Effects of Betaine, Taurine and Isethionate on Microtubule Assembly(Cell Biology)

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Two amino acid analogues, betaine and taurine, and isethionate which are major low molecular weight substances in the axoplasm of squid giant axon were studied for their pronounced effects on microtubule assembly. Betaine stabilized assembled microtubules, while both taurine and isethionate exhibited inhibitory effect on microtubule assembly, showing half maximal inhibitions at 0.5 M and 0.2 M, respectively. SDS-polyacrylamide gel electrophoresis revealed that betaine increases the contents of microtubule-associated proteins (MAPs) in the assembled microtubules while taurine causes decrease in their contents. Isethionate seemed to cause little change in the ratio of MAPs to tubulin, although less microtubules were formed with increasing isethionate concentration. The inhibitory effect of isethionate was overcome by stabilizing action of betaine as measured by turbidity change as well as by protein analyses after centrifugation. Upon measuring vinblastineinduced inhibition of microtubule assembly, taurine and isethionate accelerated inhibition while betaine diminished the vinblastine-induced inhibition. Like glycerol or dimethylsulfoxide, betaine initiated tubulin polymerization in the total absence of MAPs. Electron microscopic observation revealed that betaine caused polymerization of tubulin into microtubules with none of sheets or ribbons, being different from the action of glycerol or dimethylsulfoxide.
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