Apoptosis induced by the nuclear death domain protein p84N5 is inhibited by association with Rb protein

Rb protein inhibits both cell cycle progression and apoptosis. Interaction of specific cellular proteins, including E2F1, with Rb C-terminal domains m ediates cell cycle regulation. In contrast, the nuclear N5 protein associat es with an Rb N-terminal domain with unknown function. The N5 protein conta ins a region of sequence similarity to the death domain of proteins involve d in apoptotic signaling. We demonstrate here that forced N5 expression pot ently induces apoptosis in several tumor cell lines. Mutation of conserved residues within the death domain homology compromise N5-induced apoptosis, suggesting that it is required for normal function. Endogenous N5 protein i s specifically altered in apoptotic cells treated with ionizing radiation. Furthermore, dominant interfering death domain mutants compromise cellular responses to ionizing radiation. Finally, physical association with Rb prot ein inhibits N5-induced apoptosis. We propose that N5 protein plays a role in the regulation of apoptosis and that Rb directly coordinates cell prolif eration and apoptosis by binding specific proteins involved in each process through distinct protein binding domains.