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p73 is a p53 paralog that encodes proapoptotic (transactivation-competent (TA)) and antiapoptotic (dominant negative) isoforms. TAp73 transcription factors mediate cell cycle arrest and/or apoptosis in response to DNA damage and are involved in developmental processes in the central nervous system and the immune system, p73 proteins may also play a role in the regulation of cell growth. Indeed, p73 expression is itself modulated during the cell cycle and TAp73 proteins accumulate in S phase cells. In addition, the function of p73 proteins is also regulated by post-translational modifications and protein-protein interactions in different cellular and pathophysiological contexts. Here we show that p73 is a physiological target of the p34[sup cdc2]-cyclin B mitotic kinase complex in vivo. Both p73β and p73α isoforms are hyperphosphorylated in normal mitotic cells and during mitotic arrest induced by microtubule-targeting drugs, p34[sup cdc2]-cyclin B phosphorylates and associates with p73 in vivo, which results in a decreased ability of p73 to both bind DNA and activate transcription in mitotic cells. Indeed, p73 is excluded from condensed chromosomes in meta- and anaphase, redistributes throughout the mitotic cytoplasm, and unlike p53, shows no association with centrosomes. Together these results indicate that M phase-specific phosphorylation of p73 by p34[sup cdc2]-cyclin B is associated with negative regulation of its transcriptional activating function. [ABSTRACT FROM AUTHOR]

The GTPase Ran plays a crucial role in nucleo-cytoplasmic transport of tumor suppressors, proto-oncogenes, signaling molecules and transcription factors. It also plays direct roles in mitosis, through which it regulates faithful chromosome segregation and hence the generation of genetically stable cells. Ran operates through a group of effector proteins. In this review we summarize growing evidence suggesting that deregulated activity of Ran or its effectors can contribute to pathways of cell transformation and facilitate tumor progression.

Spatial control is a key issue in cell division. The Ran GTPase regulates several fundamental processes for cell life, largely acting through importin molecules. The best understood of these is protein import through the nuclear envelope in interphase, but roles in mitotic spindle assembly are also established. In mammalian cells, in which centrosomes are major spindle organizers, a link is emerging between the Ran network, centrosomes and spindle poles. Here, we show that, after nuclear envelope breakdown, importin beta is transported to the spindle poles in mammalian cells. This localization is temporally regulated from prometaphase until anaphase, when importin beta dissociates from poles and is recruited back around reforming nuclei. Importin beta sediments with mitotic microtubules in vitro and its accumulation at poles requires microtubule integrity and dynamics in vivo. Furthermore, RNA interference-dependent inactivation of TPX2, the major Ran-dependent spindle organizer, abolishes importin beta accumulation at poles. Importin beta has a functional role in spindle pole organization, because overexpression yields mitotic spindles with abnormal, fragmented poles. Coexpression of TPX2 with importin beta mitigates these abnormalities. Together, these results indicate that the balance between importins and spindle regulators of the TPX2 type is crucial for spindle formation. Targeting of TPX2/importin-beta complexes to poles is a key aspect in Ran-dependent control of the mitotic apparatus in mammalian cells.

By means of the yeast two-hybrid system, we have discovered a novel physical interaction between the adenovirus E1A oncoprotein and Ran, a small GTPase which regulates nucleocytoplasmic transport, cell cycle progression, and mitotic spindle organization. Expression of E1A elicits induction of S phase and centrosome amplification in a variety of rodent cell lines. The induction of supernumerary centrosomes requires functional RCC1, the nucleotide exchange factor for Ran and, hence, a functional Ran network. The E1A portion responsible for the interaction with Ran is the extreme NH(2)-terminal region (amino acids 1-36), which is also required for the induction of centrosome amplification. In an in vitro assay with recombinant proteins, wild-type E1A interferes with nucleotide exchange on Ran, whereas an E1A mutant, deleted from the extreme NH(2)-terminal region, does not. In addition, we detected an in vitro interaction between Ran and HPV-16 E7 and SV40 large T antigen, two oncoproteins functionally related to E1A. These findings suggest a common pathway of these oncoproteins in eliciting virus-induced genomic instability.