Suppression of copper-induced cellular damage by copper sequestration with S100b protein

We previously reported that S100b protein (homodimer of S100 beta subunit) can bind copper ions with a submicromolar dissociation constant (T. Nishikawa et al., J. Biol. Chem. 272, 23037-23041, 1997). In this study, a question was addressed as to whether this protein can sequester copper ions in an in vivo situation. Escherichia coli cells that had been rendered able to produce a fusion protein of rat S100 beta subunit with glutathione S-transferase displayed a marked resistance to cellular damage induced by copper alone or its combination with H2O2, compared with control cells expressing the transferase moiety only. A study by gel chromatography showed that about half of the expressed S100 beta fusion protein in the cytosol of copper-treated cells was eluted in the void volume fraction (molecular mass > 200 kDa), which contained most of the copper incorporated. The S100 beta fusion protein purified from the void volume fraction was found to contain 82% of the total copper in the fraction, while in a parallel experiment with the control cells, the glutathione S-transferase eluted in the void volume fraction contained only 18% of the total copper. Thus, it is clear that extraneously expressed S100b protein can acts as a "copper sink," thereby protecting E. coli cells from copper-induced cellular damage.