The intracellular movement and cycling of ricin.

The binding, internalization and recycling of the plant toxin ricin, was studied using electron microscopy and biochemical techniques. For the electron microscope study, ricin was visualized using a gold-labeled second antibody, in the cells of the EJ human bladder carcinoma line growing in monolayer culture. The labeled antibody/toxin complex was found to enter the cell in coated pits and to accumulate in endosomes and to a lesser extent in vesicles associated with the Golgi system. The complex recycled to the cell surface partly in uncoated vesicles, but largely in multivesicular bodies which appeared to exocytose their contents to the extracellular space. Twenty hours after the initial contact with ricin as much as 50% of the cellular label was found on the cell surface mainly associated with shed vesicles. When cells were treated with unlabeled ricin holotoxin and then after 20 h stained post-fixation, ricin molecules, partly associated with vesicles, were present on the cell surface. Biochemical studies showed that ricin was internalized by cells and then released in an intact form to the extracellular space. It was found that less than 10% of the released material had been degraded during its passage through the cells, which is in accord with the low level of label found in the lysosomal system during the morphological study.