Differential turnover of tyrosinated and detyrosinated microtubules.

Turnover of tyrosinated and detyrosinated microtubules (TyrMTs and GluMTs, respectively) was analyzed by the combined use of hapten-mediated immunocytochemistry and peptide-specific antibodies. Cells were microinjected with hapten-labeled tubulin and then processed for triple-label immunofluorescence to determine the pattern of incorporation of the injected subunits into Tyr- and Glu-MTs. Within 2 min of microinjection, hapten-labeled domains were present at the ends of virtually all TyrMTs but were absent from most GluMTs, demonstrating that TyrMTs grew, whereas most GluMTs did not. After 1 hr of incubation, all TyrMTs analyzed were copolymers of endogenous and hapten-labeled subunits, indicating complete and rapid turnover of these MTs. However, the majority of GluMTs were not hapten-labeled, indicating that they had not turned over. Even 16 hr after injection, cells that had not divided retained a small proportion of GluMTs lacking hapten, implying that some had persisted for most of a cell generation. At mitosis, all MTs were hapten-labeled, indicating that the stable interphase GluMTs had depolymerized. The results establish that the MT network is heterogeneous in its turnover rate, being composed of at least two populations: TyrMTs that turn over rapidly and GluMTs that turn over slowly.