REGULATION OF CELL POLARITY BY MICROTUBULES IN FISSION YEAST

To investigate the role of microtubules in regulating cell polarity in Schizosaccharomyces pombe, we have developed a system in which normal ly cylindrical fission yeast synchronously form branched cells at high frequency upon treatment with the microtubule-depolymerizing drug thi abendazole (TBZ), Branching depends on both elevated temperature and c ell cycle state and occurs at high frequency only when TBZ is added to cells that have not yet passed through New-End Take-Off (NETO), the n ormal transition from monopolar to bipolar growth. This suggests that microtubules may be of greatest physiological importance for the maint enance of cell shape at specific points in the cell cycle. The localiz ation of three different proteins normally found at cell ends-cortical F-actin, teal, and an ral3 (scd2)-green fluorescent protein (GFP) fus ion-is disrupted by TBZ treatment. However, these proteins can eventua lly return to cell ends in the absence of microtubules, indicating tha t although their localization to ends normally depends on microtubules , they may recover by alternative mechanisms. In addition, TBZ induces a shift in ral3-GFP distribution from cell ends to the cell middle, s uggesting that a protein complex containing ral3 map be part of the cu e that specifies the position of branch formation.