Comparative studies on internalization of gold-labelled mistletoe lectin I, its subunits, as well as of an immunotoxin in murine L 1210 leukemia cells.

The binding and internalization of gold-labelled mistletoe lectin I (ML I), its A (ML I-A) and B (ML I-B) subunits, as well as of an immunotoxin consisting of an monoclonal anti L 1210 antibody and the cytotoxic A chains of ML I, were studied on murine L 1210 leukemia cells by a preembedding electron microscopic technique. We found that receptor-mediated endocytosis differs remarkably between the whole lectin, its subunits, and the immunotoxin. Whereas ML I, its A chain, as well as the immunotoxin are internalized by coated pits/coated vesicles or in combination with uncoated membranes, the B chain is exclusively endocytosed via uncoated deep invaginations of the cell membrane. The endocytosis via clathrin-coated or uncoated vesicles is discussed taking into account the binding and internalization kinetics of ligand-receptor complexes related with the movement by the cytoskeleton.