Drosophila Naked cuticle (Nkd) engages the nuclear import adaptor Importin-α3 to antagonize Wnt/β-catenin signaling

Laboratory of Molecular Pathology, Department of Pathology, UT Southwestern Medical Center, Dallas, TX 75390-9072, USA.
Developmental Biology (Impact Factor: 3.55). 07/2008; 318(1):17-28. DOI: 10.1016/j.ydbio.2008.02.050
Source: PubMed


Precise control of Wnt/beta-catenin signaling is critical for animal development, stem cell renewal, and prevention of disease. In the fruit fly Drosophila melanogaster, the naked cuticle (nkd) gene limits signaling by the Wnt ligand Wingless (Wg) during embryo segmentation. Nkd is an intracellular protein that is composed of separable membrane- and nuclear-localization sequences (NLS) as well as a conserved EF-hand motif that binds the Wnt receptor-associated scaffold protein Dishevelled (Dsh), but the mechanism by which Nkd inhibits Wnt signaling remains a mystery. Here we identify a second NLS in Nkd that is required for full activity and that binds to the canonical nuclear import adaptor Importin-alpha3. The Nkd NLS is similar to the Importin-alpha3-binding NLS in the Drosophila heat-shock transcription factor (dHSF), and each Importin-alpha3-binding NLS required intact basic residues in similar positions for nuclear import and protein function. Our results provide further support for the hypothesis that Nkd inhibits nuclear step(s) in Wnt/beta-catenin signaling and broaden our understanding of signaling pathways that engage the nuclear import machinery.

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Available from: Raphaël Rousset
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    • "In an effort to identify novel components involved in Notch signaling and its regulation, a yeast two-hybrid screen was carried out using the portion of intracellular domain of Notch receptor as bait and we identified Drosophila Importin-α3 as binding partner of Notch. Drosophila Importin-α3 protein is known to play major role in nuclear trafficking of different Nuclear Localization Signal (NLS) containing proteins such as Germ Cell-less [18], the large subunit of DNA polymerase α [19], heat shock transcription factor (dHSF) [20], Daxx [21], Naked cuticle (Nkd) [22] etc. Since nuclear transport protein Importin-α3 directly binds to portion of Notch intracellular domain which contains NLS [7], we were prompted to examine if Notch intracellular domain translocates to nucleus using the canonical nuclear transport machinery. "
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    • "Immunostaining of salivary glands from 71B>impα1RNAi, 71B>impα2RNAi and 71B>impα3RNAi larvae revealed no changes in Cbt nuclear localization (Figure S3A–D). However, only the impα3RNAi line has been shown to reduce Impα3 immunoreactivity [77]. "
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    • "In abundance of other Dvl PDZ binding partners, the NES motif would be released making the protein more susceptible to nuclear export and resulting in an increased induction of PCP and/or calcium signaling cascade. So far, our understanding of the Wnt signaling network is very limited, and we are still discovering new possible regulatory mechanisms (Yokoyama et al., 2007; Chan et al., 2008; Wu et al., 2008) and links to other cellular processes and pathways (Luyten et al., 2008) to broaden our comprehension of the molecular basis of development. "
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