Identification of a TPX2-like microtubule-associated protein in Drosophila

Chromosome segregation during mitosis and meiosis relies on the spindle and the functions of numerous microtubule-associated proteins (MAPs). One of the best-studied spindle MAPs is the highly conserved TPX2, which has been reported to have characteristic intracellular dynamics and molecular activities, such as nuclear localisation in interphase, poleward movement in the metaphase spindle, microtubule nucleation, microtubule stabilisation, microtubule bundling, Aurora A kinase activation, kinesin-5 binding, and kinesin-12 recruitment. This protein has been shown to be essential for spindle formation in every cell type analysed so far. However, as yet, TPX2 homologues have not been found in the Drosophila genome. In this study, I found that the Drosophila protein Ssp1/Mei-38 has significant homology to TPX2. Sequence conservation was limited to the putative spindle microtubule-associated region of TPX2, and intriguingly, D-TPX2 (Ssp1/Mei-38) lacks Aurora A- and kinesin-5-binding domains, which are highly conserved in other animal and plant species, including many insects such as ants and bees. D-TPX2 uniformly localised to kinetochore microtubule-enriched regions of the metaphase spindle in the S2 cell line, and it had microtubule binding and bundling activities in vitro. In comparison with other systems, the contribution of D-TPX2 to cell division seems to be minor; live cell imaging of microtubules and chromosomes after RNAi knockdown identified significant delay in chromosome congression in only 18% of the cells. Thus, while this conserved spindle protein is present in Drosophila, other mechanisms may largely compensate for its spindle assembly and chromosome segregation functions.