Crystallization and calcium/sulfur SAD phasing of the human EF-hand protein S100A2

Human S100A2 is an EF-hand protein and acts as a major tumour suppressor, binding and activating p53 in a Ca²⁺-dependent manner. Ca²⁺-bound S100A2 was crystallized and its structure was determined based on the anomalous scattering provided by six S atoms from methionine residues and four calcium ions present in the asymmetric unit. Although the diffraction data were recorded at a wavelength of 0.90 Å, which is usually not assumed to be suitable for calcium/sulfur SAD, the anomalous signal was satisfactory. A nine-atom substructure was determined at 1.8 Å resolution using SHELXD, and SHELXE was used for density modification and phase extension to 1.3 Å resolution. The electron-density map obtained was well interpretable and could be used for automated model building by ARP/wARP.