CLEAVAGE OF RABAPTIN-5 BLOCKS ENDOSOME FUSION DURING APOPTOSIS

Cells undergoing apoptosis exhibit striking changes in membrane organi zation, including plasma membrane blebbing and invagination, vacuolati on and fragmentation of organelles, and alterations in the surface exp ression of receptors, The underlying mechanisms for these changes are unknown, though alterations in vesicular fusion are likely to play a r ole, Using a cell-free system based on Xenopus laevis egg extracts me have found that endosome fusion is blocked during apoptosis, Inhibitio n of fusion is prevented by Bcl-2 or Bcl-x(L), two negative regulators of apoptosis, or by specific inhibitors of members of the caspase fam ily of apoptotic proteases. Selective cleavage of Rabaptin-5, an essen tial and rate-limiting component of endosome fusion, is responsible fo r the loss of fusion activity, Cleavage of Rabaptin-5 also occurs in c ellular models for apoptosis. These results suggest that inactivation of Rabaptin-5 and inhibition of vesicle transport lead to fragmentatio n of endosomes and inhibition of the endocytic pathway during the exec ution phase of apoptosis, We propose that parallel changes to other me mbrane transport pathways would give rise to general membrane fragment ation in apoptotic cells. These changes are likely to play an importan t role in the generation of apoptotic bodies and their recognition by phagocytosing cells.