Role of microtubules and tea1p in establishment and maintenance of fission yeast cell polarity

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Abstract
Microtubules and the protein tea1p have important roles in regulating cell polarity in the fission yeast Schizosaccharomyces pombe. Here, using combinations of drugs, environmental perturbations and genetic mutants, we demonstrate that once a cell polarity axis is established, microtubules have at best a minor role in maintaining the cortical actin cytoskeleton and the rate and direction of cell growth. In addition, we find that after perturbations that disrupt cell polarity and the cortical actin cytoskeleton, microtubules are not required for re-establishment of polarity per se. However, after such perturbations, the distribution of cytoplasmic microtubules plays an important role in dictating the position of sites of polarity re-establishment. Furthermore, this influence of microtubule distribution on site selection during polarity re-establishment requires the presence of tea1p, suggesting that tea1p is crucial for coupling microtubule distribution to the regulation of cell polarity. Our results suggest a model in which, at the cellular level, two distinct and separable mechanisms contribute to how tea1p regulates site selection during polarity re-establishment. First, tea1p remaining at cell tips after cortical depolarization can serve as a cortical landmark for microtubule-independent site selection; second, tea1p newly targeted to the cell cortex by association with microtubules can promote the formation of polarity axes de novo.