The ability of p53 to induce apoptosis requires its sequence-specific DNA binding activity; however, the transactivation-deficient p53(Gln 22 -Ser 23 ) can still induce apoptosis. Previously, we have shown that the region between residues 23 and 97 in p53 is necessary for such activity. In an effort to more precisely map a domain necessary for apoptosis within the N terminus, we found that deletion of the N-terminal 23 amino acids compromises, but does not abolish, p53 induction of apoptosis. Surprisingly, p53(⌬1-42), which lacks the N-terminal 42 amino acids and the previously defined activation domain, retains the ability to induce apoptosis to an even higher level than wild-type p53. A more extensive deletion, which eliminates the N-terminal 63 amino acids, renders p53 completely inert in mediating apoptosis. In addition, we found that both p53(⌬1-42) and p53(Gln 22 -Ser 23 ) can activate a subset of cellular p53 targets. Furthermore, we showed that residues 53 and 54 are critical for the apoptotic and transcriptional activities of both p53(⌬1-42) and p53(Gln 22 -Ser 23 ). Taken together, these data suggest that within residues 43-63 lie an apoptotic domain as well as another transcriptional activation domain. We therefore postulate that the apoptotic activity in p53(Gln 22 -Ser 23) and p53(⌬1-42) is still transcription-dependent.

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The p53 protein contains several functional domains necessary for inducing cell cycle arrest and apoptosis. The C-terminal basic domain within residues 364 -393 and the proline-rich domain within residues 64 -91 are required for apoptotic activity. In addition, activation domain 2 within residues 43-63 is necessary for apoptotic activity when the N-terminal activation domain 1 within residues 1-42 is deleted (⌬AD1) or mutated (AD1 ؊ ). Here we have discovered that an activation domain 2 mutation at residues 53-54 (AD2 ؊ ) abrogates the apoptotic activity but has no significant effect on cell cycle arrest. We have also found that p53-(⌬AD2), which lacks activation domain 2, is inert in inducing apoptosis. p53-(AD2 ؊ ⌬BD), which is defective in activation domain 2 and lacks the C-terminal basic domain, p53-(⌬AD2⌬BD), which lacks both activation domain 2 and the C-terminal basic domain, and p53-(⌬PRD⌬BD), which lacks both the proline-rich domain and the Cterminal basic domain, are also inert in inducing apoptosis. All four mutants are still capable of inducing cell cycle arrest, albeit to a lesser extent than wild-type p53. Interestingly, we have found that deletion of the N-terminal activation domain 1 alleviates the requirement of the C-terminal basic domain for apoptotic activity. Thus, we have generated a small but potent p53-(⌬AD1⌬BD) molecule. Furthermore, we have determined that at least two of the three domains (activation domain 1, activation domain 2, and the proline-rich domain), are required for inducing cell cycle arrest. Taken together, our results suggest that activation domain 2 and the proline-rich domain form an activation domain for inducing pro-apoptotic genes or inhibiting antiapoptotic genes. The C-terminal basic domain is required for maintaining this activation domain competent for transactivation or transrepression.

The transporter associated with antigen processing (TAP) 1 is required for the major histocompatibility complex (MHC) class I antigen presentation pathway, which plays a key role in host tumor surveillance. Since more than 50% of tumors have a dysfunctional p53, evasion of tumor surveillance by tumor cells may be linked to loss of p53 function. Here we found that TAP1 is strongly induced by p53 and DNA-damaging agents through a p53-responsive element. We also found that p73, which is homologous to p53, is capable of inducing TAP1 and cooperates with p53 to activate TAP1. Furthermore, we found that by inducing TAP1, p53 enhances the transport of MHC class I peptides and expression of surface MHC-peptide complexes, and cooperates with interferon gamma to activate the MHC class I pathway. These results suggest that tumor surveillance may be a mechanism by which p53 and/or p73 function as tumor suppressors.