Functional divergence of TPX2 family members in Arabidopsis thaliana

Abstract Background TPX2 (Targeting protein for Xklp2) is an evolutionary conserved microtubule-associated protein important for microtubule nucleation and proper mitotic spindle assembly. The protein was also described as an activator of the mitotic kinase Aurora A in humans and the Arabidopsis AURORA 1 (AUR1) kinase. In contrast to animal genomes that encode only one TPX2 gene in their genome, higher plants genomes encode a family with several TPX2-LIKE gene members (TPXL). Results Arabidopsis genome comprise of 21 TPXL genes. To better understand the functional divergence within the TPXL gene family in plants, we characterized eight genes most closely related to the canonical TPX2 Arabidopsis. TPXL genes of Arabidopsis can be divided into two groups. Group A proteins (TPXL2, 3, 4 and 8) contain Aurora binding and TPX2_importin domains, while group B proteins (TPXL1, 5, 6 and 7) harbor an Xklp2 domain. Canonical Arabidopsis TPX2 contains all above mentioned domains. We confirmed using in vitro kinase assays that the group A proteins contain a functional Aurora kinase binding domain and are able to at least double the activity of recombinant AUR1 kinase. Transient expression of GFP-tagged Arabidopsis TPX2-like proteins in Nicotiana benthamiana revealed preferential localization to microtubules and nuclei, except TPXL7, which localized mainly to the nuclear envelope. Co-expression of AUR1 together with TPX2-like proteins changed the localization of AUR1, indicating that these proteins serve as targeting factors for Aurora kinases. Conclusion Taken together, we visualize the various localizations of the TPX2-LIKE family in Arabidopsis as proxy to their functional divergence and provide evidence of their role in the targeted regulation of AUR1 kinase activity.