Hippo Promotes Proliferation Arrest and Apoptosis in the Salvador/Warts Pathway

Proliferation and apoptosis must be precisely regulated to form organs with appropriate cell numbers and to avoid tumour growth 1,2. Here we show that Hippo (Hpo), the Drosophila homologue of the mammalian Ste20-like kinases 3, MST1/2, promotes proper termination of cell proliferation and stimulates apoptosis during development. hpo mutant tissues are larger than normal because mutant cells continue to proliferate beyond normal tissue size and are resistant to apoptotic stimuli that usually eliminate extra cells. Hpo negatively regulates expression of Cyclin E to restrict cell proliferation, downregulates the Drosophila inhibitor of apoptosis protein DIAP1, and induces the proapoptotic gene head involution defective (hid) to promote apoptosis. The mutant phenotypes of hpo are similar to those of warts (wts), which encodes a serine/threonine kinase of the myotonic dystrophy protein kinase family 4,5, and salvador (sav), which encodes a WW domain protein that binds to Wts 6,7. We find that Sav binds to a regulatory domain of Hpo that is essential for its function, indicating that Hpo acts together with Sav and Wts in a signalling module that coordinately regulates cell proliferation and apoptosis

Different modes of translation for hid, grim and sickle mRNAs in Drosophila

Protein synthesis is inhibited during apoptosis. However, the translation of many mRNAs still proceeds driven by internal ribosome entry sites (IRESs). Here we show that the 5'UTR of hid and grim mRNAs promote translation of uncapped-mRNA reporters in cell-free embryonic extracts and that hid and grim mRNA 5'UTRs drive IRES-mediated translation. The translation of capped-reporters proceeds in the presence of cap competitor and in extracts where cap-dependent translation is impaired. We show that the endogenous hid and grim mRNAs are present in polysomes of heat-shocked embryos, indicating that cap recognition is not required for translation. In contrast, sickle mRNA is translated in a cap-dependent manner in all these assays. Our results show that IRES-dependent initiation may play a role in the translation of Drosophila proapoptotic genes and suggest a variety of regulatory pathways

Mechanism of action of Drosophila Reaper in mammalian cells: Reaper globally inhibits protein synthesis and induces apoptosis independent of mitochondrial permeability

Drosophila Reaper can bind inhibitor of apoptosis proteins (IAP) and thereby rescue caspases from proteasomal degradation. In insect cells, this is sufficient to induce apoptosis. Reaper can also induce apoptosis in mammalian cells, in which caspases need to be activated, usually via the mitochondrial pathway. Nevertheless, we find that Reaper efficiently induces apoptosis in mammalian cells in the absence of mitochondrial permeabilisation and cytochrome c release. Moreover, this capacity was only marginally affected by deletion of Reaper's amino-terminal IAP-binding motif. Independent of this motif, Reaper could globally suppress protein synthesis. Deletion of 20 amino acids from the carboxy-terminus of Reaper fully abrogated its potential to inhibit protein synthesis and to induce apoptosis in the absence of IAP-binding. Our findings indicate that the newly identified capacity of Reaper to suppress protein translation can operate in mammalian cells and may be key to its pro-apoptotic activity