The C-terminal sterile alpha motif (SAM) domain of human p73 is a highly dynamic protein, which acquires high thermal stability through a decrease in backbone flexibility

The alpha-splice variant of p73 (p73 alpha), a homologue of the tumour suppressor p53, has close to its C terminus a sterile alpha motif (SAM), SAMp73, that is involved in protein-biomolecule interactions. The conformational stability of SAMp73 is low (similar to 5 kcal mol(-1)), although its thermal stability is high. To explain this high thermostability, we studied the dynamics of SAMp73 over a wide range of GdmCl (guanidine hydrochloride) concentrations and temperatures by NMR relaxation, NMR hydrogen-exchange (HX) and fluorescence lifetime approaches. The slowest exchanging residues of SAMp73 belong to the helical regions, and they did exchange by a global unfolding process. Moreover, SAMp73 was very flexible, with most of its amide protons affected by slow mu s-ms conformational exchange. Within this time scale, the residues of SAMp73 with the largest exchange rates (R-ex) were involved in binding with other molecules; therefore, the flexibility in the mu s-ms range was associated with biological functions. As the [GdmCl] increased, the pico-to-nanosecond flexibility of the backbone amide protons raised, but it did so differently depending on the residue. We were able to obtain, for the first time, the linear [GdmCl]-variation of the local conformational entropies, m(Si), which ranged from 5.3 to 0.3 cal mol(-1) K-1 M-1, similar to those measured by using macroscopic techniques in other proteins. Conversely, the temperature dependence of the pico-to-nanosecond dynamics of the backbone amide protons of SAMp73 indicates that the flexibility of some residues decreased with the temperature; these results explain the high thermostability of the protein.