Effect of AMN on endocytosis of IF-cobalamin in cubilin-expressing CHO cells. (A) Endocytosis of ¹²⁵I–IF-cobalamin after 4 hours at 37°C in cubilin and AMN single transfectants and in cubilin/AMN double transfectants, in the absence and presence of the lysosomal enzyme inhibitors chloroquine and leupeptin. Degradation (▪) represents the cell-mediated increase in TCA-soluble radioactivity in the medium, and uptake (▦) represents cell-associated radioactivity plus cell-mediated degradation. Error bars indicate SD. (B) Confocal immunofluoresence microscopy of the transfected cells incubated with IF-cobalamin at 37°C. Only the double cubilin/AMN transfectants take up IF-cobalamin as seen by vesicular staining of IF. Original magnification, × 630.