Chlorpromazine inhibits the calcium-mediated effects of S-100 protein(s) on assembled brain microtubule proteins, but not those on microtubule protein assembly.

We have examined the S-100-chlorpromazine interplay at the level of brain microtubule proteins in vitro. The results indicate that in the presence of 0.12 M KCl and 10 microM free Ca2+ the inhibitory effect of S-100 on microtubule assembly is additive to that of chlorpromazine, but S-100 fails to potentiate the disassembling effect of 0.1 mM Ca2+ if added to assembled microtubule proteins after chlorpromazine and Ca2+, probably because of inhibition of S-100 by the phenothiazine. Chlorpromazine does not compete with S-100 for binding to purified tubulin.