Brain tubulin polymerized with dynein isolated from bull spermatozoa forms cold-stable microtubules, in contrast with microtubules made of brain tubulin polymerized by brain microtubule-associated proteins (MAPs). The level of cold-stable microtubules depends on the concentration of dynein used. Addition of dynein to cold-unstable microtubules renders these microtubules stable to cold. Although ATP and a non-hydrolysable ATP analogue increase the formation of microtubules made of tubulin and dynein, these nucleotides have no effect on dynein cold-stabilizing properties. The data suggests that a new factor, not involving the dynein ATPase active site and present in bull sperm dynein preparations, confers cold-stability to microtubules.

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