Rat Brain S100b Protein: Purification, Characterization, and Ion Binding Properties. A Comparison with Bovine S100b Protein

We purified to homogeneity rat brain S100b protein, which constitutes about 90% of the soluble S100 protein fraction. Purified rat S100b protein comigrates with bovine S100b protein in nondenaturant system electrophoresis but differs in its amino acid composition and in its electrophoretic mobility in urea‐sodium dodecyl sulfate‐polyacrylamide gel with bovine S100b protein. The properties of the Ca2+ and Zn2+ binding sites on rat S100b protein were investigated by flow dialysis and by fluorometric titration, and the conformation of rat S100b in its metal‐free form as well as in the presence of Ca2+ or Zn2+ was studied. The results were compared with those obtained for the bovine S100b protein. In the absence of KCI, rat brain S100b protein is characterized by two high‐affinity Ca2+ binding sites with a KD of 2 + 10−5M and four lower affinity sites with KD about 10−4M. The calcium binding properties of rat S100b protein differ from bovine S100b only by the number of low‐affinity calcium binding sites whereas similar Ca2+ ‐induced conformational changes were observed for both proteins. In the presence of 120 mM KCI rat brain S100b protein bound two Zn2+‐ ions/mol of protein with a KD of 10 −7M and four other with lower affinity (KD+ 10−6M). The occupancy of the two high‐affinity Zn2+ binding sites was responsible for most of the Zn2+‐induced conformational changes in the rat S100b protein. No increase in the tyrosine fluorescence quantum yield after Zn2+ binding to rat S100b was observed. Such results significantly differ from that obtained with bovine S100b protein, which on binding of eight Zn2+ equivalents/mol undergoes maximal conformational changes associated with a sixfold increase of the tyrosine fluorescence quantum yield. Finally, we report in this work that zinc binding on rat S100b protein regulates calcium binding by increasing the calcium affinity of the protein and reducing the antagonistic effect of K+ on calcium binding.