Drosophila Decapentaplegic (Dpp) has served as a paradigm to study morphogen-dependent growth control. However, the role of a Dpp gradient in tissue growth remains highly controversial. Two fundamentally different models have been proposed: the 'temporal rule' model suggests that all cells of the wing imaginal disc divide upon a 50% increase in Dpp signalling, whereas the 'growth equalization model' suggests that Dpp is only essential for proliferation control of the central cells. Here, to discriminate between these two models, we generated and used morphotrap, a membrane-tethered anti-green fluorescent protein (GFP) nanobody, which enables immobilization of enhanced (e)GFP::Dpp on the cell surface, thereby abolishing Dpp gradient formation. We find that in the absence of Dpp spreading, wing disc patterning is lost; however, lateral cells still divide at normal rates. These data are consistent with the growth equalization model, but do not fit a global temporal rule model in the wing imaginal disc.

The role of protein localization along the apical-basal axis of polarized cells is difficult to investigate in vivo, partially due to lack of suitable tools. Here, we present the GrabFP system, a collection of four nanobody-based GFP-traps that localize to defined positions along the apical-basal axis. We show that the localization preference of the GrabFP traps can impose a novel localization on GFP-tagged target proteins and results in their controlled mislocalization. These new tools were used to mislocalize transmembrane and cytoplasmic GFP fusion proteins in the Drosophila wing disc epithelium and to investigate the effect of protein mislocalization. Furthermore, we used the GrabFP system as a tool to study the extracellular dispersal of the Decapentaplegic (Dpp) protein and show that the Dpp gradient forming in the lateral plane of the Drosophila wing disc epithelium is essential for patterning of the wing imaginal disc.DOI: http://dx.doi.org/10.7554/eLife.22549.001

Developmental biology research would benefit greatly from tools that enable protein function to be regulated, both systematically and in a precise spatial and temporal manner, In recent years, functionalized protein binders have emerged as versatile tools that can be used to target and manipulate proteins. Such protein binders can be based on various scaffolds, such as nanobodies, designed ankyrin repeat proteins (DARPins) and monobodies, and can be used to block or perturb protein function in living cells. In this Primer, we provide an overview of the protein binders that are currently available and highlight recent progress made in applying protein binder-based tools in developmental and synthetic biology.

Highlights d Asymmetric expression of a single Toll receptor leads to Myo-II polarization d The adhesion GPCR Cirl binds to Toll-8 mediating Toll-8induced Myo-II polarization d Toll-8 boundaries generate a Cirl interfacial asymmetry that can polarize Myo-II d Differences in Toll-8 levels lead to interdependent Toll-8 and Cirl planar polarity

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Investigating the role of protein localization is crucial to understand protein function in cells or tissues. However, in many cases the role of different subcellular fractions of given proteins along the apical-basal axis of polarized cells has not been investigated in vivo, partially due to lack of suitable tools. Here, we present the GrabFP system, a nanobody-based toolbox to modify the localization and the dispersal of GFP-tagged proteins along the apical-basal axis of polarized cells. We show that the GrabFP system is an effective and easy-to-implement tool to mislocalize cytosolic and transmembrane GFP-tagged proteins and thereby functionally investigate protein localization along the apical-basal axis. We use the GrabFP system as a tool to study the extracellular dispersal of the Decapentaplegic (Dpp) protein and show that the Dpp gradient forming in the lateral plane of the Drosophila wing disc epithelium is essential for patterning of the wing imaginal disc.

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Tissue morphogenesis requires spatial control over actomyosin contractility to drive cell shape changes 1,2 . How developmental patterning controls cell mechanics is poorly understood. In Drosophila embryos, Myosin-II is enriched at the interface between anteroposterior neighboring cells, driving planar polarized cell intercalation and extension of the embryonic axis 3-5 . G protein-coupled receptors (GPCRs) signaling controls Myosin-II activation at junctions 6,7 and Toll receptors provide a spatial bias to this process 8,9 . How Toll receptors polarize actomyosin contractility remains unknown. Here we report that cells expressing different levels of a single Toll receptor, Toll-8, activate Myosin-II at their interface. This does not require the intracellular domain of Toll-8, suggesting signaling by a proxy mechanism. We found that Toll-8 forms a molecular complex with the adhesion GPCR Cirl/Latrophilin that is required for junctional Myosin-II activation. We further show that Myosin-II is also activated at the interfaces between wild-type and cirl deficient cells. Since Toll-8 differentially recruits Cirl in trans and in cis, we propose that Toll-8 activates Myosin-II by inducing Cirl asymmetric localization at the boundary of its expression domain. Finally, we found that Toll-8 and Cirl exhibit dynamic interdependent planar polarization when neighboring cells express different levels of Toll-8. Through this feedback, Toll-8 and Cirl self-organize to drive planar polarized signaling. This work sheds new light on how embryo patterning induces cell polarization during morphogenesis.