Mannose receptor dependent uptake of ricin A1 and A2 chains by macrophages

The ricin A chain, the toxic subunit of ricin, consists of two forms which differ in sugar content. The major component A1 contains one high mannose chain while the minor component A2 contains an additional high mannose chain. Endocytosis of this toxin occurs in macrophages via the mannose receptor. To study the role of the sugar residues in ricin A chain cytotoxicity, we have purified the two forms by ion-exchange chromatography. The uptake of A1 and A2 by a macrophage cell line was concentration and time dependent. The total amount of A2 internalized was approximately twice the amount of A1, indicating a higher affinity of A2 for the mannose receptor. Ricin A2 was four times more toxic to macrophages than A1, in agreement with the higher affinity of A2 compared to the A1. These experiments suggest that the high mannose chains on the A chain promote mannose-receptor-mediated endocytosis by providing the initial binding to the cell surface. Once the toxin is accumulated inside the cell however, the carbohydrates do not seem to influence intracellular transport and/or translocation of the ricin A chain into the cytoplasm.