Angiogenin, an RNase A-family protein, promotes angiogenesis and has been implicated in cancer, neurodegenerative diseases, and epigenetic inheritance ^1-10. Upon activation during cellular stress, angiogenin cleaves tRNAs at the anticodon loop, resulting in translation repression ^11-15. The catalytic activity of isolated angiogenin, however, is very low, and the mechanisms of the enzyme activation and tRNA specificity have remained a puzzle ^3,16-23. Here, we uncover these mechanisms using biochemical assays and cryogenic electron microscopy. Our work reveals that the cytosolic ribosome is the long-sought activator of angiogenin. A 2.8-Å resolution cryo-EM structure features angiogenin bound in the A site of the 80S ribosome. The C-terminal tail of angiogenin is rearranged by interactions with the ribosome to activate the RNase catalytic center, making the enzyme several orders of magnitude more efficient in tRNA cleavage. Additional 80S•angiogenin structures capture how tRNA substrate is directed by the ribosome into angiogenin’s active site, demonstrating that the ribosome acts as the specificity factor. Our findings therefore suggest that angiogenin is activated by ribosomes with a vacant A site, whose abundance increases during cellular stress^24-27. These results may facilitate the development of therapeutics to treat cancer and neurodegenerative diseases.

血管生成素是一种 RNase A 家族蛋白，可促进血管生成，并与癌症、神经退行性疾病和表观遗传有关^1-10。在细胞应激过程中激活后，血管生成素会在反密码子环处切割 tRNA，导致翻译抑制^11-15。然而，分离的血管生成素的催化活性非常低，并且酶激活和tRNA特异性的机制仍然是一个谜^3,16-23。在这里，我们使用生化测定和低温电子显微镜揭示了这些机制。我们的工作表明，胞质核糖体是长期以来寻找的血管生成素激活剂。 2.8 Å 分辨率冷冻电镜结构的特点是血管生成素结合在 80S 核糖体的 A 位点上。血管生成素的 C 末端尾部通过与核糖体的相互作用进行重排，从而激活 RNase 催化中心，使该酶的 tRNA 切割效率提高几个数量级。其他80S·血管生成素结构捕获了tRNA底物如何被核糖体引导至血管生成素的活性位点，证明核糖体充当特异性因子。因此，我们的研究结果表明，血管生成素被具有空缺 A 位点的核糖体激活，其丰度在细胞应激期间增加^24-27。这些结果可能有助于开发治疗癌症和神经退行性疾病的疗法。