**3.2. Aurora-B**

**3. Expression, subcellular localization, and functions of the Aurora kinases**

The Aurora kinases play a major role during mitosis [49, 50]. As mentioned above, these proteins display distinct intracellular localizations, substrate specificity and functions, and their expression and activity are tightly regulated at the transcriptional or posttranscriptional

The expression of Aurora-A is cell cycle regulated, being very low during the G1-phase and starting to accumulate at the centrosome in the late S phase to be maximal at the G2-M transition. In this period, it localizes at the spindle poles, and it is degraded before cytokinesis [50, 53]. Aurora-A regulates centrosome separation and maturation, mitotic entry, and bipolar spindle formation. Recruitment of Aurora-A to the centrosome is controlled by the centrosome

**Figure 2.** (A) Schematic representation of the pathway induced by Aurora-A to activate the CDK1/cyclin B complex allowing the transition of the cell from the G2 to the M phase. Aurora-A in association with Bora phosphorylates the PLK1. Both Aurora-A and PLK1 phosphorylate CDC25B (cell division cycle 25 B) allowing cyclin-dependent kinase 1 (CDK1)/cyclin B complex activation and thus promoting mitotic entry. PLK1 facilitates this process also by inhibiting the CDK1 inhibitor WEE1. Inactivation of Aurora-A or Plk1 individually shows no significant effect on Cdk1 activation and entry to mitosis, while their simultaneous inactivation produces a marked delay in both Cdk1 activation and mitotic entry, suggesting that the two kinases have redundant functions. (B) Immunofluorescence showing Aurora-A localization at the spindle pole of an anaplastic thyroid cancer cell in metaphase. Adapted with permission from Ref.

level, through phosphorylation/dephosphorylation and protein degradation [57].

**3.1. Aurora-A**

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[14].

Aurora-B protein level peaks at G2/M phase, with the highest kinase activity recorded from metaphase to the end of mitosis [49, 50]. Aurora-B acts in concert with three other proteins, inner centromere protein (INCENP), Survivin, and Borealin/Dasra B, to which it associates forming the chromosomal passenger complex (CPC). In early prophase, the CPC is located on chromosomal condensing arms where it displaces the heterochromatin protein-1 from DNA

**Figure 3.** Schematic representation (A) and immunofluorescence images (B) of Aurora-B localization during mitosis in an anaplastic thyroid cancer cell. In (B) Aurora-B is in green, microtubules in red and DNA, stained by DAPI, in blue. Adapted with permission from Ref. [14].

to recruit condensin proteins (**Figure 3**) [67, 68]. From early G2 phase to prophase, Aurora-B phosphorylates histone H3, but its physiological meaning remains unclear. From late prophase to metaphase CPC localizes to the inner centromere, playing a role in formation and stability of the bipolar mitotic spindle, kinetochore assembly, correction of non-bipolar chromosomespindle attachments, and control of the spindle checkpoint (**Figure 3**). In anaphase, the CPC relocates to the midzone of the mitotic spindle and to the cell cortex, remaining evident in the midbody of telophasic cells where it modulates the activity of several proteins involved in spindle dynamics, cleavage furrow formation and completion of cytokinesis (**Figure 3**) [49, 50, 67–69].

Aurora-B activation requires the auto-phosphorylation and binding to INCENP, while all CPC components are necessary for its proper localization during mitosis. Several kinases, such as BubR1 and Bub1 (checkpoint kinases), monopolar spindle 1 (Mps1), checkpoint kinase 1 (Chkl), Tousled-like kinase-1, Plk1, and TD-60/RCC2 (regulator of chromosome condensation 2), have been shown to be involved in Aurora-B activation. The phosphorylation status and activity of Aurora-B are controlled by PP1 and PP2A phosphatases [69].
