Project description:During early embryonic cell divisions, the total nuclear volume doubles with each cycle. At a critical volume ratio between the nucleus and cytoplasm, embryos start transcribing their genome in a reproducible and sequential manner. However, the underlying molecular mechanism for these well-timed events remains unclear. Previous studies proposed that titration of a regulatory factor by the exponentially increasing DNA content of the system triggers the onset of the first transcripts. Nevertheless, a systematic investigation of nuclear proteome changes during development is still missing. In this study, we perform this investigation and find that differential nuclear import timing can largely explain the sequential onset of transcription. First, we investigated nuclear proteome changes in early development. We found that nuclear proteins sequentially titrate into the embryonic nuclei. Interestingly, transcription factors and RNA polymerases enter the nucleus at different times, which correspond to the activation of their nuclear functions. Notably, we observed a high correlation between when transcription factors enter the nucleus and the order in which they activate downstream transcription. Time-lapse imaging of protein import into nuclei encapsulated in droplets of cytoplasmic extract validated our discovery of the sequential nuclear import by mass spectrometry. Lastly, we investigated the underlying molecular mechanism responsible for the differential timing of protein entry into embryonic nuclei. To this end, we quantified the relative affinity of substrates to importin and observe that high-affinity substrates tend to enter the nucleus earlier than low-affinity substrates. Notably, we can explain > 36% of the observed time-variation of when proteins enter the nucleus in the embryo with this simple affinity assay. Together, our results suggest that embryos control protein entry into the nucleus by modulating importin affinities, which then determines the timing of essential nuclear processes in early development.

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Dynamic regulation of chromatin signatures during Drosophila embryonic glial differentiation [RNA-Seq]

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