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Vol. 17, Issue 5, 2212-2222, May 2006

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The Fission Yeast Transforming Acidic Coiled Coil–related Protein Mia1p/Alp7p Is Required for Formation and Maintenance of Persistent Microtubule-organizing Centers at the Nuclear Envelope

Liling Zheng ^* , Cindi Schwartz , Liangmeng Wee ^*, and Snezhana Oliferenko ^*

^* Cell Dynamics Group, Temasek Life Sciences Laboratory, 117604 Singapore, Singapore; Department of Biological Sciences, National University of Singapore, 117543 Singapore, Singapore; and The Boulder Laboratory for 3-D Electron Microscopy of Cells, Department of MCD Biology, University of Colorado, Boulder, CO 80309

Submitted August 30, 2005; Revised January 12, 2006; Accepted February 6, 2006
Monitoring Editor: Tim Stearns

Microtubule-organizing centers (MTOCs) concentrate microtubule nucleation, attachment and bundling factors and thus restrict formation of microtubule arrays in spatial and temporal manner. How MTOCs occur remains an exciting question in cell biology. Here, we show that the transforming acidic coiled coil–related protein Mia1p/Alp7p functions in emergence of large MTOCs in interphase fission yeast cells. We found that Mia1p was a microtubule-binding protein that preferentially localized to the minus ends of microtubules and was associated with the sites of microtubule attachment to the nuclear envelope. Cells lacking Mia1p exhibited less microtubule bundles. Microtubules could be nucleated and bundled but were frequently released from the nucleation sites in mia1 cells. Mia1p was required for stability of microtubule bundles and persistent use of nucleation sites both in interphase and postanaphase array dynamics. The -tubulin–rich material was not organized in large perinuclear or microtubule-associated structures in mia1 cells. Interestingly, absence of microtubules in dividing wild-type cells prevented appearance of large -tubulin–rich MTOC structures in daughters. When microtubule polymerization was allowed, MTOCs were efficiently assembled de novo. We propose a model where MTOC emergence is a self-organizing process requiring the continuous association of microtubules with nucleation sites.

The online version of this article contains supplemental material at MBC Online (http://www.molbiolcell.org).

Address correspondence to: Snezhana Oliferenko (snejana{at}tll.org.sg).