Homogeneous brain tubulin, free of microtubule associated proteins, forms microtubules in polymerization buffers containing 7.5--15% dextran T10 or 2--4.5% poly(ethylene glycol) type 6000. In both systems the resulting microtubules are sensitive to colchicine and low temperature. The critical tubulin concentration in both polymerization systems can be as low as 0.25 mg/ml.
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