Rabouille: Cellular stress in Drosophila Back to research group The Rabouille group studies the cell biology of stress in Drosophila. Delivery of newly synthesized proteins to the plasma membrane and the extracellular medium takes place in the membrane organelles of the secretory pathway. We focus on the ER exit sites (ERES), that are characterized by the machinery mediating COPII coat vesicle budding. This includes the large scaffold ERES protein Sec16 that optimizes COPII vesicle dynamics. This molecular machinery is tightly controlled and is regulated by signaling. In this context, we have studied the role of ERK7 in Drosophila S2 cells in response to serum starvation, and have shown that amino-acid starvation leads to the formation of a of novel membrane-less stress assembly, the Sec bodies, as well as stress granules. Amino-acid starvation of Drosophila leads to the formation of Sec bodies and stress granules, two pro-survival stress assemblies A large part of our past activity has focussed on identifying principles underlying the functional organisation of the early secretory pathway (ER exit site (ERES)-Golgi) in cultured Drosophila S2 cells (1)(2). In this respect, we have characterised the Drosophila orthologue of the large hydrophilic protein Sec16 that is critical for the biogenesis and maintenance of ERES (3). Recently, we have used an RNAi depletion approach of pre-selected putative ER proteins to identify new components involved this organisation (4). Considering the strong regulation of secretion and the organisation of the early secretory pathway by signalling (5), we have started a large project to elucidate the regulation of secretion by nutrient signalling in Drosophila S2 cells. In particular, we have shown that secretion is actively inhibited by serum starvation, leading to the release of Sec16 away from ER membrane (6). On the other hand, amino-acid starvation leads to the formation of reversible non-membrane bound pro-survival stress assemblies, the Sec bodies, that incorporate the components of the ER exit sites, the Sec16 and the COPII components (7). We have recently studied whether ADP-ribosylation plays a role in the formation of stress assemblies. We showed that PARP16 MARylates Sec16 on a small region called the SRDC and that this is sufficient to induce Sec body formation (8). We are also working on the role of PARP1 in stress granule formation. We are in the middle to identifying the signalling pathways required for formation of these stress assemblies as well as their composition. We are also testing whether Sec bodies form in vivo using the Drosophila fat body, and whether they form in mammalian cells. Another stress assembly, the stress granules, also forms upon amino-acid starvation. Stress granules form as a result of protein translation inhibition that is imposed by almost any type of stress. This leads in turn to the accumulation of untranslated mRNAs that are bound to specific RNA binding proteins to be stored in stress granules or degraded in P-bodies (7). Interestingly, we have noticed that the formation of both structures, Sec bodies and stress granules, is linked and we are investigating this. Indeed, we have shown that Sec16, a protein related to ER exit sites and protein transport, is also required in stress granule formation specifically during amino-acid starvation because it binds a modified form of the RNA binding protein Rasputin and stabilises it (9). Asymmetric distribution of mRNAs in the Drosophila egg chamber: Sub- compartmentalisation of P-bodies and translational control In the past, we have developed methods to visualise RNA localisation at the ultrastructural level based on RNA in situ hybridisation coupled to immuno-EM on frozen sections. In collaboration with the Davis group in Oxford (UK), we have investigated the localisation of gurken (10) and bicoid (11) mRNAs in the Drosophila oocyte, both endogenous, injected and MS2 tagged. We have also recently shown that although both mRNAs reside in the same large cytoplasmic structures resembling Processing Bodies in stage 9 oocytes, their sub-compartmentalisation within P-bodies correlates with different states of translation (12). We have studied how this controls gurken translation in oocytes and absence of translation in the nurse cells when gurken RNA is synthesised. We have