Crystal and Structural Chemistry, Bijvoet Center for Biomolecular Research, Department of Chemistry, Faculty of Science, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands.
Cell Rep. 2013 Jun 27;3(6):1885-92. doi: 10.1016/j.celrep.2013.06.009.
Leucine-rich repeat-containing G protein-coupled receptors 4-6 (LGR4-LGR6) are receptors for R-spondins, potent Wnt agonists that exert profound trophic effects on Wnt-driven stem cells compartments. We present crystal structures of a signaling-competent fragment of R-spondin 1 (Rspo1) at a resolution of 2.0 Å and its complex with the LGR5 ectodomain at a resolution of 3.2 Å. Ecto-LGR5 binds Rspo1 at its concave leucine-rich-repeat (LRR) surface, forming a dimeric 2:2 complex. Fully conserved residues on LGR4-LGR6 explain promiscuous binding of R-spondins. A phenylalanine clamp formed by Rspo1 Phe106 and Phe110 pinches Ala190 of LGR5 and is critical for binding. Mutations related to congenital anonychia reduce signaling, but not binding of Rspo1 to LGR5. Furthermore, antibody binding to the extended loop of the C-terminal LRR cap of LGR5 activates signaling in a ligand-independent manner. Thus, our data reveal binding of R-spondins to conserved sites on LGR4-LGR6 and, in analogy to FSHR and related receptors, suggest a direct signaling role for LGR4-LGR6 in addition to its formation of Wnt receptor and coreceptor complexes.
富含亮氨酸重复序列的 G 蛋白偶联受体 4-6（LGR4-LGR6）是 R 分泌蛋白的受体，R 分泌蛋白是一种强有力的 Wnt 激动剂，对 Wnt 驱动的干细胞区室发挥深远的营养作用。我们展示了分辨率为 2.0 Å 的 R 分泌蛋白 1（Rspo1）信号活性片段的晶体结构及其与 LGR5 外域的复合物的晶体结构，分辨率为 3.2 Å。外显子 LGR5 在其凹陷的富含亮氨酸重复（LRR）表面结合 Rspo1，形成二聚体 2:2 复合物。LGR4-LGR6 上完全保守的残基解释了 R 分泌蛋白的混杂结合。由 Rspo1 Phe106 和 Phe110 形成的苯丙氨酸夹钳夹住 LGR5 的 Ala190，对于结合至关重要。与先天性无甲相关的突变会降低信号转导，但不会降低 Rspo1 与 LGR5 的结合。此外，抗体结合到 LGR5 末端 LRR 帽的扩展环以配体非依赖性方式激活信号转导。因此，我们的数据揭示了 R 分泌蛋白与 LGR4-LGR6 上保守位点的结合，并且与 FSHR 和相关受体类似，表明 LGR4-LGR6 除了形成 Wnt 受体和共受体复合物外，还具有直接的信号转导作用。
