The specific molecular events that characterize the intrinsic apoptosis pathway have been the subject of intense research due to the pathway's fundamental role in development, homeostasis, and cancer. This pathway is defined by the release of cytochrome c from mitochondria into the cytosol and subsequent binding of cytochrome c to the caspase activator Apaf-1. Here, we report that both mitochondrial and cytosolic transfer RNA (tRNA) bind to cytochrome c. This binding prevents cytochrome c interaction with Apaf-1, blocking Apaf-1 oligomerization and caspase activation. tRNA hydrolysis in living cells and cell lysates enhances apoptosis and caspase activation, whereas microinjection of tRNA into living cells blocks apoptosis. These findings suggest that tRNA, in addition to its well-established role in gene expression, may determine cellular responsiveness to apoptotic stimuli.
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We thank Drs. K. Shogen and W. Ardelt for providing onconase, Dr. X. Wang for providing Apaf-1 −/− and wild type MEFs and anti-Apaf-1 antibody, Dr. K. Dittmar for advice on tRNA work, and A. Stonestrom and S. Slattery for help with manuscript preparation. This work was supported, in part, by NIH grants GM060911 and CA108872 and a Leukemia and Lymphoma Society Scholar Award (to X.Y.). G.D. is funded by the Howard Hughes Medical Institute.