Although plant Aurora kinases can be

Although plant Aurora kinases can be clearly grouped into functional clades, the roles of the respective clades does not seem to be evolutionarily conserved. AtAurora 1 labels kinetochore microtubules [19], similarly to mammalian Aurora A, while its targeting to the cell plate resembles features of Aurora B at the cleavage furrow. AtAurora 3 proteins function on kinetochores fits with a B-type function, but AtAurora 3 does not translocate to the forming cell plate as Aurora B does to the midbody during cytokinesis. Comparison to animal Aurora kinases shows that all AtAurora kinases contain a glycine in their kinase domain, an amino SB-334867 hydrochloride typical for human Aurora A at that position [18]. In addition, plants do not have classical centrosomes, where Aurora A is targeted, to enhance microtubule polymerization for spindle formation [12]. However, plants contain microtubule-organizing centers (MTOCs) on their nuclear surface 14, 22 where AtAurora 1 and AtAurora2 are located [13], which can be seen as a further indication for mixed features of animal and plant Aurora kinases. In addition, both AtAurora 1 and AtAurora 2 show dynamic relocalization during mitosis similarly to Aurora B. This shows that plant Aurora kinases have mixed features compared to animal-type Aurora kinases (Figure 1A,B).
Interaction Partners Confer Specificity Aurora kinases acquire their functional specificity through association with various interaction partners. A-type Aurora interacts with TPX2 (targeting protein forkinesin-like protein 2) that is important for microtubule nucleation leading to spindle assembly [23] (Figure 1B), whereas B-type Aurora operates together with the chromosomal passenger complex (CPC) subunits inner centromere protein (INCEN-P), survivin, and borealin [4] (Figure 1B). Binding to different interaction partners likely defines the spatial compartmentalization and therefore the different functions of Aurora kinases [24]. The importance of the interactors for Aurora function is underlined by the fact that a single point mutation in the human Aurora A catalytic domain (G198N) can convert it into Aurora B by promoting its interaction with the CPC [25]. In addition, Aurora B can induce mitotic arrest in HeLa cells if Aurora A is not functional [26]. In mice, Aurora C functions specifically in meiotic spindle establishment in oocytes [27] and during spermatogenesis [28]. However, Aurora C, expressed under the Aurora B promoter can rescue Aurora B depletion, indicating that both function similarly [29]. Similarly to Aurora kinases in other Kingdoms, plant Aurora kinases might also achieve functional specificity via their interaction partners. The Arabidopsis genome encodes a single TPX2 homolog [30] and eight TPX2-like proteins. TPX2 and four TPX2-like proteins contain Aurora-binding motifs 31, 32. With the exception of one TPX2-like protein in Drosophila[33], no other TPX2-like proteins have been reported in other Kingdoms to date, and it is tempting to speculate that the expansion of this family in plant genomes correlates with acentrosomal bipolar spindle establishment. AtTPX2 functions in bipolar spindle assembly and its activity is controlled via nuclear sequestration [30]. AtAurora 1 colocalizes dynamically with AtTPX2 throughout mitosis from the prophase spindle to early telophase. AtTPX2 contains two Aurora-binding motifs and targeting sites for microtubules 18, 30. It was suggested that the interaction of AtAurora 1 with TPX2 is required for its stabilization and cell cycle-dependent localization on microtubules during mitosis [18]. Recently it was shown that AtTPX2 is more likely to be a substrate of AtAurora 1 than of AtAurora 3, but the exact phosphorylation sites remain to be identified. In vitro, full-length TPX2 appears not to be phosphorylated by Aurora 3, but low levels of phosphorylation on a more stable form of TPX2 (TPX2ΔN) by AtAurora 3 could be observed [31]. It remains unknown whether this also reflects the in vivo situation. In addition, interaction of AtTPX2 and AtAurora 1 increases the phosphorylation of downstream targets, such as histone H3 [31]. In addition, TPX2 was proposed to have a function in autophosphorylation of AtAurora 1 mediated by an Aurora-binding domain in AtTPX2 [31]. However, it remains to be shown to what extent autophosphorylation of AtAurora kinases plays a role in their in vivo activity and whether, in addition to TPX2 itself, there is a functional relationship between the TPX2-like proteins and Aurora.