While the yeasts and see Glossary each contain

While the yeasts and (see Glossary) each contain only one Aurora homolog, Ipl1p (increase in ploidy 1) and Ark1 (Aurora-related kinase 1), respectively 5, 6, , and Xenopus laevis contain two Aurora kinases: Aurora A and Aurora B [4]. Mammals have three Aurora kinases, A, B, and C (Figure 1). In animals and yeast, multiple substrates of Aurora kinases were characterized [4], most of which function in mitosis with roles ranging from chromosome alignment, centrosome separation and maturation, GDC-0994 australia modifications and condensation to (bipolar) spindle assembly, spindle checkpoint control, and cytokinesis 4, 7, 8. The first Aurora phosphorylation consensus site was identified for Ipl1p [9] and was later also adapted for human Aurora A to [Arg/Lys/Asp] – Arg – X(1,2) – [Ser/Thr] – hydrophobic residue ≠Pro 10, 11, 12. In contrast to yeast and animals, the role of plant Aurora kinases is only starting to emerge. Based on phylogenetic analysis, localization, and functional divergence, plant Aurora kinases can be clustered into two subclades, α-Aurora and β-Aurora (Figure 1A,B). While higher plants have members in both groups, lower plants such as Marsilea (M. vestita) and Physcomitrella (P. patens) only contain α-Aurora kinases [13]. The Arabidopsis (A. thaliana) genome encodes three Aurora kinases, which fall into the two non-complementing groups α-Aurora (AtAurora 1 and AtAurora 2; the prefix At indicates A. thaliana) and β-Aurora (AtAurora 3) (Figure 1). All three kinases were isolated in a large-scale GFP-tagging approach 13, 14 and by homology with the animal and yeast Aurora kinases [15]. Arabidopsis α-Aurora kinases show a very similar intron–exon structure and possibly originate from an early duplication event [13]. Interestingly, the barley (Hordeum vulgare) [16] and rice (Oryza sativa) [17] genomes encode only a single Aurora kinase in each group. A conserved putative degradation box with sites for ubiquitination, sites for sumoylation as well as for the interaction with cyclins, MAPK (mitogen-activated protein kinase), PP1 (protein phosphatase 1), and pRb (retinoblastoma protein) have been predicted 13, 18. Although the presence of these domains potentially links Aurora kinases to important cell-cycle regulators, direct functional evidence for this is lacking.
Localization of Aurora Kinases Transcripts of plant Aurora kinases are abundant in young root and flower tissues 13, 19, which is in line with a role in actively dividing cells. Expression of AtAurora 1, AtAurora 2 and AtAurora 3 peaks at the onset of mitosis and decreases at the end of mitosis 13, 20. Studies of AtAurora kinases in tobacco bright yellow 2 (BY-2) cells and in Arabidopsis root meristem cells have dissected their intracellular targeting during interphase and mitosis. Before division, AtAurora 1 and AtAurora 2 are detectable in the cytoplasm, but are mostly localized in the nucleoplasm 13, 15, 18, 19. Before nuclear envelope breakdown, AtAurora 1 and 2 kinases exit the nucleus and migrate to the prophase spindle [13]. Both show dynamic localization throughout mitosis because they associate with prophase spindle microtubules and kinetochore spindle microtubules during metaphase and anaphase, and with the growing cell plate during cytokinesis 13, 14, 15, 18, 19. The localization of AtAurora 1 and AtAurora 2 appeared to be similar to that of γ-tubulin, and a role for Aurora in microtubule stability was suggested 13, 15, 18. Even though the localizations of AtAurora 1 and AtAurora 2 are similar, AtAurora 2 appears to accumulate less at the expanding cell plate than does AtAurora 1 13, 15, which might be linked with a cytokinesis-specific function of AtAurora 1 and functional diversification when both AtAurora 1 and AtAurora 2 are present. However, in absence of functional AtAurora 1, AtAurora 2 concentrates more abundantly at the cell plate [19]. The β-Aurora kinase AtAurora 3 is present in the nucleoplasm and at chromocenters of interphase nuclei in meristems. Its localization at centromeres during metaphase [13] suggests that it likely plays a role in chromosome separation. Overexpression of AtAurora 3 in BY-2 cells leads to unusual cell division patterns with abnormal division orientations and spindle formation defects, whereas overexpression of AtAurora 1 and AtAurora 2 did not show these phenotypes [15]. Abnormal spindles in AtAurora 3 overexpressing cells might hint toward a role in microtubule dynamics, whereas the formation of the pre-prophase band (PPB), which determines the future division plane in plants, is not affected [15]. It remains to be investigated whether AtAurora 3 is directly involved in microtubule dynamics or if extensively increased levels of AtAurora 3 interfere with the function of the α-Aurora group. Recently, it was shown that all three AtAurora kinases localize in male and female gametophytes, suggesting a possible prominent function during plant meiosis [21].