Quasielastic light scattering studies of tubulin aggregation

A quasielastic light scattering study of purified tubulin has resulted in a monomer diffusion coefficient of 6.0 X 10(-7) cm2/s. In an attempt to characterize the small tubulin oligomers which are predicted to form as intermediates in the self-assembly into double rings, magnesium ions (10 mM) were incorporated into the PG (10 mM NaPi, 0.1 mM GTP, pH 7.0) buffer. With magnesium-containing buffers, a second slow diffusing species was identified with D = 0.56 X 10(-7) cm2/s. This corresponds to the tubulin double rings. A study of the small oligomers was precluded by the presence in all preparations of traces of aggregates resulting from the slow aging process of the protein (V. Prakash and S. N. Timasheff (1982) J. Mol. Biol. 160, 499-515). While colchicine was shown to inhibit this aging process, the presence of these irreversible aggregates, even to the extent of 1% by mass, precludes equilibrium studies of the self-association of tubulin into small oligomers.