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Drosophila Schneider 2 (S2) cells are a simple and powerful system commonly used in cell biology because they are well suited for high resolution microscopy and RNAi-mediated depletion. However, understanding dynamic processes, such as cell division, also requires methodology to interfere with protein function with high spatiotemporal control. In this research study, we report the adaptation of an optogenetic tool to Drosophila S2 cells. Light-activated reversible inhibition by assembled trap (LARIAT) relies on the rapid light-dependent heterodimerization between cryptochrome 2 (CRY2) and cryptochrome-interacting bHLH 1 (CIB1) to form large protein clusters. An anti-green fluorescent protein (GFP) nanobody fused with CRY2 allows this method to quickly trap any GFP-tagged protein in these light-induced protein clusters. We evaluated clustering kinetics in response to light for different LARIAT modules, and showed the ability of GFP-LARIAT to inactivate the mitotic protein Mps1 and to disrupt the membrane localization of the polarity regulator Lethal Giant Larvae (Lgl). Moreover, we validated light-induced co-clustering assays to assess protein-protein interactions in S2 cells. In conclusion, GFP-based LARIAT is a versatile tool to answer different biological questions, since it enables probing of dynamic processes and protein-protein interactions with high spatiotemporal resolution in Drosophila S2 cells. This research was funded by the National Funds through Fundação para a Ciência e a Tecnologia (FCT) under the project (PTDC/BEX-BCM/0432/2014). E.M. holds an FCT Investigator position. M.O. is supported by a fellowship from the GABBA PhD program from the University of Porto, PD/BD/105746/2014.

Apical-basal polarity is a common trait that underlies epithelial function. Although the asymmetric distribution of cortical polarity proteins works in a functioning equilibrium, it also retains plasticity to accommodate cell division, during which the basolateral determinant Lgl is released from the cortex. Here, we investigated how Lgl restores its cortical localization to maintain the integrity of dividing epithelia. We show that cytoplasmic Lgl is reloaded to the cortex at mitotic exit in Drosophila epithelia. Lgl cortical localization depends on protein phosphatase 1, which dephosphorylates Lgl on the serines phosphorylated by aPKC and Aurora A kinases through a mechanism that relies on the regulatory subunit Sds22 and a PP1-interacting RVxF motif of Lgl. This mechanism maintains epithelial polarity and is of particular importance at mitotic exit to couple Lgl cortical reloading with the polarization of the apical domain. Hence, PP1-mediated dephosphorylation of Lgl preserves the apicalbasal organization of proliferative epithelia. We thank Daniel St Johnston, François Schweisguth, Guilles Hickson, Jürgen Knoblich, Torcato Martins, Yang Hong, and the Bloomington Drosophila Stock Center for providing plasmids and fly stocks. This work was funded by national funds through Fundação para a Ciência e a Tecnologia (FCT) under project PTDC/BEX-BCM/0432/2014 . This work has also received funding from the project Norte-01-0145-FEDER-000029 , supported by Norte Portugal Regional Operational Program (NORTE 2020). E.M. holds an FCT Investigator position. S.M. and M.G. are supported by FCT PhD grants. M.O. is supported by a fellowship from FCT and the GABBA PhD program from the University of Porto .

Aberrant expression of the Spectraplakin Dystonin (DST) has been observed in various cancers, including those of the breast. However, little is known about its role in carcinogenesis. In this report, we demonstrate that Dystonin is a candidate tumour suppressor in breast cancer and provide an underlying molecular mechanism. We show that in MCF10A cells, Dystonin is necessary to restrain cell growth, anchorage-independent growth, self-renewal properties and resistance to doxorubicin. Strikingly, while Dystonin maintains focal adhesion integrity, promotes cell spreading and cell-substratum adhesion, it prevents Zyxin accumulation, stabilizes LATS and restricts YAP activation. Moreover, treating DST-depleted MCF10A cells with the YAP inhibitor Verteporfin prevents their growth. In vivo, the Drosophila Dystonin Short stop also restricts tissue growth by limiting Yorkie activity. As the two Dystonin isoforms BPAG1eA and BPAG1e are necessary to inhibit the acquisition of transformed features and are both downregulated in breast tumour samples and in MCF10A cells with conditional induction of the Src proto-oncogene, they could function as the predominant Dystonin tumour suppressor variants in breast epithelial cells. Thus, their loss could deem as promising prognostic biomarkers for breast cancer. The authors acknowledge the support of the Bloomington Drosophila Stock Centre, the National Institute of Genetics (NIG-Fly) the services of the Animal, Imaging and Cytometry and Genomics facilities at Instituto Gulbenkian de Ciência, and of the i3S Scientific Platforms, including the Cell Culture and Genotyping (CCGen) and the Genomics (GenCore) platforms, as well as the Bioimaging and the Advanced Light Microscopy platforms. We are also grateful to M. J. Amorim and N. Tapon for reagents and to A. Monteiro for fly food preparation. We specially thank K. Struhl for providing the TAM-inducible ER-Src and PBabe cell lines and Rafeeq Mir, Eurico Morais-de-Sá, Archana Pawar and Carla Oliveira for comments on the manuscript. This work was supported by funds from Fundação para a Ciência e Tecnologia (FCT), co-financed by Fundo Europeu de Desenvolvimento Regional (FEDER) through Programa Operacional Competitividade e Internacionalização (POCI) (POCI-01-0145-FEDER-016390) and the Laço Grant in breast cancer 2015 to F.J. The i3S Bioimaging and Advanced Light Microscopy scientific platforms are both member of the national infrastructure PPBI-Portuguese Platform of BioImaging, supported by POCI-01-0145-FEDER-022122. P.J. was the recipient of fellowships from FCT (PD/ BD/52439/2013). F.J. was the recipient of IF/01031/2012.

