TAU CONFERS DRUG STABILITY BUT NOT COLD STABILITY TO MICROTUBULES IN LIVING CELLS

We previously defined two classes of microtubule polymer in the axons of cultured sympathetic neurons that differ in their sensitivity to no codazole by roughly 35-fold (Baas and Black (1990) J. Cell Biol. 111, 495-509). Here we demonstrate that virtually all of the microtubule po lymer in these axons, including the drug-labile polymer, is stable to cold. What factors account for the unique stability properties of axon al microtubules? In the present study, we have focused on the role of tan, a microtubule-associated protein that is highly enriched in the a xon, in determining the stability of microtubules to nocodazole and/or cold in living cells. We used a baculovirus vector to express very hi gh levels of tau in insect ovarian Sf9 cells. The cells respond by ext ending processes that contain dense bundles of microtubules (Knops et al. (1991) J. Cell Biol. 114, 725-734). Cells induced to express tau w ere treated with either cold or 2 mu g/ml nocodazole for times ranging from 5 minutes to 6 hours. The results with each treatment were very different from one another. Virtually all of the polymer was depoly-me rized within the first 30 minutes in cold, while little or no microtub ule depolymerization was detected even after 6 hours in nocodazole. Ba sed on these results, we conclude that tau is almost certainly a facto r in conferring drug stability to axonal microtubules, but that factor s other than or in addition to tau are required to confer cold stabili ty.