Purification and crystallization of the human EF-hand tumour suppressor protein S100A2

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 A, 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 A resolution using SHELXD, and SHELXE was used for density modification and phase extension to 1.3 A resolution. The electron-density map obtained was well interpretable and could be used for automated model building by ARP/wARP.

S100A2 in cancerogenesis: a friend or a foe?

Owing to the exceptional intracellular distribution and the heterogeneous expression pattern during transformation and metastasis in various tumors, the EF-hand calcium-binding protein S100A2 attracts increasing attention. Unlike the majority of S100 proteins, S100A2 expression is downregulated in many cancers and the loss in nuclear expression has been associated with poor prognosis. On the other hand, S100A2 is upregulated in some cancers. This mini review highlights the general characteristics of S100A2 and discusses recent findings on its putative functional implication as a suppressor or promoter in cancerogenesis.

Metal ions modulate the folding and stability of the tumor suppressor protein S100A2

The EF-hand protein S100A2 is a cell cycle regulator involved in tumorigenesis, acting through regulation of the p53 activation state. Metal ion-free S100A2 is homodimeric and contains two Ca(2+)-binding sites and two Zn(2+)-binding sites per subunit, whereby the Zn(2+) ion binding to one of the sites is coordinated by residues from two homodimers. The effect of selective binding of these metal ions was investigated using site-specific mutants which lacked one or both zinc sites. CD analysis of secondary structure changes on metallation showed that Zn(2+) binding was associated with a decrease in the secondary structure content, whereas Ca(2+) had the opposite effect in two of the three S100A2 mutants studied. The energy of unfolding (DeltaG(U)) of the apo wild-type S100A2 was determined to be 89.9 kJ mol(-1), and the apparent midpoint transition temperature (T(m)(app))) was 58.4 degrees C. In addition, a detailed study of the urea and thermal unfolding of the S100A2 mutants in different metallation states (apo, Zn(2+) and Ca(2+)) was performed. Thermal denaturation experiments showed that Zn(2+) acts as a destabilizer and Ca(2+) as a stabilizer of the protein conformation. This suggests a synergistic effect between metal binding, protein stability and S100A2 biological activity, according to which Ca(2+) activates and stabilizes the protein, the opposite being observed on Zn(2+) binding.