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J. Cell Biol.,
Volume 140, Number 3, February 9, 1998 591-601
Department of Clinical Biochemistry, University of Cambridge, Addenbrooke's Hospital, Cambridge, CB2 2QR, United
Kingdom
Using a cell-free content mixing assay containing rat liver endosomes and lysosomes in the presence of pig brain cytosol, we demonstrated that after incubation at 37°C, late endosome-lysosome hybrid
organelles were formed, which could be isolated by
density gradient centrifugation. ImmunoEM showed
that the hybrids contained both an endocytosed marker
and a lysosomal enzyme. Formation of the hybrid organelles appeared not to require vesicular transport between late endosomes and lysosomes but occurred as a
result of direct fusion. Hybrid organelles with similar
properties were isolated directly from rat liver homogenates and thus were not an artifact of cell-free incubations. Direct fusion between late endosomes and
lysosomes was an N-ethylmaleimide-sensitive factor- dependent event and was inhibited by GDP-dissociation inhibitor, indicating a requirement for a rab protein. We suggest that in cells, delivery of endocytosed
ligands to an organelle where proteolytic digestion occurs is mediated by direct fusion of late endosomes with
lysosomes. The consequences of this fusion to the maintenance and function of lysosomes are discussed.
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