Abstract
Treatment of neurons with taxol leads to the formation of microtubule bundles in which individual microtubules are much more closely spaced than in untreated neurons (Letourneau and Ressler, 1984). This suggests that taxol interferes with the mechanisms that regulate microtubule spacing in situ. I have determined whether treatment of neurons with taxol alters the composition of their microtubules, reasoning that such alterations may be related to the taxol-induced alterations in microtubule spacing. Cultures of sympathetic neurons were incubated with 35S-methionine and either taxol, podophyllotoxin, a potent microtubule-depolymerizing agent, or dimethyl sulfoxide (DMSO), the solvent for the drugs. The levels of labeled microtubule-associated proteins (MAPs) assembled into microtubules in the various cultures were then assayed biochemically. I focused on 4 MAPs: tau, chartins, MAP-2, and the MAP with a molecular mass of 210,000 Da (210 kDa). In untreated cultures, these MAPs are prominent components of microtubules. The levels of all MAPs, as well as tubulin, in microtubules were greatly reduced in cultures treated with podophyllotoxin, compared to controls. Taxol had varied effects on the interaction of MAPs with microtubules in situ. Microtubules formed in the presence of taxol contained normal or slightly elevated levels of tau and the 210 kDa MAP compared to microtubules in control cultures. In contrast, microtubules formed in the presence of taxol were almost completely devoid of chartin MAPs and MAP-2 compared to controls. These results show that taxol interferes with the interaction of some, but not all, MAPs with microtubules in situ. The altered MAP composition of microtubules in taxol-treated neurons may contribute to the abnormal spacing of microtubules seen in such neurons.
