The receptor for asialoglycoproteins (ASGPR) was localized in human hepatoma Hep G2 cells by means of quantitative immunoelectron microscopy. Without ligand added to the culture medium, we found 34% of the total cellular receptors on the plasma membrane, 37% in compartment of uncoupling receptor and ligand (CURL), and 21% in a trans-Golgi reticulum (TGR) that was defined by the presence of albumin after immuno-double labeling. A small percent of the ASGPR was associated with coated pits, the Golgi stacks, and lysosomes. After incubation of the cells with saturating concentrations of the ligand asialo-orosomucoid (ASOR), the number of cell surface receptors decreased to 20% of total cellular receptors, whereas the receptor content of CURL increased by a corresponding amount to 50%. The ASGPR content of TGR remained constant. In contrast, after treatment of the cells with 300 microM of the weak base primaquine (PMQ), cell surface ASGPR had decreased dramatically to only 4% of total cellular receptors whereas label in the TGR had increased to 42%. ASGPR labeling of CURL increased only to 47%. The labeling of other organelles remained unchanged. This affect of PMQ was independent of the presence of additional ASOR. Implications for the intracellular pathway of the ASGPR are discussed.

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