The white button mushroom Agaricus bisporus is one of the most widely cultivated mushrooms in the world and an important component of human diet. Apart from its high nutritional value, it has a significant ecological role (e.g. involvement in the decay of a leaf litter). Here we report the structure-functional characterization of 45 kDa dimeric protein from A. bisporus. The protein shares no detectable sequence homology to any protein of known function and it is the first characterized member of the protein family. The cDNA sequence was amplified by the reverse transcription PCR and the protein was expressed in E. coli in a recombinant form. The X-ray structure was solved to a resolution of 1.6 A and it showed an elongated helical bundle structure with rather low structural similarity to other proteins. Using different biophysical techniques (AUC, CD, DSF) it was shown that the protein displays high thermostability and is stable over a wide range of pH. Finally, with DSC and ITC we identified and characterized its ability to bind divalent heavy metal ions (nickel, zinc, cadmium and cobalt), which might suggest its possible role in homeostasis, regulational pathways or sequestration and removal of heavy metals.