Structural mechanism of angiogenin activation by the ribosome

Abstract

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.