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Vol. 17, Issue 12, 5075-5093, December 2006

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Noncore Components of the Fission Yeast -Tubulin Complex

Wellcome Trust Centre for Cell Biology, Edinburgh University, Edinburgh EH9 3JR, United Kingdom

Submitted November 3, 2005; Revised September 8, 2006; Accepted September 25, 2006
Monitoring Editor: Trisha Davis

Relatively little is known about the in vivo function of individual components of the eukaryotic -tubulin complex (-TuC). We identified three genes, gfh1+, mod21+, and mod22+, in a screen for fission yeast mutants affecting microtubule organization. gfh1+ is a previously characterized -TuC protein weakly similar to human -TuC subunit GCP4, whereas mod21+ is novel and shows weak similarity to human -TuC subunit GCP5. We show that mod21p is a bona fide -TuC protein and that, like gfh1 mutants, mod21 mutants are viable. We find that gfh1 and mod21 mutants have qualitatively normal microtubule nucleation from all types of microtubule-organizing centers (MTOCs) in vivo but quantitatively reduced nucleation from interphase MTOCs, and this is exacerbated by mutations in mod22+. Simultaneous deletion of gfh1p, mod21p, and alp16p, a third nonessential -TuC protein, does not lead to additive defects, suggesting that all three proteins contribute to a single function. Coimmunoprecipitation experiments suggest that gfh1p and alp16p are codependent for association with a small "core" -TuC, whereas mod21p is more peripherally associated, and that gfh1p and mod21p may form a subcomplex independently of the small -TuC. Interestingly, sucrose gradient analysis suggests that the major form of the -TuC in fission yeast may be a small complex. We propose that gfh1p, mod21p, and alp16 act as facultative "noncore" components of the fission yeast -TuC and enhance its microtubule-nucleating ability.

This article was published online ahead of print in MBC in Press (http://www.molbiolcell.org/cgi/doi/10.1091/mbc.E05-11-1009) on October 4, 2006.

Address correspondence to: Kenneth E. Sawin (ken.sawin{at}ed.ac.uk)

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