R. Bianchi et al., S-100 PROTEIN, BUT NOT CALMODULIN, BINDS TO THE GLIAL FIBRILLARY ACIDIC PROTEIN AND INHIBITS ITS POLYMERIZATION IN A CA2-DEPENDENT MANNER(), The Journal of biological chemistry, 268(17), 1993, pp. 2669-2674
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.