MOLECULAR SIZE-FRACTIONATION DURING ENDOCYTOSIS IN MACROPHAGES

The sorting of macromolecules within and between membranous organelles is often directed by information contained in protein primary or seco ndary structure. We show here that absent such structural information, macromolecules internalized by endocytosis in macrophages can be sort ed by size. After endocytosis, small solute probes of fluid-phase pino cytosis were recycled to the extracellular medium more efficiently tha n large solutes. Using macropinosomes pulse labeled with fluorescent d extrans, we examined the ability of organelles to exchange solute cont ents. Dextran exchange was optimal between organelles of similar age, and small dextrans exchanged more efficiently than large dextrans. Eff erent solute movement, from lysosomes or phagolysosomes toward the pla sma membrane, occurred through the same endocytic vesicles as afferent movement, toward lysosomes and this movement was solute size dependen t. Remarkably, uniform mixtures of different-sized dextrans delivered into lysosomes separated into distinct organelles containing only one dextran or the other. Thus, the dynamics of endosomes and lysosomes we re sufficient to segregate macromolecules by size. This intracellular size fractionation could explain how, during antigen presentation, pep tides generated by lysosomal proteases recycle selectively from lysoso mes to endosomes for association with class II MHC molecules.