Ricin Triggers Apoptotic Morphological Changes through Caspase-3 Cleavage of BAT3

Ricin, one of type II ribosomal inactivating proteins, inhibits protein biosynthesis by its RNA N-glycosidase activity. By yeast two-hybrid screening, the human BAT3 (HLA-B-associated transcript 3) was isolated as a ricin A-chain interacting protein. A canonical caspase-3 cleavage site, DEQD(1001) was found at the C-terminal region of BAT3. Ricin induced the apoptosis by activating caspase-3 and leading to the cleavage of BAT3 at 4 h after treatment while DNA laddering at 24 h. The cleavage is completely inhibited by zDEVD-fmk, a caspase-3 specific inhibitor. In addition, cleavage of BAT3 is blocked in caspase-3-deficient MCF-7 cells, indicating that BAT3 is a novel caspase-3 substrate. Evidence indicates that caspase-3 activated by ricin acts on BAT3 at the caspase cleavage site, DEQD(1001) to release a C-terminal fragment designated CTF-131. The CTF-131 induces phosphatidylserine exposure, cell rounding, and chromatin condensation as ricin does. Moreover, silencing expression of endogenous BAT3 concomitantly suppresses ricin-induced apoptosis. Together, our results suggest a model that ricin triggers morphological changes of apoptosis by caspase-3-mediated proteolytic activation of BAT3.