The crystal structure of metal-free human EF-hand protein S100A3 at 1.7-Å resolution

S100A3 is a unique member of the EF-hand superfamily of Ca2+-binding proteins. It binds Ca2+ with poor affinity (K-d = 4-35 mm) but Zn2+ with exceptionally high affinity (K-d = 4 nm). This high affinity for Zn2+ is attributed to the unusual high Cys content of S100A3. The protein is highly expressed in fast proliferating hair root cells and astrocytoma pointing toward a function in cell cycle control. We determined the crystal structure of the protein at 1.7 Angstrom The high resolution structure revealed a large distortion of the C-terminal canonical EF-hand, which most likely abolishes Ca2+ binding. The crystal structure of S100A3 allows the prediction of one putative Zn2+ binding site in the C terminus of each subunit of S100A3 involving Cys and His residues in the coordination of the metal ion. Zn2+ binding induces a large conformational change in S100A3 perturbing the hydrophobic interface between two S100A3 subunits, as shown by size exclusion chromatography and CD spectroscopy.