The function of many eukaryotic proteins is regulated by highly dynamic changes in their nucleocytoplasmic distribution. The ability to precisely and reversibly control nuclear translocation would, therefore, allow dissecting and engineering cellular networks. Here we develop a genetically encoded, light-inducible nuclear localization signal (LINuS) based on the LOV2 domain of Avena sativa phototropin 1. LINuS is a small, versatile tag, customizable for different proteins and cell types. LINuS-mediated nuclear import is fast and reversible, and can be tuned at different levels, for instance, by introducing mutations that alter AsLOV2 domain photo-caging properties or by selecting nuclear localization signals (NLSs) of various strengths. We demonstrate the utility of LINuS in mammalian cells by controlling gene expression and entry into mitosis with blue light.

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13 during embryonic development and early induction of organ 14 progenitors. Besides free diffusion to form signalling gradients, 15 extracellular vesicle-(EV-) mediated morphogen transport was 16 identified as a central mechanism for Wnt-and Hh-signalling. Here, we 17 investigated EVs isolated from whole zebrafish embryos as a potential 18 morphogen transport mechanism. Inhibition of EV-secretion during 19 development leads to severe dorsalization phenotypes, reminiscent of 20 disrupted BMP-signalling. Subsequently, we found that EVs isolated 21 2 from zebrafish embryos at bud stage contain biologically active BMP2/4 22 protein. Embryos with inhibited EV secretion display reduced 23 Smad1/5/9-phosphorylation and downstream gene expression activity. 24 We further show that BMP-containing EVs are secreted by endodermal 25 cells in vitro, and inhibition of endodermal-EV release in vivo causes 26 signs of BMP signalling loss. Our data provides evidence that establishes 27