Structural Analysis of S100A8 Complex with Zinc and Calcium: A General Protocol for the Study of S100 Proteins in the Presence of Divalent Cations by X-Ray Crystallography.

Ions are important regulators for the cellular function of many proteins. This holds particularly true for S100 proteins whose function is not only calcium-dependent but also appears to be modulated by other divalent cations such as zinc, manganese, or copper. One way ions are thought to influence the function of S100 proteins (and any protein in general) is by changing their three-dimensional organization, through modifications in either their monomeric shape, their oligomeric state, or both. X-ray crystallography is a very powerful technique to study the effect of ions on the 3D architecture of macromolecules since it gives a direct visualization of where ions bind and how the protein structure is affected upon ion binding. Taking the example of human S100A8, I describe here the complete procedure to obtain a highly pure and homogenous S100 protein sample, crystallize it in the presence of divalent cations, and derive a 3D structural model from diffraction images. I further detail computational methods used to determine precisely the nature and position of the divalent cations within S100A8 structure. This methodology can easily be applied to any ion-binding protein, provided that the ion anomalous scattering properties allow to identify it unambiguously.