According to the prevailing ‘clock’ model, chromosome decondensation and nuclear envelope reformation when cells exit mitosis are byproducts of Cdk1 inactivation at the metaphase-anaphase transition, controlled by the spindle assembly checkpoint. However, mitotic exit was recently shown to be a function of chromosome separation during anaphase, assisted by a midzone Aurora B phosphorylation gradient-the ‘ruler’ model. Here we found that Cdk1 remains active during anaphase due to ongoing APC/CCdc20- and APC/CCdh1-mediated degradation of B-type Cyclins in Drosophila and human cells. Failure to degrade B-type Cyclins during anaphase prevented mitotic exit in a Cdk1-dependent manner. Cyclin B1-Cdk1 localized at the spindle midzone in an Aurora B-dependent manner, with incompletely separated chromosomes showing the highest Cdk1 activity. Slowing down anaphase chromosome motion delayed Cyclin B1 degradation and mitotic exit in an Aurora B-dependent manner. Thus, a crosstalk between molecular ‘rulers’ and ‘clocks’ licenses mitotic exit only after proper chromosome separation. We thank Eric Griffis, Jean-René Huynh, Claudio Sunkel, Jonathon Pines, Melina Schuh and Christian Lehner for the kind gift of reagents, and Marco Gonzalez-Gaitán for supporting OA during the final stages of this work. LPC is the recipient of a Marie Skłodowska-Curie Action fellowship (grant agreement 746515). EMS holds an FCT Investigator position and his work is supported by Fundac¸ ão para a Ciência e a Tecnologia (PTDC/BEX-BCM/0432/2014). This work was supported by R01GM107026 grant to TJM and a Commonwealth Honors College grant to CMC Confocal and FLIM microscopy data collection was performed in the Light Microscopy Facility and Nikon Center of Excellence at the Institute for Applied Life Sciences, University of Massachusetts Amherst with support from the Massachusetts Life Science Center. Work in the HM lab is supported by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 681443) and FLAD Life Science 2020.

Since microRNAs (miRNAs) influence the expression of many genes in cells, understanding how the miRNA pathway is regulated is an important area of investigation. We found that the Drosophila miRNA-induced silencing complex (miRISC) exists in multiple forms. A constitutive form, called G-miRISC, is comprised of Ago1, miRNA, and GW182. Two distinct miRISC complexes that lack GW182 are regulated by mitogenic signaling. Exposure of cells to serum, lipids, or the tumor promoter PMA suppressed formation of these complexes. P-miRISC is comprised of Ago1, miRNA, and Loqs-PB, and it associates with mRNAs assembled into polysomes. The other regulated Ago1 complex associates with membranous organelles, and is likely an intermediate in miRISC recycling. The formation of these novel complexes is correlated with a five- to ten-fold stronger repression of target gene expression inside cells. Taken together, these results indicate that mitogenic signaling regulates the miRNA effector machinery to attenuate its repressive activities.

International audience; Grass is a clip domain serine protease (SP) involved in a proteolytic cascade triggering the Toll pathway activation of Drosophila during an immune response. Epistasic studies position it downstream of the apical protease ModSP and upstream of the terminal protease Spaetzle-processing enzyme. Here, we report the crystal structure of Grass zymogen. We found that Grass displays a rather deep active site cleft comparable with that of proteases of coagulation and complement cascades. A key distinctive feature is the presence of an additional loop (75-loop) in the proximity of the activation site localized on a protruding loop. All biochemical attempts to hydrolyze the activation site of Grass failed, strongly suggesting restricted access to this region. The 75-loop is thus proposed to constitute an original mechanism to prevent spontaneous activation. A comparison of Grass with clip serine proteases of known function involved in analogous proteolytic cascades allowed us to define two groups, according to the presence of the 75-loop and the conformation of the clip domain. One group (devoid of the 75-loop) contains penultimate proteases whereas the other contains terminal proteases. Using this classification, Grass appears to be a terminal protease. This result is evaluated according to the genetic data documenting Grass function.