Small Molecule Inhibitors of Ca2+-S100B Reveal Two Protein Conformations

The drug pentamidine inhibits calcium-dependent complex formation with p53 ((Ca)S100Bp53) in malignant melanoma (MM) and restores p53 tumor suppressor activity in vivo. However, off-target effects associated with this drug were problematic in MM patients. Structure-activity relationship (SAR) studies were therefore completed here with 23 pentamidine analogues, and X-ray structures of (Ca)S100Binhibitor complexes revealed that the C-terminus of S100B adopts two different conformations, with location of Phe87 and Phe88 being the distinguishing feature and termed the FF-gate. For symmetric pentamidine analogues ((Ca)S100B5a, (Ca)S100B6b) a channel between sites 1 and 2 on S100B was occluded by residue Phe88, but for an asymmetric pentamidine analogue ((Ca)S100B17), this same channel was open. The (Ca)S100B17 structure illustrates, for the first time, a pentamidine analog capable of binding the open form of the FF-gate and provides a means to block all three hot spots on (Ca)S100B, which will impact next generation (Ca)S100Bp53 inhibitor design.