Mutated C448Y spastin isoforms do not sever MTs. Control RFL-6/Tet-On cells and stably transfected TRE/M87 WT, TRE/M87 CY, or TRE/M1 CY cells were cultured in medium with 2 μg/ml Dox for 18 h. After fixation, cells were stained with anti-spastin and anti-tubulin antibodies. A, Control RFL-6/Tet-On cells cultured with Dox did not express spastin as indicated by immunostaining with anti-spastin antibody and MTs were not severed as indicated by staining with anti-tubulin antibody. B, Moderate levels of WT M87 spastin expressed in TRE/M87 WT cells exposed to Dox were sufficient to sever all MTs. C, High levels of mutated M87 spastin were synthesized in TRE/M87 CY cells grown in medium with Dox, but MTs were not severed even in the regions of very high M87 accumulation. D, MTs in TRE/M1 CY cells expressing mutated M1 after Dox treatment were also not severed, but mutated M1 clearly decorated a subset of MTs around the nucleus. Scale bar, 10 μm.