shown that the transcriptor activator Orb, which is enriched in the oocyte and depleted in the nurse cells, is a key determinant in gurken translation (13). GRASP mutant mice In most cases, newly synthesised proteins destined for the plasma membrane are synthesised in the ER and use the classical secretory pathway (ER>ER exit sites>Golgi>PM) (14). A few years ago, we have shown that in Drosophila, some of these proteins bypass the Golgi in a dGRASP dependent manner (15)(16). Surprisingly, dGRASP is a peripheral protein of the Golgi apparatus and is required for many aspects of its biology (17). To understand the relevance of this pathway in mammals, we have generated a KO mouse for one of the mammalian homologues, GRASP65, that displays no phenotype under state conditions (18). In collaboration with the Malhotra lab in Barcelona (Spain), we are now generating a double KO GRASP65/GRASP55 mouse. References 1. Kondylis V. Van Nispen tot Pannerden, H.E., Herpers, B., Friggi-Grelin, F. and Rabouille, C. The Golgi comprises a paired stack that is separated at G2 by modulation of the actin cytoskeleton through Abi and Scar:WAVEDevelopmental Cell 12:901 (2007) 2. Kondylis V. and Rabouille C. The Golgi apparatus - lessons from DrosophilaFEBS lett 583:3827 (2009) 3. Ivan V., De Voer, G., Xanthakis, D., Spoorendonk, K.M., Kondylis, V. and Rabouille, C. Drosophila Sec16 mediates the biogenesis of tER sites upstream of Sar1 through an arginine-rich motifMol Biol Cell 19:4352 (2008) 4. Kondylis V., Tang, Y., Fuchs, F., Boutros, M. and Rabouille, C. Identification of ER proteins involved in the functional organisation of the early secretory pathway in drosophila cells by a targed RNAi screenPLOS One 6: e17173 (2011) 5. Farhan H. and Rabouille C. Signaling to and from the secretory pathwayJ Cell Sci 124:171 (2011) 6. Zacharogianni, M., V. Kondylis, Y. Tang, D. Xanthakis, H. Farhan, F. Fuchs, M. Boutros and C. Rabouille. ERK7 is a negative regulator of protein secretion in response to amino-acid starvation by modulating Sec16 membrane associationEMBO J 30:3684-700 (2011) 7. Zacharogianni, M., A. Aguilera, J. Smout, T. Veenendaal, and C. Rabouille. A stress assembly that confers cell viability by preseriving ERES components during amino-acid starvationElife 3: e04132 (2014) 8. Aguilera-Gomez A., Van Oorschot, M.M., Veenendaal, T. and Rabouille, C. In vivo vizualisation of mono-ADP-riboylation by dPARP16 upon amino-acid starvationElife 3: e21475 (2016) 9. Aguilera-Gomez, A., Zacharogianni, M., Van Oorschot, M.M., Genau, H., Grond, R., Veenendaal, T., Sinsimer, K.S., Gavis, E.A., Behrends, C., Rabouille, C. Phospho-rasputin stabilization by Sec16 is required for stress granule formation upon amino acid starvationCell Reports 20:935-948 (2017) 10. Delanoue R., Herpers, B., Davis, I. and Rabouille, C. Drosophila Squid-hnRNP helps dynein switch from a gurken mRNA transport motor to an ultrastructural static anchor in sponge bodiesDevelopmental Cell 13:523 (2007) 11. Weil T.T., Xanthakis, D., Parton, R., Dobbie, I., Rabouille, C., Gavis, E.R. and Davis, I. Distinguishing direct from indirect roles for bicoid mRNA localization factorsDevelopment 137:169 (2010) 12. Weil, T.T., Parton, R.M., Herpers, B., Soetaert, J., Veenendaal, T., Xanthakis, D., Dobbie, I.M., Halstead, J., Hayashi, R., Rabouille, C.* and Davis, I.* Drosophila patterning is established by differential association of mRNAs with P bodiesNature Cell Biology 14:1305-13 (2012) 13. Davidson, A., Parton, R.M., Rabouille, C., Weil, T.T. and Davis, I. Localized Translation of gurkenTGF-a mRNA during Axis Specification is Controlled by Access to OrbCPEB on Processing BodiesCell Reports 14:2451-2462 (2016) 14. Mellman I. and Warren W. The road taken - past and future foundations of membrane trafficCell 100:99 (2000). 15. Rabouille, C. Pathways of unconventional protein secretionTrends Cell Biol. 27:230-240 (2017). 16. Grieve AG and Rabouille C Golgi Bypass - Skirting Around the Heart of Classical SecretionCold Spring Harb Perspect Biol. 1;3(4) (2011). 17. Vinke F.P., Grieve, A.G. and Rabouille, C. The multiple facets of the Golgi reassembly stacking proteinsBiochem J 433(3):423-433 (2011) 18. Veenendaal, T., Jarvela, T., Grieve, A.G., Van Es, J.H., Linstedt, A.D. and Rabouille, C. GRASP65 controls the cis Golgi integrity in vivoBiology Open 3:431 (2014).