Fig. 7. Schematic diagram of protein–protein interactions mediated by X-type four-helix bundles. RIIα is unique in binding one protomer per dimer at a surface hydrophobic groove. HNF-1α presents a hydrophobic face and forms a dimer of dimers. Likewise, the stoichiometry of P53 binding to S100 is two peptides per dimer. The binding interactions with SAM are under investigation.