Drosophila miR2 induces pseudo-polysomes and inhibits translation initiation

Abstract

MicroRNAs (miRs) inhibit protein synthesis by mechanisms that are as yet unresolved1,2,3,4,5,6,7,8,9,10,11. We developed a cell-free system from Drosophila melanogaster embryos that faithfully recapitulates miR2-mediated translational control by means of the 3′ untranslated region of the D. melanogaster reaper messenger RNA. Here we show that miR2 inhibits translation initiation without affecting mRNA stability. Surprisingly, miR2 induces the formation of dense (heavier than 80S) miRNPs (‘pseudo-polysomes’) even when polyribosome formation and 60S ribosomal subunit joining are blocked. An mRNA bearing an ApppG instead of an m7GpppG cap structure escapes the miR2-mediated translational block. These results directly show the inhibition of m7GpppG cap-mediated translation initiation as the mechanism of miR2 function, and uncover pseudo-polysomal messenger ribonucleoprotein assemblies that may help to explain earlier findings.

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Figure 1: Translational repression of a miR2 reporter mRNA in vitro.
Figure 2: miR2 inhibits 80S complex formation and induces heavy mRNP assemblies.
Figure 3: miR2 induces the formation of heavy mRNP assemblies even when 60S ribosomal subunit joining is blocked.
Figure 4: The m 7 GpppN cap structure is essential for miR2-mediated translational repression but not for the formation of ‘pseudo-polysomes’.

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Acknowledgements

We thank S. Cohen and J. Brennecke for advice, and R. Jackson and the members of the Hentze laboratory for discussions. This work was supported by a grant from the Deutsche Forschungsgemeinschaft to M.W.H.

Author Contributions R.T. performed the experiments. R.T. and M.W.H. designed, analysed and interpreted the experiments and wrote the paper.

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Correspondence to Matthias W. Hentze.

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Thermann, R., Hentze, M. Drosophila miR2 induces pseudo-polysomes and inhibits translation initiation. Nature 447, 875–878 (2007). https://doi.org/10.1038/nature05878

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