A monoclonal antibody (OSW2) was prepared by using human osteosarcoma cells. OSW2 was found to be directed toward the 116 (also called 100)- kD protein that uniquely associates to the vacuolar-type proton pump. The antibody specifically localized acidic membrane compartments that could be visualized with acridine orange in many types of human cells. It also reacted with the surface and was internalized along the endosomal pathway. Monitoring the endosome pH by using FITC-dextran and acridine orange suggested that the antibody interfered with low pH. Cell-free experiments indicated that the ATP-dependent acidification was inhibited in endosomes associated with OSW2. In contrast, the antibody gave little effect on the ATPase activity of the solubilized H+ pump. The internalization of OSW2 reduced infectivity of certain enveloped viruses (influenza, SFV, VSV) by 50 to 80%. Inhibition of viral fusion was directly demonstrated by monitoring the fate of octadecylrhodamine-labeled influenza virus fluorescence. These results indicate that the 116 (100)-kD protein is necessary for the control of pH. The antibody represents a novel probe for understanding the role of the endosomal compartments in cellular physiology.

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