Detyrosinated (Glu) tubulin was prepared from porcine brain and microinjected into human fibroblasts and Chinese hamster ovary (CHO) cells. Glu tubulin assembled onto the ends of preexisting microtubules and directly from the centrosome within minutes of its microinjection. Incorporation into the cytoskeleton continued until almost all of the microtubules were copolymers of Glu and tyrosinated (Tyr) tubulin. However, further incubation resulted in the progressive and ultimately complete loss of Glu-staining microtubules. Glu tubulin injected into nocodazole-treated cells was converted to Tyr tubulin by a putative tubulin/tyrosine ligase activity. The observed decrease in staining with the Glu antibody over time was used to analyze microtubule turnover in microinjected cells. The mode of Glu disappearance was analyzed quantitatively by tabulating the number of Glu-Tyr copolymers and Tyr-only microtubules at fixed times after injection. The proportion of Glu-Tyr copolymers decreased progressively over time and no segmentally labeled microtubules were observed, indicating that microtubules turn over rapidly and individually. Our results are consistent with a closely regulated tyrosination-detyrosination cycle in living cells and suggest that microtubule turnover is mediated by dynamic instability.

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