Activation of a temperature-sensitive form of mouse p53 in murine erythroleukaemia cells rapidly inhibits protein synthesis and causes early dephosphorylation and cleavage of protein synthesis initiation factor eIF4GI and the eIF4E-binding protein 4E-BP1. Dephosphorylated 4E-BP1 and the cleaved products of 4E-BP1 and eIF4GI associate with eIF4E under these conditions, concomitant with decreased interaction of full-length eIF4GI with eIF4E. These changes may play an important role in preventing formation of the eIF4F complex and thus the initiation of protein synthesis. As observed previously for eIF4GI, the cleavage of 4E-BP1 is insensitive to the general caspase inhibitor z-VAD.FMK, consistent with a caspase-independent mechanism of factor modification and regulation of protein synthesis. Comparison of the p53-induced patterns of eIF4GI and 4E-BP1 dephosphorylation and cleavage with those caused by the mTOR inhibitor rapamycin indicates that p53 activation and rapamycin have distinct but additive effects. Moreover, p53 activation inhibits rapamycin-insensitive protein kinase activity against 4E-BP1. P53 and rapamycin have additive effects on the inhibition of overall protein synthesis. These data suggest that the inhibition of protein synthesis by p53 is largely independent of the regulation of rapamycin-sensitive mTOR in the system under investigation.