MATURATION OF EARLY ENDOSOMES AND VESICULAR TRAFFIC TO LYSOSOMES IN RELATION TO MEMBRANE RECYCLING

The controversy whether endocytic processing occurs by organellar matu ration or by vesicular traffic has not been resolved, It is also not c lear whether maturation continues to the stage of lysosomes, to what e xtent it involves a decrease in organellar fusogenicity, and how it re lates to membrane recycling, Maturation and vesicular traffic imply di stinct kinetics for the intermingling of endocytic markers after seque ntial endocytic uptake. We have studied the kinetics of intermingling of fluid-phase markers (fluorescein-labelled dextran and horseradish p eroxidase) and cell surface-derived membrane (labelled by galactosylat ion) in organelles at early and late stages of the endocytic pathway i n macrophage-like P388D1 cells, Intermingling declined by sigmoid kine tics, indicating that endosomes matured within about 3 minutes to beco me non-fusogenic towards early endosomes, During maturation about 60% of internalized membrane was recycled with T1/2 approximate to 2 minut es, Whereas matured endosomes were non-fusogenic towards early endosom es and towards each other, a second phase of intermingling was observe d upon delivery to lysosomes, This intermingling occurred by a first-o rder process (T1/2 approximate to 4 minutes), concurrent with recyclin g of the remaining 40% of internalized membrane marker, These kinetic observations suggest a model for endocytic processing which reconciles maturation of early endosomes with the known function of carrier vesi cles: Endocytic carrier vesicles do not bud off from permanent early e ndosomes as proposed for vesicular traffic, but are derived, together with recycling vesicles, from the maturation of early endosomes which are consumed by this process; these carrier vesicles subsequently medi ate delivery to lysosomes by vesicular traffic during which the remain ing surface-derived membrane is recycled.