Sorting out interphase microtubules

Proper organization of microtubule polymers is crucial to the form and function of all eukaryotic cells. Whether growing, dividing or polarizing, different cell types—be they neurons, plant cells or fungal cells—organize specialized microtubule patterns appropriate for their needs. Is it possible to identify the set of factors sufficient to organize microtubules in a specific cell, and to understand how the cell regulates those factors spatially and temporally such that they collectively sustain that particular cellular pattern of microtubules? In a recent issue of Cell, Marcel Janson and collaborators (Janson et al, 2007) use an elegant combination of quantitative microscopy, in vitro assays and computer simulations to try to elucidate the minimal set of components required to stably organize microtubule patterns in a unicellular eukaryote, the fission yeast Schizosaccharomyces pombe. Microtubule organization in a given cell is partly dependent on the intrinsic turnover of each of its polymers. But it is the collective interaction of microtubules with microtubule-interacting proteins—nucleators determining polymer number and localization; stabilizing or destabilizing proteins regulating average polymer length; static or dynamical crosslinkers mediating polymer connectivity—that mostly determines the global spatial pattern and ‘systemic’ cellular function of microtubules. Individually, every component of this ‘system’ appears to behave independently; collectively, they generate an overall stable, organized and functional microtubule pattern. In the fission yeast, the proper growth and form of each cell relies on the presence in its cytoplasm of ‘bipolar’ bundles of antiparallel microtubules, organized with their more dynamic ‘plus’ ends towards the cell tips and their ‘minus’ ends overlapping at the cell centre (Drummond and Cross, 2000; Tran et al, 2001). It is generally thought that the organization of these bipolar microtubule arrays relies on two evolutionarily conserved microtubule interactors: the Prc1-related protein Ase1, which statically crosslinks or ‘bundles’ microtubules (Loiodice et al, 2005; Yamashita et al, 2005); and the Kar3/Ncd-related, minus-end-directed kinesin motor Klp2, which mediates the transport (‘sliding’) of short microtubules towards the cell centre along longer microtubules (Carazo-Salas et al, 2005). Recently, it has been suggested that these two interactors might suffice for bipolar microtubule arrays to ‘self-organize’ without preexisting organizational templates (Carazo-Salas and Nurse, 2006; Daga et al, 2006).