Microtubule stabilization in vivo by nucleation-incompetent gamma-tubulin complex

Although the fission yeast Schizosaccharomyces pombe contains many of the gamma-tubulin ring complex (gamma-TuRC)-specific proteins of the gamma-tubulin complex (gamma-TuC), several questions about the organizational state and function of the fission yeast gamma-TuC in vivo remain unresolved. Using 3 x GFP-tagged gamma-TuRC-specific proteins, we show here that gamma-TuRC-specific proteins are present at all microtubule organizing centers in fission yeast and that association of gamma-TuRC-specific proteins with the gamma-tubulin small complex (gamma-TuSC) does not depend on Mto1, which is a key regulator of the gamma-TuC. Through sensitive imaging in mto1 Delta mutants, in which cytoplasmic microtubule nucleation is abolished, we unexpectedly found that gamma-TuC incapable of nucleating microtubules can nevertheless associate with microtubule minus-ends in vivo. The presence of gamma-TuC at microtubule ends is independent of gamma-TuRC-specific proteins and strongly correlates with the stability of microtubule ends. Strikingly, microtubule bundles lacking gamma-TuC at microtubule ends undergo extensive treadmilling in vivo, apparently induced by geometrical constraints on plus-end growth. Our results indicate that microtubule stabilization by the gamma-TuC, independently of its nucleation function, is important for maintaining the organization and dynamic behavior of microtubule arrays in vivo.