Structural characterization of human S100A16, a low-affinity calcium binder
Elena Babini • Ivano Bertini • Valentina Borsi • Vito Calderone • Xiaoyu Hu • Claudio Luchinat • Giacomo Parigi[1]
Molecular Tour
S100A16 is a special member of the S100 class of calcium binding proteins, because it performs a conformational change upon calcium(II) binding much smaller than experienced by most S100 proteins. 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 for minimal conformational change in S100A16 is the lower calcium binding affinity and stronger hydrophobic interaction between helix III and IV 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 because of the presence of a Gly-Gly-Ile-Thr-Gly-Pro sequence motif in helix IV. 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 important Glu residue in the last position of first EF-hand calcium binding loop is missing, these experimental data indicated that S100A16 can still bind one calcium(II) ion in such loop. 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.
PDB references: X-ray structure of the apo form of human S100A16 3nxa; Solution structure of calcium bound S100A16 2l51; Solution structure of apo S100A16 2l50.
References
- ↑ Babini E, Bertini I, Borsi V, Calderone V, Hu X, Luchinat C, Parigi G. Structural characterization of human S100A16, a low-affinity calcium binder. J Biol Inorg Chem. 2010 Nov 3. PMID:21046186 doi:10.1007/s00775-010-0721-3
