Plant-unique receptor kinases harbor conserved cytoplasmic kinase domains and sequence-diverse ectodomains. Here we report crystal structures of CRINKLY4-type ectodomains from Arabidopsis ACR4 and Physcomitrella patens PpCR4 at 1.95 Å and 2.70 Å resolution, respectively. Monomeric CRINKLY4 ectodomains harbor a N-terminal WD40 domain and a cysteine-rich domain (CRD) connected by a short linker. The WD40 domain forms a seven-bladed β-propeller with the N-terminal strand buried in its center. Each propeller blade is stabilized by a disulfide bond and contributes to the formation of a putative ligand binding groove. The CRD forms a β-sandwich structure stabilized by six disulfide bonds and shares low structural homology with tumor necrosis factor receptor domains. Quantitative binding assays reveal that ACR4 is not a direct receptor for the peptide hormone CLE40. An ACR4 variant lacking the entire CRD can rescue the known acr4-2 mutant phenotype, as can expression of PpCR4. Together, an evolutionary conserved signaling function for CRINKLY4 receptor kinases is encoded in its WD40 domain.

中文翻译：

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拟南芥和Physcomitrella CR4的晶体结构揭示了CRINKLY4受体激酶的分子结构。

植物独特的受体激酶具有保守的胞质激酶结构域和序列多样的胞外域。在这里，我们报道了来自拟南芥ACR4和Physcomitrella patens PpCR4的CRINKLY4型胞外域的晶体结构，其分辨率分别为1.95Å和2.70Å。单体CRINKLY4胞外域包含一个N末端WD40域和一个通过短接头连接的富含半胱氨酸的域（CRD）。WD40结构域形成一个七叶形的β螺旋桨，其N端链埋在其中心。每个螺旋桨叶片均通过二硫键稳定，并有助于形成假定的配体结合槽。CRD形成由六个二硫键稳定的β夹心结构，并且与肿瘤坏死因子受体域的结构同源性低。定量结合测定表明，ACR4不是肽激素CLE40的直接受体。缺乏完整CRD的ACR4变体可以挽救已知的acr4-2突变体表型，PpCR4的表达也可以。一起，在其WD40域中编码CRINKLY4受体激酶的进化保守信号功能。