The Aurora family kinases are pivotal to the successful execution of cell division. Together they ensure the formation of a bipolar mitotic spindle, accurate segregation of chromosomes and the completion of cytokinesis. They are also attractive drug targets, being frequently deregulated in cancer and able to transform cells in vitro. In this review, we summarize current knowledge about the three family members, Aur-A, Aur-B and Aur-C. We then focus on Aur-A, its roles in mitotic progression, and its emerging roles in checkpoint control pathways. Aur-A activity can be controlled at several levels, including phosphorylation, ubiquitin-dependent proteolysis and interaction with both positive regulators, such as TPX2, and negative ones, like the tumor suppressor protein p53. In addition, work in Xenopus oocytes and early embryos has revealed a second role for Aur-A, directing the polyadenylation-dependent translation of specific mRNAs important for cell cycle progression. This function extends to post-mitotic neurons, and perhaps even to cycling somatic cells.

The mitotic kinase Aurora A (Aur-A) is overexpressed in a high proportion of human tumors, often in the absence of gene amplification. In somatic cells, Aur-A protein levels fall following mitosis or upon overexpression of Cdh1, an activator of the ubiquitin ligase APC/C. Thus, mutations that reduce or block the rate of Aur-A destruction might also be expected to contribute to its oncogenic potential. Previous work had defined two short sequences of Xenopus Aur-A that are required for its Cdh1-inducible destruction in extracts of Xenopus eggs, an N-terminal A box and a C-terminal D box, and a serine residue within the A box whose phosphorylation might inhibit destruction. Here, we show that these same sequences are required for the destruction of human Aur-A during mitotic exit and G1 in the somatic cell cycle. Expression of a dominant negative Cdh1 protein leads to accumulation of Aur-A, further indicating that the Cdh1-activated form of the APC/C is responsible for destruction of Aur-A during the somatic cell cycle in vivo. During the course of this work, we found some previously unsuspected problems in commonly used in vitro destruction assays, which can result in misleading results. Potentially confounding factors include: (i) the presence of D-box- and A-box-dependent destruction-promoting activities in the reticulocyte in vitro translation mix that is used to produce radiolabeled substrates for destruction assays; and (ii) the ability of green-fluorescent-protein tags to reduce the destruction rate of Aur-A substantially. These findings have direct relevance for studies of Aur-A destruction itself, and for broader approaches that use in vitro translation products in screens for additional APC/C targets.

Through a screen to identify genes that induce multi-drug resistance when overexpressed, we have identified a fission yeast homolog of Int-6, a component of the human translation initiation factor eIF3. Disruption of the murine Int-6 gene by mouse mammary tumor virus (MMTV) has been implicated previously in tumorigenesis, although the underlying mechanism is not yet understood. Fission yeast Int6 was shown to interact with other presumptive components of eIF3 in vivo, and was present in size fractions consistent with its incorporation into a 43S translation preinitiation complex. Drug resistance induced by Int6 overexpression was dependent on the AP-1 transcription factor Pap1, and was associated with increased abundance of Pap1-responsive mRNAs, but not with Pap1 relocalization. Fission yeast cells lacking the int6 gene grew slowly. This growth retardation could be corrected by the expression of full length Int6 of fission yeast or human origin, or by a C-terminal fragment of the fission yeast protein that also conferred drug resistance, but not by truncated human Int-6 proteins corresponding to the predicted products of MMTV-disrupted murine alleles. Studies in fission yeast may therefore help to explain the ways in which Int-6 function can be perturbed during MMTV-induced mammary tumorigenesis.

Eukaryotic translation initiation factor 3 (eIF3) is a multisubunit complex that plays a central role in translation initiation. We show that fission yeast Sum1, which is structurally related to known eIF3 subunits in other species, is essential for translation initiation, whereas its overexpression results in reduced global translation. Sum1 is associated with the 40S ribosome and interacts stably with Int6, an eIF3 component, in vivo, suggesting that Sum1 is a component of the eIF3 complex. Sum1 is cytoplasmic under normal growth conditions. Surprisingly, Sum1 is rapidly relocalized to cytoplasmic foci after osmotic and thermal stress. Int6 and p116, another putative eIF3 subunit, behave similarly, suggesting that eIF3 is a dynamic complex. These cytoplasmic foci, which additionally comprise eIF4E and RNA components, may function as translation centers during environmental stress. After heat shock, Sum1 additionally colocalizes stably with the 26S proteasome at the nuclear periphery. The relationship between Sum1 and the 26S proteasome was further investigated, and we find cytoplasmic Sum1 localization to be dependent on the 26S proteasome. Furthermore, Sum1 interacts with the Mts2 and Mts4 components of the 26S proteasome. These data indicate a functional link between components of the structurally related eIF3 translation initiation and 26S proteasome complexes.