Endosomes can undergo an ATP-dependent density increase in the absence of dense lysosomes

On the basis of evidence that lysosomal enzymes and membrane proteins are present in endosomes, we have previously suggested that the production of lysosomes involves maturation rather than vesicle fusion (Roederer, M., R. Bowser, R. F. Murphy, J. Cell. Physiol. 131, 200-209 (1987)). Since the appearance of endocytosed material in lysosomes is associated with an increase in buoyant density from that of endosomes, a prediction of the model is that endosomes should be capable of undergoing such an increase in vitro. We observe that under appropriate conditions, isolated endosomes containing [125I]EGF can undergo an increase in density in vitro to that of dense lysosomes, mimicking the density change which occurs in vivo. This occurs in the absence of dense lysosomes with which to fuse. The density increase requires ATP and can be efficiently inhibited in vitro by the presence of benzylamine, suggesting that vesicular acidification is required. Since low pH has previously been shown to induce formation of a matrix by lysosomal enzymes in vitro (Buckmaster, M. J., A. L. Ferris, B. Storrie, Biochem. J. 249, 921-923 (1988)), we propose that a mechanism by which endosomes and/or lysosomes increase their density is a low pH induced aggregation of vesicle contents which decreases the osmotic pressure inside the vesicle. Together with previous data, the results provide highly suggestive evidence that the pathway to lysosomes includes a maturation of the postsorting compartment into what has classically been termed a lysosome.