Fission yeast Aip3p (spAip3p) is required for an alternative actin-directed polarity program

Aip3p is an actin-interacting protein that regulates cell polarity in buddi ng yeast. The Schizosaccharomyces pombe-sequencing project recently led to the identification of a homologue of Aip3p that we have named spAip3p. Our results confirm that spAip3p is a true functional homologue of Aip3p. When expressed in budding yeast, spAip3p localizes similarly to Aip3p during the cell cycle and complements the cell polarity defects of an aip3 Delta stra in. Two-hybrid analysis shows that spAip3p interacts with actin similarly t o Aip3p. In fission yeast, spAip3p localizes to both cell ends during inter phase and later organizes into two rings at the site of cytokinesis. spAip3 p localization to cell ends is dependent on microtubule cytoskeleton, its l ocalization to the cell middle is dependent on actin cytoskeleton, and both patterns of localization require an operative secretory pathway. Overexpre ssion of spAip3p disrupts the actin cytoskeleton and cell polarity, leading to morphologically aberrant cells. Fission yeast, which normally rely on t he microtubule cytoskeleton to establish their polarity axis, can use the a ctin cytoskeleton in the absence of microtubule function to establish a new polarity axis, leading to the formation of branched cells. spAip3p localiz es to, and is required for, branch formation, confirming its role in actin- directed polarized cell growth in both Schizosaccharomyces pombe and Saccha romyces cerevisiae.