Metal analysis by energy dispersive x-ray fluorescence of bovine brain tubulin and microtubule-associated proteins prepared by phosphocellulose chromatography

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+.

Characterization of acid and alkaline phosphatase activity in preparations of tubulin and microtubule-associated proteins

Acid and alkaline phosphatase activity, determined by the hydrolysis of p-nitrophenyl phosphate, was found in preparations of microtubules purified from bovine brain by temperature-dependent assembly-disassembly and ion-exchange chromatography. Phosphocellulose-purified tubulin contained an associated acid phosphatase activity, stimulated by Mg2+ and by Zn2+. Alkaline phosphatase activity with a pH optimum of 10.4 was measured in a fraction of microtubule-associated proteins (MAPs). Kinetics and the effects of sodium fluoride, sodium tartrate, sulfhydryl-blocking agents, EDTA and Zn2+ are reported.

Zinc binding to tubulin

1. Zinc binding to tubulin was studied by incubating microtubules with 65Zn and measuring the 65Zn retained with tubulin on DEAE Sephadex. 2. Data showed two classes of binding site, one of very low affinity and a second class binding 0.86 g-atoms Zn/mol tubulin with a dissociation constant of approx. 110 microM. 3. Binding of zinc to the higher affinity site was 50% inhibited by 2 mM N-ethyl maleimide.

Zinc-stimulated microtubule assembly and evidence for zinc binding to tubulin

1. Microtubule reassembly was studied in supernatant fluids from rat brain. Tubulin in extracts from zinc-deficient animals showed an impaired ability to repolymerize compared to extracts from controls; 10 microM zinc stimulated reassembly of tubulin in extracts from zinc-deficient animals. 2. Low zinc concentrations (250-900 microM zinc in the presence of 1 mM EGTA) stimulated reassembly of tubulin in brain extracts from control rats; similar concentrations of nickel had no effect whilst cobalt was inhibitory. In the absence of EGTA 20-40 microM zinc stimulated reassembly in brain extracts from normal rats. 3. Zinc-induced changes in reassembly were associated with changes in the free sulphydryl group content of the assembled crude microtubule protein; increased assembly was associated with a higher free sulphydryl group content, decreased assembly with a lower content. 4. 65Zn was found to bind to tubulin. This binding was partly inhibited by N-ethylmaleimide.