Mitotic Spindle Formation in Plants

Mitotic spindles are microtubule-based struc- tures that separate chromosomes during mitosis. In most animals and fungal cells, spindle microtubules nucleate from centro- somes or spindle pole bodies. Plant cells lack such structured microtubule organizing centers, and some of their microtubules appear to nucleate from near the nuclear envelope, but very little is known about spindle formation in plants (reviewed in Bannigan et al., 2008). Vos et al. (pp. nnn) provide evidence that, despite their differ- ences, plant mitotic spindles could form via similar molecular mechanisms to those found in animals. Targeting Protein for Xklp2 (TPX2) is a reg- ulator of spindle assembly in animal cells that induces microtubule nucleation and targets certain proteins to spindle microtubules. Vos et al. identified a potential TPX2 ortholog in Arabidopsis by sequence similarity. Modeling based on the crystal structure of human TPX2 with one of its binding partners shows that At TPX2 could carry out a similar interaction. The authors also found that At TPX2 can replace endogenous Xenopus laevis TPX2 to induce the formation of microtubule asters in extracts from X. laevis oocytes. In addition, At TPX2 can coimmunoprecipitate one of Xl TPX2's binding partners. In tobacco BY-2 cells, TPX2 antibodies label the inside of the nucleus from interphase to G2, a perinuclear region early in prophase, and the spindle later in mitosis (see figure). Using various constructs and organisms, the authors found that At TPX2 has nuclear localization signals and a microtubule binding domain. Unlike other TPX2s, however, At TPX2 also has a signal for nuclear export. In ad- dition, a movie of GFP-tagged TPX2 in BY-2 cells shows increasing TPX2 signal in the nucleus as chromatin condenses, then export of the signal from the nucleus and labeling in two foci before nuclear envelope breakdown. These data are consistent with At TPX2 func- tioning in the formation of the spindle, which in plants begins outside of the nucleus before