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S100A8 and S100A9 are two calcium binding Myeloid Related Proteins, and important mediators of inflammatory diseases. They were recently introduced as partners for phagocyte NADPH oxidase regulation. However, the precise mechanism of their interaction remains elusive. We had for aim (i) to evaluate the impact of S100 proteins on NADPH oxidase activity; (ii) to characterize molecular interaction of either S100A8, S100A9, or S100A8/S100A9 heterocomplex with cytochrome  b   558 ; and (iii) to determine the S100A8 consensus site involved in cytochrome  b   558 /S100 interface. Recombinant full length or S100A9-A8 truncated chimera proteins and ExoS-S100 fusion proteins were expressed in  E. coli  and in  P. aeruginosa  respectivel y . Our results showed that S100A8 is the functional partner for NADPH oxidase activation contrary to S100A9, however, the loading with calcium and a combination with phosphorylated S100A9 are essential  in vivo . Endogenous S100A9 and S100A8 colocalize in differentiated and PMA stimulated PLB985 cells, with Nox2/gp91 phox  and p22 phox . Recombinant S100A8, loaded with calcium and fused with the first 129 or 54 N-terminal amino acid residues of the  P. aeruginosa  ExoS toxin, induced a similar oxidase activation  in vitro,  to the one observed with S100A8 in the presence of S100A9  in vivo . This suggests that S100A8 is the essential component of the S100A9/S100A8 heterocomplex for oxidase activation. In this context, recombinant full-length rS100A9-A8 and rS100A9-A8 truncated 90 chimera proteins as opposed to rS100A9-A8 truncated 86 and rS100A9-A8 truncated 57 chimeras, activate the NADPH oxidase function of purified cytochrome  b   558  suggesting that the C-terminal region of S100A8 is directly involved in the molecular interface with the hemoprotein. The data point to four strategic  87 HEES 90  amino acid residues of the S100A8 C-terminal sequence that are involved directly in the molecular interaction with cytochrome  b 558   and then in the phagocyte NADPH oxidase activation.

Molecular Interface of S100A8 with Cytochrome b558 and NADPH Oxidase Activation