Emerging evidences suggest that transcription-independent mechanism of p53 appears to make an important contribution to the overall p53-dependent apoptosis. Recently, it has been postulated that the DNA-binding domain of p53 can interact with Bcl-XI, and subsequently the proposed molecular interaction has been shown by NMR studies. Interestingly, Chipuk et al. [Cancer Cell 4 (2003) 371] reported that the N-terminal domain of p53 (p53NTD) alone is necessary and sufficient to induce transcription-independent apoptosis. To further define and understand the nature of the molecular recognition between p53 and Bcl-XI, our current study focuses on p53NTD. We first demonstrated the molecular interaction between p53NTD and Bcl-XI by co-expressing and purifying the complex. Second, to define the binding interface of the molecular interaction, which is not previously characterized, in the current we employed a NMR-based binding study, showing that the binding site on Bcl-XI is located in the region including alpha 4, the N- and C-termini of alpha 3, the N-terminus of alpha 5, and the central part of alpha 2. To further probe this observation, we then performed fluorescence resonance energy transfer (FRET) assay in cells. The FRET efficiency detected between the donor and acceptor molecules appears to suggest the presence of molecular interaction of p53NTD with Bcl-XI in cells. Taken together, our data suggest that p53NTD interacts with Bcl-XI but the characteristic of the molecular interaction appears to be different from that of the DNA-binding domain of p53. (c) 2006 Elsevier Inc. All rights reserved.