The redox state of specific cysteine residues in the p53 tumor suppressor protein plays a critical role in its ability to bind to consensus deoxyribonucleic acid (DNA) sequences and to function as a transcriptional regulator of target genes, including those involved in suppressing cellular transformation (1-3). Three cysteines (Cys-176, Cys-238, and Cys-242) along with His-179 are directly involved in binding of a zinc ion, which is essential for maintaining the active structural conformation of p53 (4). Other cysteines (Cys-124, Cys-135, Cys-141, Cys-275, and Cys-277) are located in the DNA-binding domain of p53 and may

John T. Hancock (ed.), Methods in Molecular Biology, Redox-Mediated Signal Transduction, vol. 476 © 2008 Humana Press, a part of Springer Science + Business Media, Totowa, NJ DOI: 10.1007/978-1-59745-129-1_9

form a redox-sensitive site subject to modulation by the redox conditions within the cell (1, 4). Oxidation-reduction of these cysteines would affect the conformation of the protein and in turn would affect the ability of p53 to arrest the cell cycle or trigger apoptosis in response to stress signals, either genotoxic or nongenotoxic in nature.

The conformation of p53 has been redox-modulated in vitro by treating purified p53 with the reducing agent dithiothreitol (DTT) or in vivo by treating p53-positive cells with a reducing agent such as aminothiol WR1065 or an oxidizing agent such as diamide. Redox changes in p53 structure have been analyzed by alterations in gel mobility. Structural redox perturbations of p53 have been shown to affect the ability of the protein to bind to consensus DNA in vitro and to function in G1 cell cycle arrest in vivo. In an alternative approach to studying the effect of redox on p53 function, p53-negative malignant cells were co-transfected with a plasmid expressing wild-type p53 and a plasmid +/- thiore-doxin (TRX), and the effect of enhanced expression of the TRX on p53-induced apoptosis was examined. TRX is a small redox-active protein that helps maintain the overall redox state of the cell and control the redox status of important redox-sensitive transcription factors, such as NF-kB and API (5). The ability of p53 to induce apoptosis of the malignant cells was decreased in the presence of TRX, suggesting that the redox state of p53 plays a role in apoptosis.

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