Journal:JBIC:3

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S100A16 is a special S100 member because it <scene name='Journal:JBIC:3/Calcium_binding_start/6'>does not perform a significant conformational change</scene> upon <scene name='Journal:JBIC:3/Binding_calcium/2'>calcium(II) binding</scene>. This was observed after determination of the solution structures of apo and calcium(II)-bound S100A16 and the crystal structure of apo S100A16. The likely reason is the lower calcium binding affinity and <scene name='Journal:JBIC:3/Hydrophobic_residues/1'>stronger hydrophobic interaction between helix III and IV</scene> present in this protein with respect to other S100 proteins. Another characteristic of S100A16 is that the helix IV has the same length in both apo and calcium(II) forms <scene name='Journal:JBIC:3/Gly-gly-ile-thr-gly-pro/1'>because of the presence</scene> of a <scene name='Journal:JBIC:3/Gly-gly-ile-thr-gly-pro_zoom/1'>Gly-Gly-Ile-Thr-Gly-Pro sequence motif</scene>. Based on the available structures of S100 members, we analyzed and summarized all their conformational changes due to calcium(II) binding by a principal component analysis. Calcium binding was proved by both NMR titration and Isothermal Titration Calorimetry (ITC) experiments. Even if the <scene name='Journal:JBIC:3/Gly_without/1'>important Glu residue in the last position of first EF-hand calcium binding loop is missing</scene>, these experimental data indicated that S100A16 can <scene name='Journal:JBIC:3/Gly_without_full/2'>still bind one calcium(II) ion in such loop</scene>. NMR relaxation studies showed that the first calcium binding loop and the beginning of the second helix are the most flexible regions in both the apo and calcium(II)-bound S100A16. Although the biological function of S100A16 is still unclear yet, these structural and dynamic properties can provide useful information for further functional studies.