Department of Biological Structure, University of Washington School of Medicine, Seattle, WA 98195, USA.
Proc Natl Acad Sci U S A. 2012 Jan 31;109(5):1500-5. doi: 10.1073/pnas.1116618109. Epub 2012 Jan 17.
Axin is a tumor suppressor and a key negative regulator of the Wnt/β-catenin signaling pathway. Axin turnover is controlled by its poly-ADP-ribosylation catalyzed by tankyrase (TNKS), which requires the direct interaction of Axin with TNKS. This interaction is thus an attractive drug target for treating cancers, brain injuries, and other diseases where β-catenin is involved. Here we report the crystal structure of a mouse TNKS1 fragment containing ankyrin-repeat clusters 2 and 3 (ARC2-3) in a complex with the TNKS-binding domain of mouse Axin1. Surprisingly, we found that Axin contains two discrete TNKS-binding segments, both of which bind simultaneously to the two ARC2 domains in the ARC2-3 homodimer. Our crystal structure shows that in each TNKS-binding segment of Axin there is a conserved glycine residue that lies in the bottom of a narrow "gate" formed by two parallel tyrosine side chains on the TNKS surface. This glycine-selection gate is crucial for TNKS-Axin interactions, as mutation of the TNKS gate-forming residues, or mutation of either glycine residue in the two Axin segments, completely abolishes the binding of the corresponding Axin segment to TNKS. The bivalent binding of Axin to TNKS is required for Axin turnover, since mutations in either gate-binding glycine residue in Axin lead to Axin stabilization in the cell. In addition, our analyses also reveal the structural basis for TNKS substrate recruitment, and shed light on the overall structure of TNKS that should help in developing specific inhibitors of Wnt/β-catenin signaling.
Axin 是一种肿瘤抑制因子，也是 Wnt/β-连环蛋白信号通路的关键负调控因子。Axin 的周转受到其被 Tankyrase（TNKS）多聚 ADP-核糖基化的控制，这需要 Axin 与 TNKS 的直接相互作用。因此，这种相互作用是治疗癌症、脑损伤和其他涉及β-连环蛋白的疾病的有吸引力的药物靶点。在这里，我们报告了一个包含ankyrin 重复簇 2 和 3（ARC2-3）的小鼠 TNKS1 片段与小鼠 Axin1 的 TNKS 结合域复合物的晶体结构。令人惊讶的是，我们发现 Axin 包含两个离散的 TNKS 结合片段，这两个片段都同时结合到 ARC2-3 同源二聚体的两个 ARC2 结构域上。我们的晶体结构显示，在 Axin 的每个 TNKS 结合片段中，都有一个保守的甘氨酸残基，该残基位于 TNKS 表面两个平行的酪氨酸侧链形成的狭窄“门”的底部。这个甘氨酸选择门对于 TNKS-Axin 相互作用至关重要，因为 TNKS 门形成残基的突变，或者 Axin 两个片段中任何一个甘氨酸残基的突变，都会完全消除相应 Axin 片段与 TNKS 的结合。Axin 对 TNKS 的二价结合对于 Axin 的周转是必需的，因为 Axin 中的任何一个门结合甘氨酸残基的突变都会导致 Axin 在细胞中的稳定。此外，我们的分析还揭示了 TNKS 底物募集的结构基础，并阐明了 TNKS 的整体结构，这应该有助于开发针对 Wnt/β-连环蛋白信号的特异性抑制剂。
