Toward an understanding of the conformational plasticity of S100A8 and S100A9 Ca2+-binding proteins

S100A8 and S100A9 are small, human, Ca2+-binding pro-teins with multiple intracellular and extracellular functions in signaling, regulation, and defense. The two proteins are not detected as monomers but form various noncovalent homo-or hetero-oligomers related to specific activities in human physi-ology. Because of their significant roles in numerous medical conditions, there has been intense research on the conforma-tional properties of various S100A8 and S100A9 proteoforms as essential targets of drug discovery. NMR or crystal structures are currently available only for mutated or truncated protein complexes, mainly with bound metal ions, that may well reflect the proteins' properties outside cells but not in other biological contexts in which they perform. Here, we used structural mass spectrometry methods combined with molecular dynamics simulations to compare the conformations of wildtype full-length S100A8 and S100A9 subunits in biologically relevant homo-and heterodimers and in higher oligomers formed in the presence of calcium or zinc ions. We provide, first, ratio-nales for their functional response to changing environmental conditions, by elucidating differences between proteoforms in flexible protein regions that may provide the plasticity of the binding sites for the multiple targets, and second, the key factors contributing to the variable stability of the oligomers. The described methods and a systematic view of the confor-mational properties of S100A8 and S100A9 complexes provide a basis for further research to characterize and modulate their functions for basic science and therapies.

Automated summary

S100A8 and S100A9 are small calcium-binding proteins with important roles in signaling, regulation, and defense. Through structural mass spectrometry methods and simulations, this study compares the conformational properties of wildtype full-length S100A8 and S100A9 subunits in biologically relevant homo-and heterodimers and higher oligomers. It provides insights into the functional responses of these proteins to changing environmental conditions and identifies key factors contributing to the stability of the oligomers. These findings lay the foundation for further research on the functions and modulation of S100A8 and S100A9 complexes for basic science and therapies.