S-100 PROTEIN, BUT NOT CALMODULIN, BINDS TO THE GLIAL FIBRILLARY ACIDIC PROTEIN AND INHIBITS ITS POLYMERIZATION IN A CA2-DEPENDENT MANNER()

S-100 protein, a Ca2+-binding protein of the EF-hand type, interacts w ith the glial fibrillary acidic protein (GFAP) in a Ca2+-dependent man ner. The binding of S-100 protein to GFAP was investigated by fluoresc ence spectroscopy using acrylodan-S-100 protein and cross-linking expe riments 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 pro tein 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/GF AP molar ratio of 0.2 and maximal effect at a molar ratio of 0.5. Iden tical results were obtained irrespective of whether the unfractionated bovine brain S-100 protein mixture (S-100a plus S-100b), S-100a0, S-1 00a, or S-100b was used. S-100 protein was observed to be maximally ef fective as an inhibitor of GFAP polymerization at approximately 3 muM free Ca2+. Calmodulin neither bound to GFAP nor inhibited its polymeri zation. Altogether, the present results suggest that S-100 protein mig ht be involved in the regulation of the state of assembly of glial fil aments by binding to and sequestering unpolymerized GFAP.