A membrane-bound form of the acute-phase protein C-reactive protein is the galactose-specific particle receptor on rat liver macrophages.

Rat liver macrophages express a galactose-specific receptor which mediates endocytosis of particles or neuraminidase-treated blood cells. From rat serum we now have isolated a galactose-specific lectin by affinity chromatography. Comparative analysis of this serum galactose-binding protein with the galactose-specific particle receptor protein purified from rat liver macrophages and with the acute-phase protein C-reactive protein (CRP) revealed a close relation or identity of these proteins. An apparent molecular weight of 30 kilodaltons was determined for all three proteins by SDS-PAGE under reducing conditions and of about 130 kilodaltons by native PAGE. All three proteins exhibit the same pentameric, ring-shaped structure. Antibodies raised against the serum galactose-binding protein or against the macrophage receptor did cross-react. Monoclonal antibodies raised against rat CRP labeled liver macrophage but not hepatocyte surfaces and reacted with all three isolated proteins in a Western blot assay. Furthermore, the galactose-specific particle receptor could be functionally replaced by purified CRP. Northern blot analysis showed that the CRP is not synthesized in the macrophages but appears to be acquired from hepatocytes or blood. We now conclude that a membrane-bound form of CRP functions as the recycling galactose-specific particle receptor in rat liver Kupffer cells.