Thrombopoietin (Tpo) is the primary regulator of megakaryocyte and platelet numbers and is required for haematopoetic stem cell maintenance. Tpo functions by binding its receptor (TpoR, a homodimeric Class I cytokine receptor) and initiating cell proliferation or differentiation. Here we characterise the murine Tpo:TpoR signalling complex biochemically and structurally, using cryo-electron microscopy. Tpo uses opposing surfaces to recruit two copies of receptor, forming a 1:2 complex. Although it binds to the same, membrane-distal site on both receptor chains, it does so with significantly different affinities and its highly glycosylated C-terminal domain is not required. In one receptor chain, a large insertion, unique to TpoR, forms a partially structured loop that contacts cytokine. Tpo binding induces the juxtaposition of the two receptor chains adjacent to the cell membrane. The therapeutic agent romiplostim also targets the cytokine-binding site and the characterisation presented here supports the future development of improved TpoR agonists.

血小板生成素 (Tpo) 是巨核细胞和血小板数量的主要调节剂，是造血干细胞维持所必需的。 Tpo 通过结合其受体（TpoR，同型二聚体 I 类细胞因子受体）并启动细胞增殖或分化来发挥作用。在这里，我们使用冷冻电子显微镜从生化和结构上表征了小鼠 Tpo:TpoR 信号复合物。 Tpo 使用相对的表面来招募受体的两个副本，形成 1:2 复合物。尽管它与两个受体链上相同的膜远端位点结合，但它的亲和力显着不同，并且不需要其高度糖基化的 C 末端结构域。在一条受体链中，TpoR 特有的大插入形成了接触细胞因子的部分结构化环。 Tpo 结合诱导邻近细胞膜的两条受体链并置。治疗剂 romiplostim 还针对细胞因子结合位点，此处介绍的表征支持改进的 TpoR 激动剂的未来开发。