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PP1 binds Sara and negatively regulates Dpp signaling in Drosophila melanogaster

Nature Genetics volume 31, pages 419–423 (2002)Cite this article

Abstract

In signaling involving the transforming growth factor-β (TGF-β) superfamily of proteins, ligand binding brings the constitutively active type II receptor kinase into close proximity to its substrate, the type I receptor kinase, which it then activates by phosphorylation1. The type I receptor kinase in turn phosphorylates one of the Smad family of transcription factors, which translocates to the nucleus and regulates gene expression2,3,4,5. Smads are recruited to the receptor complex by an anchor protein, SARA (Smad anchor for receptor activation)6. Although several protein kinases in this pathway were known, including the receptors themselves, the relevant phosphatases had not previously been identified. Here we report the isolation of a Drosophila melanogaster homolog of SARA (Sara) in a screen for proteins that bind the catalytic subunit of type 1 serine/threonine protein phosphatase (PP1c). We identified a PP1c-binding motif in Sara, disruption of which reduced the ability of Sara to bind PP1c. Expression of this non-PP1c-binding mutant resulted in hyperphosphorylation of the type I receptor and stimulated expression of a target of TGF-β signaling. Reducing PP1c activity enhanced the increase in the basal level of expression of genes responsive to Dpp (Decapentaplegic) caused by ectopic expression of the type II receptor Punt. Together these data suggest that PP1c is targeted to Dpp receptor complexes by Sara, where it acts as a negative regulator of Dpp signaling by affecting the phosphorylation state of the type I receptor.

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Figure 1: Predicted amino-acid sequence of Sara and alignment with vertebrate homologs.
Figure 2: PP1c binding of Sara in mammalian cells.
Figure 3: Enhanced effect of increased type II receptor kinase (Punt) in D. melanogaster wings resulting from reduction of PP1c.
Figure 4: Activation of the Dpp signaling pathway as a result of SaraF678A expression in wings.
Figure 5: PP1 Antagonism of TGF-β signaling by PP1 through reduced phosphorylation of the type I receptor.
Figure 6: A model of PP1c in the TGFβ receptor complex.

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Acknowledgements

We thank S. Elledge for providing the yeast two-hybrid system, M. Axton and D. Glover for PP1 cDNAs, M. O'Connor, T. Tabata and J. Gausz for fly strains, and M. Kawabata and J. Massague for tissue-culture cells and constructs. This work was supported by UK Medical Research Council (MRC) and BBSRC grants to L.A. L.A. is an MRC Senior Research Fellow.

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  1. Department of Zoology, Oxford University, South Parks Road, Oxford, OX1 3PS, UK

    Daimark Bennett & Luke Alphey

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Correspondence to Luke Alphey.

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Bennett, D., Alphey, L. PP1 binds Sara and negatively regulates Dpp signaling in Drosophila melanogaster. Nat Genet 31, 419–423 (2002). https://doi.org/10.1038/ng938

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