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¹ Cell Biology Group and CREST/JST, Kansai Advanced Research Center, National Institute of Information and Communications Technology, Kobe, 651-2492, Japan
² Laboratory of Cell Regulation, Cancer Research UK London Research Institute, Lincoln's Inn Fields Laboratories, London WC2A 3PX, UK

The -tubulin complex acts as a nucleation unit for microtubule assembly. It remains unknown, however, how spatial and temporal regulation of the complex activity affects microtubule-mediated cellular processes. Alp4 is one of the essential components of the S. pombe-tubulin complex. We show here that overproduction of a carboxy-terminal form of Alp4 (Alp4C) and its derivatives tagged to a nuclear localization signal or to a nuclear export signal affect localization of -tubulin complexes and induces novel phenotypes that reflect distinct functions of nuclear and cytoplasmic -tubulin complexes. Nuclear Alp4C induces a Wee1-dependent G2 delay, reduces the levels of the -tubulin complex at the spindle pole body, and results in defects in mitotic progression including spindle assembly, cytoplasmic microtubule disassembly, and chromosome segregation. In contrast, cytoplasmic Alp4C induces oscillatory nuclear movement and affects levels of cell polarity markers, Bud6 and Tip1, at the cell ends. These results demonstrate that regulation of nuclear -tubulin complex activity is essential for cell cycle progression through the G2/M boundary and M phase, whereas regulation of cytoplasmic -tubulin complex activity is important for nuclear positioning and cell polarity control during interphase.

^* Correspondence: E-mail: hmasuda{at}nict.go.jp