Fig. 3. Huntingtin coassembles with purified microtubules. A rat brain homogenate was clarified by high-speed centrifugation and the soluble tubulin present in the supernatant was allowed to polymerize in the presence of paclitaxel and GTP. Microtubules were then sedimented through a sucrose cushion. The pellet (P) containing microtubules and the supernatant (S) were examined by immunoblotting using monoclonal antibodies directed against either ß-tubulin, huntingtin, MAP-2, actin or caspase-1. Huntingtin and MAP-2 cosediment with microtubules, whereas actin and caspase-1, two proteins not known to be associated with microtubules, remained virtually entirely in the supernatant. In the absence of paclitaxel, no microtubules were formed and both huntingtin and MAP-2 remained in the supernatant.