The C-terminal SAM domain of p73 binds to the N terminus of MDM2

Background: The p53, p63 and p73 proteins belong to the p53 family of transcription factors, playing key roles in tumour suppression. The alpha-splice variant of p73 (p73 alpha) has at its C terminus a sterile alpha motif (SAM); this domain, SAMp73, formed by five helices (alpha 1 to alpha 5), is thought to mediate in protein-protein interactions. The E3-ligase MDM2 binds to p73 at its N terminus transactivation domain (TA), but it does not promote its degradation via ubiquitination; however, the details of such MDM2/p73 interaction are not fully known. Methods: We studied the binding of SAMp73 with N-terminal MDM2, by several biophysical techniques, namely, fluorescence, far-UV circular dichroism (CD), NMR and bio-layer interferometry (BLI). Results: Our results obtained by fluorescence, T-2-relaxation measurements and BLI show that there was binding between both proteins with a dissociation constant of similar to 10 mu M. Furthermore, the binding region of SAMp73 involved mainly residues in the major alpha-helix, alpha 5, and the nearby alpha 4, as shown by HSQC-NMR. The binding was so specific that an isolated peptide comprising alpha 4 and alpha 5 helices of SAMp73, alpha 4 alpha 5, did also bind to the N terminus of MDM2, although with weaker affinity than the entire domain. Conclusions: A new interaction between MDM2 and SAMp73 has been found, which could have potential therapeutic applications in cancers involving inactivated p53. General significance: A novel interaction between the C-terminal SAM of p73 and N-terminal MDM2 is described. The interaction could be used to modulate the functions where the wild-type, intact p73 is involved.