Human S100b protein: Formation of a tetramer from synthetic calcium‐binding site peptides

Abstract

Human brain S100b protein is a unique calcium‐binding protein comprised of two identical 91‐amino acid polypeptide chains that each contain two proposed helix‐loop‐helix (EF‐hand) calcium‐binding sites. In order to probe the assembly of the four calcium‐binding sites in S100b, a peptide comprised of the N‐terminal 46 residues of S100b protein was synthesized and studied by CD and 1H NMR spectroscopies as a function of concentration and temperature. At relatively high peptide concentrations and in the absence of calcium, the peptide exhibited a significant proportion of α‐helix (45%). Decreasing the peptide concentration led to a loss of α‐helix as monitored by CD spectroscopy and coincident changes in the 1H NMR spectrum. These changes were also observed by 1H NMR spectroscopy as a function of temperature where it was observed that the Tm of the peptide was lowered approximately 14 °C with a 17‐fold decrease in peptide concentration. Sedimentation equilibrium studies were used to determine that the peptide formed a tetramer in solution in the absence of calcium. It is proposed that this tetrameric fold also occurs in S100b and is a result of the interaction of portions of all four calcium‐binding sites. Copyright © 1995 The Protein Society