Povelones Michael, Nusse Roel
Department of Developmental Biology, Beckman Center, Howard Hughes Medical Institute, Stanford University School of Medicine, Stanford, CA 94305-5428, USA.
EMBO J. 2005 Oct 5;24(19):3493-503. doi: 10.1038/sj.emboj.7600817. Epub 2005 Sep 15.
The Frizzled (Fz) receptors contain seven transmembrane helices and an amino-terminal cysteine-rich domain (CRD) that is sufficient and necessary for binding of the ligands, the Wnts. Recent genetic experiments have suggested, however, that the CRD is dispensable for signaling. We engineered fz CRD mutant transgenes and tested them for Wg signaling activity. None of the mutants was functional in cell culture or could fully replace fz in vivo. We also show that replacing the CRD with a structurally distinct Wnt-binding domain, the Wnt inhibitory factor, reconstitutes a functional Wg receptor. We therefore hypothesized that the function of the CRD is to bring Wg in close proximity with the membrane portion of the receptor. We tested this model by substituting Wg itself for the CRD, a manipulation that results in a constitutively active receptor. We propose that Fz activates signaling in two steps: Fz uses its CRD to capture Wg, and once bound Wg interacts with the membrane portion of the receptor to initiate signaling.
卷曲蛋白（Fz）受体含有7个跨膜螺旋和一个氨基末端富含半胱氨酸的结构域（CRD），该结构域对于配体Wnt的结合是充分且必要的。然而，最近的遗传学实验表明，CRD对于信号传导是可有可无的。我们构建了fz CRD突变转基因并测试它们的Wg信号活性。这些突变体在细胞培养中均无功能，也不能在体内完全替代fz。我们还表明，用结构不同的Wnt结合结构域——Wnt抑制因子取代CRD，可重建功能性Wg受体。因此，我们推测CRD的功能是使Wg与受体的膜部分紧密靠近。我们通过用Wg本身替代CRD来测试该模型，这种操作会产生组成型活性受体。我们提出Fz通过两个步骤激活信号传导：Fz利用其CRD捕获Wg，一旦结合，Wg就与受体的膜部分相互作用以启动信号传导。
