Plant TPX2 and related proteins.

At the onset of mitosis, microtubules form a bipolar spindle around the prophase nucleus. TPX2 is phosphorylated during mitosis and acts as a spindle assembly factor that nucleates microtubules in the close vicinity of chromosomes, independent of the centrosomes. Furthermore, it activates the kinase Aurora A and targets the Xenopus kinesin-like protein 2 to spindle poles. We have characterized the plant orthologue of TPX2 that possesses all identified functional domains of its animal counterpart. Moreover, we have demonstrated that it is exported before nuclear envelope breakdown and that its activity around the nuclear envelope is essential for prospindle assembly. Here, we compare the sequences of several characterized TPX2 domains, allowing us to define TPX2. We propose that true TPX2 orthologues share simultaneously all these conserved domains and that other proteins possessing only some of these functional blocks may be considered as TPX2-related proteins.