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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.cell.2006.12.030 Byline: Marcel E. Janson (1), Rose Loughlin (2), Isabelle LoA[macron]odice (1), Chuanhai Fu (1), Damian Brunner (2), Francois J. Nedelec (2), Phong T. Tran (1) Abstract: Microtubule (MT) nucleation not only occurs from centrosomes, but also in large part from dispersed nucleation sites. The subsequent sorting of short MTs into networks like the mitotic spindle requires molecular motors that laterally slide overlapping MTs and bundling proteins that statically connect MTs. How bundling proteins interfere with MT sliding is unclear. In bipolar MT bundles in fission yeast, we found that the bundler ase1p localized all along the length of antiparallel MTs, whereas the motor klp2p (kinesin-14) accumulated only at MT plus ends. Consequently, sliding forces could only overcome resistant bundling forces for short, newly nucleated MTs, which were transported to their correct position within bundles. Ase1p thus regulated sliding forces based on polarity and overlap length, and computer simulations showed these mechanisms to be sufficient to generate stable bipolar bundles. By combining motor and bundling proteins, cells can thus dynamically organize stable regions of overlap between cytoskeletal filaments. Author Affiliation: (1) Cell and Developmental Biology, University of Pennsylvania, Philadelphia, PA 19104, USA (2) Cell Biology and Biophysics Program, European Molecular Biology Laboratory, 69117 Heidelberg, Germany Article History: Received 29 June 2006; Revised 4 October 2006; Accepted 26 December 2006 Article Note: (miscellaneous) Published: January 25, 2007

The mechanism for forming linear microtubule (MT) arrays in cells such as neurons, polarized epithelial cells, and myotubes is not well understood. A simpler bipolar linear array is the fission yeast interphase MT bundle, which in its basic form contains two MTs that are bundled at their minus ends. Here, we characterize mto2p as a novel fission yeast protein required for MT nucleation from noncentrosomal [gamma]-tubulin complexes ([gamma]-TuCs). In interphase mto2[DELTA] cells, MT nucleation was strongly inhibited, and MT bundling occurred infrequently and only when two MTs met by chance in the cytoplasm. In wild-type 2, we observed MT nucleation from [gamma]-TuCs bound along the length of existing MTs. We propose a model on how these nucleation events can more efficiently drive the formation of bipolar MT bundles in interphase. Key to the model is our observation of selective antiparallel binding of MTs, which can both explain the generation and spatial separation of multiple bipolar bundles.

The fission yeast Schizosaccharomyces pombe serves as a good genetic model organism for the molecular dissection of the microtubule (MT) cytoskeleton. However, analysis of the number and distribution of individual MTs throughout the cell cycle, particularly during mitosis, in living cells is still lacking, making quantitative modelling imprecise. We use quantitative fluorescent imaging and analysis to measure the changes in tubulin concentration and MT number and distribution throughout the cell cycle at a single MT resolution in living cells. In the wild-type cell, both mother and daughter spindle pole body (SPB) nucleate a maximum of 23 ± 6 MTs at the onset of mitosis, which decreases to a minimum of 4 ± 1 MTs at spindle break down. Interphase MT bundles, astral MT bundles, and the post anaphase array (PAA) microtubules are composed primarily of 1 ± 1 individual MT along their lengths. We measure the cellular concentration of αβ-tubulin subunits to be ~5 µM throughout the cell cycle, of which one-third is in polymer form during interphase and one-quarter is in polymer form during mitosis. This analysis provides a definitive characterization of αβ-tubulin concentration and MT number and distribution in fission yeast and establishes a foundation for future quantitative comparison of mutants defective in MTs. [ABSTRACT FROM AUTHOR]

The article offers information on self healing property of mictrotubules and self repair characteristic of these biological filaments. Topics discussed include damaging effect on physiology of molecules due to mechanical loads, reliable healing processes based on tissues' regeneration ability and composition of microtubules' lattice walls.