Aurora at the pole and equator: overlapping functions of Aurora kinases in the mitotic spindle

Abstract

The correct assembly and timely disassembly of the mitotic spindle is crucial for the propagation of the genome during cell division. Aurora kinases play a central role in orchestrating bipolar spindle establishment, chromosome alignment and segregation. In most eukaryotes, ranging from amoebas to humans, Aurora activity appears to be required both at the spindle pole and the kinetochore, and these activities are often split between two different Aurora paralogues, termed Aurora A and B. Polar and equatorial functions of Aurora kinases have generally been considered separately, with Aurora A being mostly involved in centrosome dynamics, whereas Aurora B coordinates kinetochore attachment and cytokinesis. However, double inactivation of both Aurora A and B results in a dramatic synergy that abolishes chromosome segregation. This suggests that these two activities jointly coordinate mitotic progression. Accordingly, recent evidence suggests that Aurora A and B work together in both spindle assembly in metaphase and disassembly in anaphase. Here, we provide an outlook on these shared functions of the Auroras, discuss the evolution of this family of mitotic kinases and speculate why Aurora kinase activity may be required at both ends of the spindle microtubules.

Figure 1.

Specific and combined Aurora kinase functions. (a) Overview of functions of Aurora kinases in the mitotic spindle and images of cells lacking Aurora A, Aurora B and Aurora A+B activity [2]. (b) Centrosomal and centromeric localization of A and B-type Aurora in human HeLa cells. Immuno-fluorescent image of a formaldehyde fixed metaphase cell. (c) Nuclear phenotypes of HeLa cells expressing GFP-Histone H2B following 24 h incubation with Aurora A inhibitor (500 nM MLN8237), Aurora B inhibitor (60 nM AZD 1152) and Aurora A+B inhibitors (500 nM MLN8237+60 nM AZD1152). Aurora A inhibition causes micronuclei formation (see arrow) and aneuploidy [3,4], Aurora B inhibition results in a cytokinesis defect leading to binucleate cells [5,6], whereas inactivation of both Aurora A and B causes an abrogation in chromosome segregation and mitotic exit with a single quatroploid nucleus [2,7].

Figure 2.

Aurora A and B kinase synergies. (a) CenpE phosphorylation by Aurora kinases results in kinetic proofreading (model adapted from Kim et al. [92]) Aurora kinase phosphorylation destabilizes CenpE binding to MTs. The protein is morel likely to re-attach to a neighbouring MT in the dense K-fibre bundles than on single astral MTs. (b) Control of Kif18b/MCAK interaction by Aurora- kinases (model adapted from Tanenbaum et al. [95]). The plus-ended motor Kif18b transports MCAK to the MT tip, where it depolymerizes the tubulin polymers. Aurora kinases jointly negatively regulate this interaction by an unknown mechanism. (c) Aurora controlled MT depolymerization in anaphase (model adapted from Hegarat et al. [2]). In metaphase, the kinetochores are attached to kinetochore MT fibres (K-fibres) that reach to the spindle poles. Minus-end depolymerization causes a constant flux of tubulin towards the spindle poles that is counteracted by plus-end MT polymerization at the kinetochore to achieve constant spindle length. In anaphase K-fibres are rapidly depolymerized at both plus and minus ends. This MT depolymerization releases energy that is used to pull the kinetochores along the shrinking K-fibre. Possible targets in this pathway are proteins that regulate MT stability. Among these, the end-binding proteins have been shown to be Ipl1 substrates in yeast, and this phosphorylation is linked to spindle disassembly [96]. MT-severing enzymes and MT depolymerases have also been shown to be targeted by Aurora kinases, but these phosphorylations are thought to negatively regulate their enzymatic activities [66,67,97].

Figure 3.

Aurora kinase evolution. (a) Eukaryotic tree [–122]. (b) Presence and type of structure at the spindle pole [123,124]. Cylindrical structures (centrioles) exist in animals in the context of centrosomes, but they exist also in other organisms, albeit with unclear role in spindle formation. Additionally, many organisms have different structures at the spindle poles. (c) Number of Aurora kinases in the genome, identified via recursive BLAST searches from known Aurora/Ipl1 proteins, and classified as such by being monophyletic with Aurora/Ipl1 and not other related protein kinases. Red indicates an equatorial localization, green a spindle pole/centrosomal localization, and grey indicates that the localization of that kinase has not been described in the literature. Localization data were obtained from H. sapiens [19,25,125], M. musculus [126], C. elegans [21,127], Xenopus sp. [22,45,128], P. pectinifera [116], S. cerevisiae [129], S. pombe [130], D. discoideum [117], A. thaliana [115], P. falciparum [131], T. brucei [132]. (d) Dendograms (maximum likelihood) of Aurora kinases and related kinases for each evolutionary group except for fungi. Branches in grey have less than 60% bootstrap support.