INVARIANT CHAIN INDUCES A DELAYED TRANSPORT FROM EARLY TO LATE ENDOSOMES

Invariant chain associated with class II molecules is proteolytically processed in several distinct intermediates during its transport throu gh the endocytic pathway. Using subcellular fractionation, early and l ate endosomal compartments were separated in human fibroblasts transfe cted with HLA-DR (4N5 cells) and supertransfected with invariant chain (4N5Li cells) or invariant chain lacking most of the cytoplasmic tail (4N5 Delta 20Ii cells). Early and late endosome membrane fractions we re characterized by morphology and by analyzing the presence of the Ra b5 and Rab7 GTPases as markers of early and late endosomes, respective ly. The transfer of endocytosed horseradish peroxidase from early to l ate endosomes proceeded relatively rapid both in 4N5 and 4N5 Delta 20I i cells (t(1/2) = 25 min), whereas this transfer was significantly del ayed (t(1/2) = 2 h) in 4N5Ii cells. Pulse-chase experiments showed tha t invariant chain and its degradation products were first observed in early endosomes and thereafter in late endosomes. Our results strongly suggest that invariant chain induces a retention mechanism in the end ocytic pathway.