Interactions of S100 Proteins with Proteins Kinase Substrates. Biological Implication

Abstract

S-100 proteins are a group of low molecular weight (10 kDa) acidic proteins highly concentrated in brain tissues (for a recent review see Kligman and Hild, 1988). S100 proteins purified from bovine brain, are a mixture of hetero- and homodimer of two types of subunit, α and ß with different amino acid composition (Isobe et al., 1977). The amino acid sequence of the α and ß subunits revealed the structural relationship of S100 with the calcium binding proteins of the EF-hand type (Isobe and Okuyama, 1978). Both subunits have one 30-residue putative EF-hand calcium binding domain (site I) in the N-terminal part and one typical 28-residue domain (site II) in the C-terminal part. Calcium-binding studies on bovine brain S100αα and S100ßß (S100b) proteins confirmed the presence of two specific calcium-binding sites per subunits (Baudier et al., 1986a). The calcium binding sequence on the α and ß subunit have been studied by means of intrinsic fluorescence and absorption spectroscopy, binding of the Ca²⁺ analoge Tb³⁺ and H-NMR (in preparation), and showed that in both cases saturation of the typical Ca²⁺ -binding sites (site II) occured first followed by the binding of Ca²⁺ to the putative Ca²⁺ - binding site (site I). In the absence of monovalent cation the affinities of the typical sites II α and IIß range between 10–20 μM and those of the putative sites Iα and Iß range between 100–400 μM. In the presence of physiological intracellular KCl concentrations the S100 protein affinities for calcium drastically decrease to 500 μM -1 mM, which become probably not compatible with intracellular calcium concentrations. However, it has been shown that S100 protein affinities for calcium may also depend greatly on the quaternary and tertiary protein structures. Conformational effectors such as Zn²⁺ ions in the peculiar case of S100b, alkylation of Cys 85α or Cys 84ß, or interaction with target protein were proved to greatly increase the calcium binding affinities of sites II α and IIß in the range of .5 –10 μM and to decrease the antagonistic effect of KCl on calcium binding (Baudier et al., 1986a, 1986b, 1987a).