Fig. 4. Ectopic expression of the Op18 N-terminus is sufficient to partially destabilize the interphase MT system in KA8 cells. (A) Cells were transfected with the indicated NR-helix derivative or vector alone (Vec-Co) and induced for 6 hours as in Fig. 3. The fraction of polymerized tubulin was determined as described in Materials and Methods and the mean of two independent transfection experiments is shown. (B) A histogram derived from flow cytometric analysis of MT content in cells induced to express wild-type Op18 and Op18(1-57)-NR-helix or vector alone for 6 hours as in A. Open graphs depict -tubulin-specific fluorescence of cells transfected with the indicated pMEP4 derivative and the filled graph show control staining, in the absence of anti--tubulin but in the presence of fluorescein-conjugated rabbit anti-mouse immunoglobulin. The data are representative for three independent experiments.