Mixtures of pre-formed microtubules, polymerized from chicken erythrocyte and brain tubulin, rapidly anneal end-to-end in vitro in standard microtubule assembly buffer. The erythrocyte tubulin segments in annealed heteropolymers can be distinguished by an immunoelectron microscopic assay that uses an antibody specific for chicken erythrocyte beta-tubulin. An annealing process is consistent with the following observations: (a) Microtubule number decreases while the polymer mass remains constant. (b) As the total number of microtubules declines, the number of heteropolymers, and the number of segments contained in each heteropolymer, increases. (c) The size of the segments determined after annealing and antibody labeling is the same as the original microtubule polymers. (d) Points of discontinuity in the annealing heteropolymers can be observed directly by electron microscopy, and correspond to type-specific polymer domains. The junctions probably represent initial contact points during the annealing process. Microtubule annealing occurs rapidly in vitro and may be significant for determining properties of microtubule dynamics in vivo.

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