Analysis of the structure of human apo-S100B at low temperature indicates a unimodal conformational distribution is adopted by calcium-free S100 proteins.

S100B is one of the best-characterized members of the calcium-signaling S100 protein family. Most S100 proteins are dimeric, with each monomer containing two EF-hand calcium-binding sites (EF1, EF2). S100B and other S100 proteins respond to calcium increases in the cell by coordinating calcium and undergoing a conformational change that allows them to interact with a variety of cellular targets. Although several three dimensional structures of S100 proteins are available in the calcium-free (apo-) state it has been observed that these structures appear to adopt a wide range of conformations in the EF2 site with respect to the positioning of helix III, the helix that undergoes the most dramatic calcium-induced conformational change. In this work, we have determined the structure of human apo-S100B at 10 degrees C to examine whether temperature might be responsible for these structural differences. Further, we have used this data, and other available apo-S100 structures, to show that despite the range of interhelical angles adopted in the apo-S100 structures, normal Gaussian distributions about the mean angles found in the structure of human apo-S100B are observed. This finding, only obvious from the analysis of all available apo-S100 proteins, provides direct structural evidence that helix III is a loosely packed helix. This is likely a necessary functional property of the S100 proteins that facilitates the calcium-induced conformational change of helix III. In contrast, the calcium-bound structures of the S100 proteins show significantly smaller variability in the interhelical angles. This shows that calcium binding to the S100 proteins causes not only a conformational change but results in a tighter distribution of helices within the EF2 calcium binding site required for target protein interactions.