Activin receptor-like kinase 1 (ALK1)-mediated endothelial cell signalling in response to bone morphogenetic protein 9 (BMP9) and BMP10 is of significant importance in cardiovascular disease and cancer. However, detailed molecular mechanisms of ALK1-mediated signalling remain unclear. Here, we report crystal structures of the BMP10:ALK1 complex at 2.3 Å and the prodomain-bound BMP9:ALK1 complex at 3.3 Å. Structural analyses reveal a tripartite recognition mechanism that defines BMP9 and BMP10 specificity for ALK1, and predict that crossveinless 2 is not an inhibitor of BMP9, which is confirmed by experimental evidence. Introduction of BMP10-specific residues into BMP9 yields BMP10-like ligands with diminished signalling activity in C2C12 cells, validating the tripartite mechanism. The loss of osteogenic signalling in C2C12 does not translate into non-osteogenic activity in vivo and BMP10 also induces bone-formation. Collectively, these data provide insight into ALK1-mediated BMP9 and BMP10 signalling, facilitating therapeutic targeting of this important pathway.

激活素受体样激酶 1 (ALK1) 介导的内皮细胞信号转导响应骨形态发生蛋白 9 (BMP9) 和 BMP10 在心血管疾病和癌症中具有重要意义。然而，ALK1 介导的信号传导的详细分子机制仍不清楚。在这里，我们报告了 2.3 Å 的 BMP10:ALK1 复合物的晶体结构和 3.3 Å 的前结构域结合的 BMP9:ALK1 复合物的晶体结构。结构分析揭示了定义 BMP9 和 BMP10 对 ALK1 特异性的三重识别机制，并预测 crossveinless 2 不是 BMP9 的抑制剂，这已得到实验证据的证实。将 BMP10 特异性残基引入 BMP9 中会产生 BMP10 样配体，其在 C2C12 细胞中的信号传导活性减弱，从而验证了三方机制。C2C12 中成骨信号的丧失不会转化为体内非成骨活性，并且 BMP10 也会诱导骨形成。总的来说，这些数据提供了对 ALK1 介导的 BMP9 和 BMP10 信号传导的深入了解，有助于针对这一重要途径的治疗靶向。