Antibodies against a lysosomal membrane antigen (A-Ly-M) have recently been obtained and characterized (Reggio, H., D. Bainton, E. Harms, E. Coudrier, and D. Louvard, 1984, J. Cell Biol., 99:1511-1526). They recognize a 100,000-mol-wt antigen immunologically related to a purified [H+,K+]ATPase from pig gastric mucosa. In the present study, we have localized this antigen during adsorptive endocytosis in rat prolactin cells in culture using cationized ferritin (CF) as a tracer. CF was rapidly internalized (after 5 min) in coated pits and vesicles that were labeled by antibodies against clathrin. The tracer was then delivered (after 15 min) to vacuoles and multivesicular bodies. These structures were labeled with A-Ly-M. These organelles were devoid of acid phosphatase activity. At later stages (after 30 min) CF was observed within larger structures that were strongly stained by A-Ly-M and displayed a strong acid phosphatase activity. These findings clearly indicate that A-Ly-M react with prelysosomal and lysosomal compartments involved in the endocytic pathway in cultured prolactin cells. The membrane of these structures therefore contains antigenic determinant(s) related to the 100,000-mol-wt polypeptide. Our results suggest that the prelysosomal structure stained by A-Ly-M may represent in GH3 cells the acidic prelysosomal compartment recently described in the early steps of endocytosis in other cell types (Tycko, B., and F. R. Maxfield, 1982, Cell, 28:643-651).

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