The crystal structure of full-length Sizzled from Xenopus laevis yields insights into Wnt-antagonistic function of secreted Frizzled-related proteins
The crystal structure of full-length Sizzled from Xenopus laevis yields insights into Wnt-antagonistic function of secreted Frizzled-related proteins
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非洲爪蟾全长 Sizzled 的晶体结构有助于深入了解分泌的卷曲相关蛋白的 Wnt 拮抗功能
DOI：
10.1074/jbc.m117.791756
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发表时间：
2017-09-29
影响因子：
4.8
通讯作者：
Liu, Heli
中科院分区：
文献类型：
--
作者：
Bu, Qixin;Li, Zhiqiang;Liu, Heli
Wnt信号通路对细胞增殖、分化和迁移至关重要。分泌型卷曲相关蛋白（sFRPs）代表分泌型Wnt抑制剂的最大家族。然而，它们在拮抗Wnt信号传导中的功能仍然存在一些争议。在这里，我们报告的晶体结构Sizzled从非洲爪蟾，第一个全长结构的sFRP。Sizzled的N-末端富含半胱氨酸的结构域（CRD）和C-末端网蛋白样结构域（NTR）通过结构域间二硫键和接头连接，以串联方式排列，其中NTR结构域封闭CRD的凹槽以便于Wnt接近。双荧光素酶测定表明，去除NTR结构域并用芳香族残基取代CRD沟残基His-116和His-118可以显著增强Sizzled在抑制Wnt 3A信号传导中的拮抗功能。Sizzled是溶液中的单体，Sizzled CRD在晶体中表现出不同的包装，表明sFRP不具有保守的CRD二聚化模式。与经典的NTR结构域不同，Sizzled NTR采用了一种新的α/β折叠，其中两个垂直的螺旋面向中心混合β折叠。人sFRP 1/2/5和Sizzled的亚组应该具有类似的NTR结构域，其特征在于与NTR-CRD界面相对的高度带正电荷的区域，这表明人sFRPs中的NTR结构域，至少sFRP 1/2/5，不太可能与Wnt结合，但可能参与双相Wnt信号调节。总之，Sizzled结构提供了关于CRD和NTR结构域如何彼此相关以调节sFRP的Wnt拮抗功能的第一见解。
The Wnt-signaling pathway is crucial to cell proliferation, differentiation, and migration. The secreted Frizzled-related proteins (sFRPs) represent the largest family of secreted Wnt inhibitors. However, their function in antagonizing Wnt signaling has remained somewhat controversial. Here, we report the crystal structure of Sizzled from Xenopus laevis, the first full-length structure of an sFRP. Tethered by an inter-domain disulfide bond and a linker, the N-terminal cysteine-rich domain (CRD) and the C-terminal netrin-like domain (NTR) of Sizzled are arranged in a tandem fashion, with the NTR domain occluding the groove of CRD for Wnt accessibility. A Dual-Luciferase assay demonstrated that removing the NTR domain and replacing the CRD groove residues His-116 and His-118 with aromatic residues may significantly enhance antagonistic function of Sizzled in inhibiting Wnt3A signaling. Sizzled is a monomer in solution, and Sizzled CRD exhibited different packing in the crystal, suggesting that sFRPs do not have a conserved CRD dimerization mode. Distinct from the canonical NTR domain, the Sizzled NTR adopts a novel α/β folding with two perpendicular helices facing the central mixed β-sheet. The subgroup of human sFRP1/2/5 and Sizzled should have a similar NTR domain that features a highly positively charged region, opposite the NTR–CRD interface, suggesting that the NTR domain in human sFRPs, at least sFRP1/2/5, is unlikely to bind to Wnt but is likely involved in biphasic Wnt signaling modulation. In summary, the Sizzled structure provides the first insights into how the CRD and the NTR domains relate to each other for modulating Wnt-antagonistic function of sFRPs.