The effect of S-100a and S-100b proteins and Zn2+ on the assembly of brain microtubule proteins in vitro.

The homologous proteins S-100a and S-100b affect the microtubule system in a distinctly different way in the presence of low molar ratios of Zn2+. Assembly of brain microtubule proteins can be almost completely inhibited and rapid disassembly can be induced by low molar amounts of S-100b in the presence of low molar ratios [2-4] of Zn2+. Higher molar ratios per S-100b (greater than 4) potentiate the general Zn2+ effect, promoting the formation of sheets of microtubules. However, the effect of S-100a is quite different, no inhibition of assembly can be observed and the presence of S-100a seems to protect the microtubule proteins against the effect of Zn2+ by chelating the Zn2+ and decreasing the free metal-ion concentration. S-100a or S-100b cannot bind to the microtubule polymer-form, either in the absence or in the presence of Zn2+.