794) with quantitative data using different methodology and ligands, and provide additional evidence indicating the receptor on sinusoidal cells to be distinct from the hepatic binding protein. A possible role of the mannan-binding protein in the intracellular transport of intermediates of glycoprotein biosynthesis is also discussed.