S100 protein, but not calmodulin, binds to and inhibits the polymerization of glial fibrillary acidic protein in a Ca2+-dependent manner.

S-100 protein, a Ca(2+)-binding protein of the EF-hand type, interacts with the glial fibrillary acidic protein (GFAP) in a Ca(2+)-dependent manner. The binding of S-100 protein to GFAP was investigated by fluorescence spectroscopy using acrylodan-S-100 protein and cross-linking experiments using the bifunctional cross-linker, disuccinimidyl suberate. The binding affinity was observed to be in the nanomolar range with a stoichiometry of 2 mol of GFAP/mol of S-100 protein (dimer). S-100 protein was found to inhibit the polymerization of GFAP in a dose- and Ca(2+)-dependent manner, with a half-maximal effect at an S-100 protein/GFAP molar ratio of 0.2 and maximal effect at a molar ratio of 0.5. Identical results were obtained irrespective of whether the unfractionated bovine brain S-100 protein mixture (S-100a plus S-100b), S-100ao, S-100a, or S-100b was used. S-100 protein was observed to be maximally effective as an inhibitor of GFAP polymerization at approximately 3 microM free Ca2+. Calmodulin neither bound to GFAP nor inhibited its polymerization. Altogether, the present results suggest that S-100 protein might be involved in the regulation of the state of assembly of glial filaments by binding to and sequestering unpolymerized GFAP.