Ubiquitin-dependent protein processing controls radiation-induced apoptosis through the N-end rule pathway

The ubiquitination of nuclear proteins activated in human lymphocytes under going radiation-induced apoptosis and the subsequent downstream proteasomal protein processing, shown to be involved in apoptotic death control, may b e dependent on an aminoterminal sequence identity of ubiquitin target prote ins, the "N-end rule" pathway. Here we report that this selective pathway c ontrols radiation-induced apoptosis and that it is involved in the initiati on of this type of cell death. Dipeptide competitors of protein ubiquitinat ion/processing dependent solely on the basic amino-terminal residues (type I) efficiently inhibited the radiation-induced apoptotic death phenotype, i ndicating that only the substrates of ubiquitination with basic NH2-termina l amino acids are involved in apoptotic death control. This selective inhib ition was followed by an early, overall but also target-specific inhibition of ubiquitination and by an activation and stabilization of poly(ADP-ribos e) polymerase (PARP) that occurs through inhibition of ubiquitination of it s cleaved form (85 kDa). Interestingly, caspases-3 and -7 were not activate d following irradiation, further suggesting that PARP cleavage may be regul ated by an N-end rule pathway in a caspase-independent manner. These result s highly suggest involvement of this subset of the ubiquitin system in the apoptotic death control and in the specific regulation of PARP activity. (C ) 2000 Academic Press.