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Depolymerization products of purified microtubules from porcine brain were examined by sedimentation analysis and electron microscopy. The complete depolymerization mixture exhibited 36S and 6S components in concentration-dependent equilibrium, whether depolymerization was caused by low temperature or by calcium ion. These components were recognized by electron microscopy as spirals and rings, and small particles. Agarose column chromatography yielded two major fractions, a leading one comprising mostly 36S and some 6S material and a trailing one of solely 6S material. The latter had high specific colchicine-binding activity and no tendency to polymerize. For the leading peak material these properties were the converse. It is proposed that tubulin molecules (of mass 110,000 daltons) exist in two states, here called X and Y, with those of the X-state equivalent to the material found predominantly in the trailing fraction, and those of the Y-state equated with the material in the leading fraction. Participation of tubulin molecules in microtubule assembly and disassembly is discussed, based on the assumption that those of both states have longitudinal and lateral binding domains whose strengths differentially depend upon temperature and calcium-ion concentration.

Microtubules from Mammalian Brain: Some Properties of Their Depolymerization Products and a Proposed Mechanism of Assembly and Disassembly