Structural dissection of the C-terminal sterile alpha motif (SAM) of human p73

The alpha splice variant of p73 (p73α), a homologue of the tumour suppressor p53, has at its C terminus a sterile alpha motif (SAM); this domain, SAMp73, is involved in lipid binding and it is thought to mediate in protein-protein interactions. SAMp73 is composed of five helices (α1-α5). In this work, we dissected SAMp73 in fragments encompassing the different helices, to study the conformational stability of the isolated elements of secondary structure. There was no evidence of stable residual helical structure in the isolated α1, α4 and α5 helices in aqueous solution, as shown by 2D-(1)H NMR and far-UV CD spectroscopies; those helices acquired native-like helical structure in the presence of 40% trifluoroethanol (TFE). The population of helical structure in α5 seemed to be driven by the indole moiety of Trp542, and it was enhanced by the presence of α4. On the other hand, helices α2 and 310(α3) had a tendency to self-associate even in TFE-water solutions. However, the short, aggregation-prone 310(α3) helix was key to attain the native-like fold of SAMp73, as suggested by experiments with non-covalent complexes among the peptides.