Structural Insight into Human Zn2+-bound S100A2 from NMR and Homology Modeling

The S100 subfamily of EF-hand proteins is distinguished by the binding of Zn2+ in addition to Ca2+. In an effort to understand the role of Zn2+ in modulating the activity of S100 proteins, we have carried out heteronuclear NMR studies of Zn2+-bound S100A2 and obtained near complete resonance assignments. This analysis revealed an equilibrium between multiple isoforms due to cis-trans isomerism of proline residues in flexible regions of the protein. The secondary structure of S100A2 has been determined based on the NMR chemical shift index (CSI) technique. The protein is found to possess essentially the same secondary structure found in other S100 proteins such as S100A6 and S100B. Homology models have been built based on the high resolution three-dimensional structures of other S100 proteins. The models predict two Zn2+ binding clusters, one involving residues His17-Cys21-Cys93 and the other Cys2-His39, and with Cys86 participating in either the N-terminal or the C-terminal binding site.